 "INTEGRAL (I)"
 (ii)
Main
Objectives
X-ray Transients
SFXTs
New Classes of HMXRBs
Gamma-Astronomie mit Radionukliden
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INTEGRAL (i)
K1.1 
Multiwavelength
K1.2
Counterparts for IBIS/ISGRI Survey Sources
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Authors: J. B. Stephen, L. Bassani, M. Molina, A. Malizia, A. Bazzano, P. Ubertini, A.J. Dean, A.J. Bird, R. Much, R. Walter |
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A&A 432 (2005) L49 [astro-ph/0502158] |
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Using the ROSAT Bright Source Catalogue to find Counterparts for IBIS/ISGRI Survey Sources
The IBIS/ISGRI first year galactic plane survey has produced a catalogue containing 123 hard X-ray sources visible down to a flux limit of a few milliCrabs. The point source location accuracy of typically 1-3 arcminutes has allowed the counterparts for 95 of these sources to be found at other wavelengths. In order to identify the remaining 28 objects, we have cross-correlated the ISGRI catalogue with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for ISGRI sources which have a counterpart in soft X-rays, we can use the, much smaller, ROSAT error box to search for identifications. As expected, we find a strong correlation between the two catalogues and calculate that there are 75 associations with the number expected by chance to be almost zero. Of these 75 sources, ten are in the list of unidentified objects. Using the ROSAT error boxes we provide tentative associations for 8 of these. |
and a
projected semimajor axis of axsini = 68 lt-s.
We conclude that the donor is an OB-supergiant with a mass of
18.8-29.3 M
K
1.3
INTEGRAL: The first Gamma-Ray Burst Catalogue
Definition: time intervals (duration) T90 & T50
T90
measures the duration of the time interval (cf.
) during which 90% of the total observed
counts have been detected.
The start of the T90 interval is defined by the time at which 5% of the total counts have
been detected, and the end of the T90 interval is defined by the time at which
95% of the total counts have been detected.
T50 is similarly defined using the times at which 25% and 75% of the counts have been detected.
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Authors: A. Rau, A.v. Kienlin, K. Hurley, G.G. Lichti |
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A&A 438 (2005) pp.1175-1183 [astro-ph/0504357
] |
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The 1st INTEGRAL SPI-ACS Gamma-Ray Burst Catalogue We present the sample of gamma-ray bursts detected with the anti-coincidence shield ACS of the spectrometer SPI on-board INTEGRAL for the first 26.5 months of mission operation (up to Jan 2005). SPI-ACS works as a nearly omnidirectional gamma-ray burst detector above ~80 keV but lacks spatial and spectral information. In this catalogue, the properties derived from the 50 ms light curves (e.g., T90, Cmax, Cint, variability, V/Vmax) are given for each candidate burst in the sample. A strong excess of very short events with durations < 0.25 s is found. This population is shown to be significantly different from the short- and long-duration burst sample by means of the intensity distribution and V/Vmax test and is certainly connected with cosmic ray hits in the detector. A rate of 0.3 true gamma-ray bursts per day is observed. |
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K
1.4 An emerging population of dark afterglows
| GRB 040223 and GRB 040624 — dark afterglows | |
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Authors: P. Filliatre, S. Covino, P. D'Avanzo, A. De Luca, D. Gotz, S. McGlynn, S. McBreen, D. Fugazza, A. Antonelli, S. Campana, G. Chincarini, A. Cucchiara, M. Della Valle, S. Foley, P. Goldoni, L. Hanlon, G. Israel, B. McBreen, S. Mereghetti, L. Stella, G. Tagliaferri |
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Journal-ref: A&A (2005) [astro-ph/0511722] |
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Title: The weak INTEGRAL bursts GRB 040223 and GRB 040624: an emerging population of dark afterglows |
| Abstract: We report here gamma-ray, X-ray and near-infrared observations of GRB 040223 along with gamma-ray and optical observations of GRB 040624. GRB 040223 was detected by INTEGRAL close to the Galactic plane and GRB 040624 at high Galactic latitude. Analyses of the prompt emission detected by the IBIS instrument on INTEGRAL are presented for both bursts. The two GRBs have long durations, slow pulses and are weak. The gamma-ray spectra of both bursts are best fit with steep power-laws, implying they are X-ray rich. GRB 040223 is among the weakest and longest of INTEGRAL GRBs. The X-ray afterglow of this burst was detected 10 hours after the prompt event by XMM-Newton. The measured spectral properties are consistent with a column density much higher than that expected from the Galaxy, indicating strong intrinsic absorption. We carried out near-infrared observations 17 hours after the burst with the NTT of ESO, which yielded upper limits. Given the intrinsic absorption, we find that these limits are compatible with a simple extrapolation of the X-ray afterglow properties. For GRB 040624, we carried out optical observations 13 hours after the burst with FORS 1 and 2 at the VLT, and DOLoRes at the TNG, again obtaining upper limits. We compare these limits with the magnitudes of a compilation of promptly observed counterparts of previous GRBs and show that they lie at the very faint end of the distribution. These two bursts are good examples of a population of bursts with dark or faint afterglows that are being unveiled through the increasing usage of large diameter telescopes engaged in comprehensive observational programmes. | |
K
1.5 Global characteristics of GRBs
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Faint gamma-ray bursts do actually exist
Gamma-ray bursts, powerful glares of high-energy that wash through the Universe once
every day or so are, for a brief time, the brightest objects in the gamma-ray sky. ESA’s Integral gamma-ray
observatory has observed several low-luminosity gamma-ray bursts, confirming the existence of an entire
population of weaker bursts hardly noticed so far.
Astronomers estimate that about 1400 GRBs per year occur but, because no one knows when and where they are going to appear, only a part of them happen to be detected. Integral detects an average of 10 GRBs per year, and has collected data about 47 of them during four and a half years of operations. When studying IBIS gamma-ray burst data, Hanlon and her colleagues, realised that some of the faintest bursts have distinctive gamma-ray emission, and also present faint afterglows in the lower-energy X-ray and visible wavelengths. Since, in general, GRBs are colossal explosions of energy triggered by the collision of very massive and compact objects such as neutron stars or black holes, or by the explosion of incredibly powerful supernovae, or hypernovae, one may think that these bursts are perceived as faint just because they take place very far away from us, in the remote corners of the Universe. However, Hanlon and colleagues noticed that these faint bursts, just at the sensitivity threshold of IBIS, seem to originate in our cosmic neighbourhood, within the nearby clusters of galaxies. “If the bursts we have studied are so ‘close’ in cosmological terms, it means that they are faint from the beginning,” says Hanlon. “From this we can deduce that the processes triggering them could be less energetic than those generating the more powerful bursts we are more used to observing.” The study team suggests that the faint bursts may be generated by the collapse of a massive star that does not present the characteristics of a supernova, or by the merger of two white dwarfs (small and dense stars about the size of Earth), or by the merger of a white dwarf with a neutron star or a black hole. “Past observations had already hinted the existence of faint GRBs, and thanks to Integral’s sensitivity we can now say that an entire population of them exist,” added Hanlon. “Actually, their rate may even be higher than that of the most luminous GRBs but, just because they are weaker, we may be only able to see those which are relatively close by.” |
K1.6.1
GRB 070707: the first short GRB observed by INTEGRAL
| GRB 070707 — T90 = 0.8 s | |
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Authors: S. McGlynn, S. Foley, S. McBreen, L. Hanlon, R. O'Connor, A.Martin Carrillo, B. McBreen |
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Journal-ref: A&A 486 (2008) 405 [0805.2880  ] |
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Title: GRB 070707: the first short gamma-ray burst observed by INTEGRAL |
| Abstract:
• Context. INTEGRAL has observed 47 long-duration GRBs (T90 > 2 s) and 1 short-duration GRB (T90 < 2 s) in five years of observation since October 2002. • Aims. This work presents the properties of the prompt emission of GRB 070707, which is the first short hard GRB observed by INTEGRAL. • Methods. The spectral and temporal properties of GRB 070707 were determined using the two sensitive coded-mask g-ray instruments on board INTEGRAL, IBIS and SPI. • Results. The T90 duration was 0.8 s, and the spectrum of the prompt emission was obtained by joint deconvolution of IBIS and SPI data to yield a best fit power-law with photon index a = -1.19, which is consistent with the characteristics of short-hard g-ray bursts. The peak flux over 1 second was f(20–200 keV) = 1.79 photons cm-2 s-1 and the fluence over the same interval was Fg(20–200 keV) = 2.07 × 10-7 erg cm-2. The spectral lag measured between 25–50 keV and 100–300 keV is 20 ± 5 ms, consistent with the small or negligible lags measured for short bursts. • Conclusions. The spectral and temporal properties of GRB 070707 are comparable to those of the short hard bursts detected by other g-ray satellites, including BATSE and Swift. We estimate a lower limit on the Lorentz factor G > 25 for GRB070707, assuming the typical redshift for short GRBs of z = 0.35. This limit is consistent with previous estimates for short GRBs and is smaller than the lower limits of G > 100 calculated for long GRBs. If GRB 070707 is a member of the recently postulated class of short GRBs at z ~ 1, the lower limit on G increases to G > 35. 1. Introduction
Two different types of progenitor are thought to be responsible for short and long gamma ray bursts (GRBs). Short
GRBs can be produced by the merger of two compact objects (e.g. neutron star–neutron star (NS–NS) or NS–black
hole (BH), Lee & Ramirez-Ruiz (2007)) while the core collapse of a massive star may give rise to a long duration
GRB (Woosley & MacFadyen 1999).
The merger of two neutron stars produces a rapidly spinning BH with huge energy reservoirs, orbited by a neutron-rich high density torus (Rosswog & Ramirez-Ruiz 2003). The binding energy of the accretion disk and the spin energy of the BH represent the two main energy reservoirs. The conventional view is that the released energy is quickly and continuously transformed into a radiation-dominated fluid, with a high entropy per baryon. This fireball is then collimated into a pair of jets, similar to the long GRB model. Host galaxies of short GRBs include both early and late type galaxies, as well as field and cluster galaxies (e.g. Prochaska et al. 2006). In contrast, the host galaxies of long GRBs are typically dwarf starburst galaxies with sub-solar metallicites (e.g. Bloom et al. 2002; Castro Ceron et al. 2008; Savaglio et al. 2008). References Bloom, J.S., Kulkarni, S.R., & Djorgovski, S.G. 2002, AJ 123, 1111 [astro-ph/0010176 | |
K1.6.2
Afterglow and very faint host galaxy
| GRB 070707 — afterglow — Fo ~ 1.4 × 10-6 erg cm-2 — Epeak ~ 400 keV | ||
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Authors: Piranomonte, S.; D'Avanzo, P.; Covino, S.; Antonelli, L. A.; Beardmore, A. P.; Campana, S.; Chincarini, G.; D'Elia, V.; Della Valle, M.; Fiore, F.; Fugazza, D.; Guetta, D.; Guidorzi, C.; Israel, G. L.; Lazzati, D.; Malesani, D.; Parsons, A. M.; Perna, R.; Stella, L.; Tagliaferri, G.; Vergani, S. D. | |
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Journal-ref: A&A 491 (2008) 183 [0807.1348 ] | |
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Title: The short GRB 070707 afterglow and its very faint host galaxy | |
| Abstract:
We present the results from an ESO/VLT campaign aimed at studying the afterglow properties of
the short/hard gamma ray burst GRB070707.
Observations were carried out at ten different epochs from ~ 0.5 to ~ 80 days after the event. The optical flux decayed steeply with a power-law decay index greater than 3, later levelling off at R ~ 27.3 mag; this is likely the emission level of the host galaxy, the faintest yet detected for a short GRB. Spectroscopic observations did not reveal any line features/edges that could unambiguously pinpoint the GRB redshift, but could set a limit z <~ 2.5. In the range of allowed redshifts, the host has a low luminosity, comparable to that of long-duration GRBs. The existence of such faint host galaxies suggests caution when associating short GRBs with bright, offset galaxies, where the true host might just be too dim for detection. The steepness of the decay of the optical afterglow of GRB070707 challenges external shock models for the optical afterglow of short/hard GRBs. We argue that this behaviour might results from prolonged activity of the central engine.
1. Introduction
Gamma-ray bursts (GRBs) are among the most powerful explosions in the universe. They are revealed in the
hard X-ray/soft gamma-ray band and are followed in many cases by a fading afterglow observable from radio
to X-ray wavelengths.
GRBs are empirically classified in two groups (Kouveliotou et al. 1993; Tavani 1996): short GRBs last less than 2 s and have a hard spectrum; long GRBs have longer durations (typically tens to hundreds seconds) and somewhat softer spectra. The emergence of a typical supernova (SN) spectrum superposed on the rapidly decaying non-thermal afterglow weeks after the events and the association with blue, highly star-forming galaxies provided strong evidence that a significant fraction of long GRBs originates in the gravitational collapse of massive stars (but see Della Valle et al. 2006; Fynbo et al. 2006; Gal-Yam et al. 2006a). Short GRBs are revealed less frequently than long GRBs (they comprise about 1/4 and 1/10 of the BATSE and Swift samples, respectively; Kouveliotou et al. 1993; Berger 2007); moreover their afterglows are weaker and, thus, more difficult to detect and follow up. These are among the reasons why the origin of short GRBs is still under debate, despite the important progresses made in the Swift era. The tight upper limits on any associated SN (Hjorth et al. 2005; Covino et al. 2006; Kann et al. 2008) as well as the association with a broad variety of Hubble types hosts, from elliptical (Berger et al. 2005) to moderate star forming galaxies (e.g. Covino et al. 2006), rules out the core-collapse mechanism as the main channel for short-GRBs production and strongly suggest that the explosion mechanism and/or progenitors of short GRBs are different from those of long GRBs (for a recent review, see Lee & Ramirez-Ruiz 2007). The leading model for short GRBs involves the merging of a system composed of two collapsed objects, a double neutron star (DNS) or a black hole/neutron star binary. In those systems that evolve out of massive stars that were born in a binary system (we term these “primordial binaries”), the delay between formation and merging is dominated by the gravitational wave inspiral time, ranging from tens of Myr to a few Gyr (Perna & Belczynski 2002), strongly dependent on the initial system separation. Short GRBs that result from them are expected to: (a) have a redshift distribution which broadly follows that of star formation and (b) drift away in some cases from the star-forming regions in which they were born, and merge outside, or in the outskirts, of galaxies (Belczynski et al. 2002). In another scenario, a fraction of the short GRBs is due to hyperflares from soft gamma-ray repeaters in the local universe (distances up to 100 Mpc; Hurley et al. 2005; Tanvir et al. 2005; Mazets et al. 2008). So far, a dozen short GRBs were localized with subarcsecond precision and host galaxies were firmly detected with small offset. Only in a few cases (e.g. GRB 061201: Stratta et al. 2007; GRB 080503: Perley et al. 2008) no host galaxy was found down to R = 26–28 after the optical afterglow had faded. For other short GRBs at unknown redshift, a putative host galaxy was proposed with magnitude R ~ 23-26. Spectroscopy of the brightest four of these galaxies indicates that they lie at 0.4 < z < 1.1. A comparison with field galaxy magnitudes suggests that the rest of the sample lies at z > 1 (Berger et al. 2007). The unambiguously localized hosts are both early- and late-type galaxies, with very different star formation rates and masses (Nakar 2007). References | ||
K1.7
Catalogue of INTEGRAL GRBs
| 56 GRBs — 50% of the GRBs have detected afterglows: ( 4 radio — 21 opt. — 25 X-ray ) | ||
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Authors: G. Vianello, D. Götz, S. Mereghetti | |
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Journal-ref: A&A (2008) [0812.3349 ] | |
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Title: The updated spectral catalogue of INTEGRAL Gamma-Ray Bursts | |
| Abstract:
We present a catalogue with the properties of all the bursts detected and localized by the IBIS
instrument onboard the INTEGRAL satellite from November 2002 to September 2008. The sample is composed
of 56 bursts, corresponding to a rate of ~ 0.8 GRB per month.
Thanks to the performances of the INTEGRAL Burst Alert System, 50% of the IBIS GRBs have detected afterglows, while 5% have redshift measurements. A spectral analysis of the 43 bursts in the INTEGRAL public archive has been carried out using the most recent software and calibration, deriving an updated, homogeneous and accurate catalogue with the spectral features of the sample. When possible also a time-resolved spectral analysis has been carried out. The GRBs in the sample have 20-200 keV fluences Fg in the range 5 × 10-8—2.5 × 10-4 erg cm-2, and peak fluxes in the range 0.11–56 photons cm-2 s-1. While most of the spectra are well fitted by a power law with photon index G ~ 1.6, we found that 9 bursts are better described by a cut-off power law, resulting in Ep values in the range 35–190 keV. Altough these results are comparable with those obtained with BAT onboard Swift , there is a marginal evidence that ISGRI detects dimmer bursts than Swift /BAT. Using the revised spectral parameters and an updated sky exposure map that takes into account also the effects of the GRB trigger efficiency, we strengthen the evidence for a spatial correlation with the super galactic plane of the faint bursts with long spectral lag (Foley et al., 2008).
1. Introduction
Enormous progress in the study of Gamma-Ray Bursts (GRBs) has been achieved after the results of the Satellite
per Astronomia X (the Italian for “Satellite for X-Ray Astronomy”) BeppoSAX , that enabled the discovery of
their counterparts at lower energy (Costa et al., 1997; van Paradijs et al., 1997; Frail et al., 1997).
Our understanding of GRBs has mainly profited from dedicated instruments, or even satellites, specifically designed for the study of these enigmatic sources. For example, thanks to the very large GRB sample obtained with the Burst And Transient Sources Experiment (BATSE) onboard the Compton Gamma-Ray Observatory (CGRO), we learned that GRBs are isotropically distributed across the sky (Briggs et al., 1996), and that their LogN-LogS is compatible with a cosmological origin (Meegan et al., 1992). BATSE also provided accurate spectra (e.g. Band et al., 1993), and confirmed that GRBs can be divided in two categories as a function of their duration and hardness, with about 25% of the GRBs being shorter than 2 s and harder compared to bursts of longer duration (Kouveliotou et al., 1993). Quick localizations with the High Energy Transient Explorer (HETE-2) satellite allowed to study the GRB emission in the X-rays (e.g. Vanderspek et al., 2004), and to detect the first afterglow of a short GRB (Fox & React GRB Team, 2005). Swift, launched in 2004, is currently localizing about 100 bursts per year, and is able to investigate the early phases of the X-ray afterglow, which were unaccessible to former instrumentation. Many key parameters are derived from the afterglow observations. Among them one of the most important is the distance, which is essential to infer the intrinsic energy and luminosity. GRBs are the most powerful explosions in the Universe, with isotropic equivalent energy Eiso in the range from 1050 to 1054 erg (Amati, 2006), although such values can be reduced with the hypothesis that GRBs are collimated sources (Rhoads, 1997), as supported by the detection of achromatic breaks in some afterglow light curves. The jet opening angles inferred from the afterglow break times (Sari et al., 1999) indicate lower energy values, clustered around 1051 erg with a reduced spread (Frail et al., 2001; Bloom et al., 2003). However, the observation of non-achromatic breaks in some afterglows, and the apparent lack of any kind of break in some others, poses doubts on this argument (see Curran et al., 2007, and reference therein). In only a few years, the Swift satellite has more than doubled the number of GRBs with known redshift, and discovered the most distant burst to date at z ~ 6.7 (GRB 080913, Fynbo et al., 2008). 4.4. The rate of short bursts
The short hard bursts detected by the IBAS system are 2 out of 56, corresponding to ~ 3.5 %,
while Swift/BAT found ~ 8%. This small difference is consistent with the small statistics of the
samples. However, the BATSE experiment onboard CGRO found that 1 burst out of 4 was a short hard GRB (25 %).
We are going to have soon an independent measurement of this rate by the Gamma-ray Burst Monitor (GBM) onboard
Fermi.
References Bloom, J.S., Frail, D.A., & Kulkarni, S.R. 2003, ApJ 594, 674 | ||
K
2.1
Antimatter in the Galaxy - positron annihilation radiation
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Authors: Weidenspointner, G.; Lonjou, V.; Knödlseder, J.; Jean, P.; et al |
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Journal-ref: ESA SP-552 (2004) 133 [astro-ph/0406178 ] |
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Title: SPI Observations of Positron Annihilation Radiation from the 4th Galactic Quadrant: Sky Distribution |
| Abstract:
During its first year in orbit the INTEGRAL observatory performed deep
exposures of the Galactic Center region and scanning observations of the
Galactic plane. We report on the status of our analysis of the positron
annihilation radiation from the 4th Galactic quadrant with the
spectrometer SPI, focusing on the sky distribution of the 511 keV line
emission. The analysis methods are described; current constraints and
limits on the Galactic bulge emission and the bulge-to-disk ratio are presented.
1. Introduction
Imaging analyses of the 511 keV line emission with
Richardson-Lucy and Bayesian algorithms clearly show extended emission from the Galactic bulge with a radial
profile that is approximately Gaussian. The sky maps  do not provide any evidence for emission from point
sources, a PLE component, or (at this stage) the Galactic disk.
A more quantitative approach for studying the Galactic distribution of the emission is model fitting. As a first step we have modelled the bulge emission by a spherical distribution with a Gaussian radial profile, located at the GC. We obtain a best fit FWHM of 8°. The corresponding bulge flux is f511 keV ~ 0.96 × 10-3 photons cm-2 s-1, with the uncertainty being dominated by the uncertainty of the width of the Gaussian intensity distribution. Spectroscopy of 511 keV line emission from the bulge resulted in a best fit energy of 511 keV and an intrinsic line width of 2.67 keV FWHM. | |
| positron annihilation radiation — line emission | ||
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Authors: G. Weidenspointner, V. Lonjou, J. Knoedlseder, P. Jean, et al | |
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Journal-ref: A&A 441 (2005) 513 [astro-ph/0506026 ] | |
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Title: SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution | |
The analysis methods are described; current constraints and limits on the Galactic bulge emission and the bulge-to-disk ratio are presented. | ||
Fig.: positronium annihilation (line)
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The SPI camera on the INTEGRAL satellite observed the
0.511 MeV gamma-ray emission line arising from positron annihilation in the Galaxy.
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Mit INTEGRAL wurde die charakteristische g-Emission der Annihilation von
Positronen aus dem Innenbereich der Galaxis vermessen. Aus SPI Daten
ergibt sich die Gammalinie deutlich verbreitert; daraus lassen sich die
physikalischen Kenngrößen (r,T) der Annihilationsregion bestimmen.
Im Übrigen erscheint deren räumliche Verteilung unerwartet: Von
den dort bekannten Jet-Quellen würde man erkennbare Einzelquellen
erwarten; Positronenquellen im kugelförmigen Innenbereich der Galaxie
würden eine ~5-15° ausgedehnte Emission erzeugen, während
Positronenquellen innerhalb der galaktischen Scheibe diese erkennbar
machen sollten, mit einer im Zentralbereich gleichförmigen Emission wie in radioaktivem 26Al gesehen.
INTEGRAL findet eine zum Zentrum symmetrische Emissionsregion mit deutlicher Ausdehnung im Bereich
~ 6°-18° (2s Grenzwerte), wie von einer
zentralen kugelfärmigen Positronenquelle erwartet (Abb.). Mit
INTEGRAL wird diese Annihilations-Emission erstmals entlang der galaktischen Ebene kartographiert werden.
| positronium annihilation — continuum emission | |
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Authors: G. Weidenspointner, C.R. Shrader, J. Knoedlseder, P. Jean, et al |
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Journal-ref: A&A 450 (2006) 1013 [astro-ph/0601673 ] |
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Title: The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL |
| Abstract:
We present a measurement of the sky distribution of positronium (Ps)
annihilation continuum emission obtained with the SPI spectrometer on board ESA's INTEGRAL observatory.
The only sky region from which significant Ps continuum emission is detected is the Galactic bulge. The Ps continuum emission is circularly symmetric about the Galactic centre, with an extension of about 8 deg FWHM. Within measurement uncertainties, the sky distribution of the Ps continuum emission is consistent with that found by us for the 511 keV electron-positron annihilation line using SPI. Assuming that 511 keV line and Ps continuum emission follow the same spatial distribution, we derive a Ps fraction of 0.92. These results strengthen our conclusions regarding the origin of positrons in our Galaxy based on observations of the 511 keV line. In particular, they suggest that the main source of Galactic positrons is associated with an old stellar population, such as Type Ia supernovae, classical novae, or low-mass X-ray binaries. Light dark matter is a possible alternative source of positrons. | |
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Authors: J. Knödlseder, P. Jean, V. Lonjou, G. Weidenspointner, et al |
|
A&A 441 (2005) 513 [astro-ph/0506026] |
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The all-sky distribution of 511 keV electron-positron annihilation emission
We present a map of 511 keV electron-positron annihilation emission, based on data accumulated with the SPI spectrometer aboard ESA's INTEGRAL gamma-ray observatory, that covers approximately 95% of the celestial sphere. 511 keV line emission is significantly detected towards the galactic bulge region and, at a very low level, from the galactic disk. The bulge emission is highly symmetric and is centred on the galactic centre with an extension of 8 deg. The emission is equally well described by models that represent the stellar bulge or halo populations. The disk morphology is only weakly constrained by the present data, being compatible with both the distribution of young and old stellar populations. The 511 keV line flux from the bulge and disk components is 1.05·10-3 ph cm-2 s-1 and 0.7·10-3 ph cm-2 s-1, respectively, corresponding to a bulge-to-disk flux ratio in the range 1-3. Assuming a positronium fraction of 0.93 this translates into annihilation rates of 1.5·1043 s-1 and 3·1042 s-1, respectively. The ratio of the bulge luminosity to that of the disk is in the range 3-9. We find no evidence for a point-like source in addition to the diffuse emission, down to a typical flux limit of 1·10-4 ph cm-2 s-1. We also find no evidence for the positive latitude enhancement that has been reported from OSSE measurements; our 3 sigma upper flux limit for this feature is 1.5·10-4 ph cm-2 s-1. The disk emission can be attributed to the beta+ decay of the radioactive species 26Al and 44Ti. The bulge emission arises from a different source which has only a weak or no disk component. We suggest that Type Ia supernovae and/or low-mass X-ray binaries are the prime candidates for the source of the galactic bulge positrons. Light dark matter annihilation could also explain the observed 511 keV bulge emission characteristics. |
Map of the effective SPI exposure at 511 keV for the dataset analysed in this work.
The contours are labelled
in units of 107 cm2 s,
corresponding to 13 ks (0.1), 133 ks, 667 ks (5), and 1.3 Ms (10)
of effective exposure times.
|
K
2.2GC — asymmetric distribution of positrons
| — | |
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Authors: Weidenspointner, G; Skinner, G; Jean, P; Knödlseder, J; von Ballmoos, P; Bignami, G; Diehl, R; Strong, A.W.; Cordier, B; Schanne, S; Winkler, C |
|
Journal-ref: Nature 451 (2008) 159-162 [ ] |
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Title: An asymmetric distribution of positrons in the galactic disk revealed by gamma rays |
| Abstract: Gamma-ray line radiation at 511keV is the signature of electron-positron annihilation. Such radiation has been known for 30years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (~10-50° from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20keV (`hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 1041 positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter. | |
|
Integral discovers the galaxy’s antimatter cloud is lopsided
[9 January 2008]
The shape of the mysterious cloud of antimatter in the central regions of the Milky Way has been
revealed by ESA’s orbiting gamma-ray observatory Integral.
The unexpectedly lopsided shape is a new clue to the origin of the antimatter. The observations have significantly decreased the chances that the antimatter is coming from the annihilation or decay of astronomical dark matter. |
Image credit: ESA/ Integral/ MPE (G. Weidenspointner et al.)
The left-hand panel shows the glow of 511 keV gamma rays coming from the annihilation of electrons by their
antimatter counterparts, the positrons. The map shows the entire sky, with the galactic centre at the middle.
The emission can be seen extending towards the right-hand side of the map. The right-hand panel shows the
distribution of hard low mass X-ray binary stars. This stellar population has a distribution that matches the
extent of the 511 keV map.
Image credit: Integral/ MPE (Weidenspointner et al.)
This computer model of the 511 keV gamma rays coming from the central region of the galaxy matches the
asymmetry around the galactic centre. The gamma rays from the galactic centre are symmetrical and the only
way to fit the observations was to assume that the asymmetry was caused by gamma rays coming from the inner
disc region of the galaxy, not throughout the galaxy. This ties in with the observed distribution of hard
low mass X-ray binaries.
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Georg Weidenspointner at the MPE and an international team of
astronomers made the discovery using four-years-worth of data from Integral. The cloud shows up because of
the gamma rays it emits when individual particles of antimatter, in this case positrons, encounter electrons,
their normal matter counterpart, and annihilate one another.
One signature of positron-electron annihilation is gamma rays carrying 511 thousand electron-volts (keV) of energy. There has been a vigorous debate about the origin of these positrons ever since the discovery of the 511 keV emission from the centre of the galaxy by gamma-ray detectors flown on balloons during the 1970s. Some astronomers have suggested that exploding stars could produce the positrons. This is because radioactive nuclear elements are formed in the giant outbursts of energy, and some of these decay by releasing positrons. However, it is unclear whether these positrons can escape from the stellar debris in sufficient quantity to explain the size of the observed cloud. Other astronomers wondered whether more exotic processes were at work. From earlier results using much less data, the positron cloud seemed to be spherical and centred on the centre of the galaxy. Such a shape and position corresponds to the expected distribution of dark matter in the centre of our galaxy, so it was suggested that dark matter was annihilating or decaying into pairs of electrons and positrons, which then annihilated to produce the gamma rays. The trouble with this idea, however, was that the dark matter particles needed to be much less massive than most theories were predicting. The new results give astronomers a valuable new clue and point away from dark matter as the origin of the antimatter. Beyond the galactic centre, the cloud is not entirely spherical. Instead it is lopsided with twice as much on one side of the galactic centre as the other. Such a distribution is highly unusual because gas in the inner region of the galaxy is relatively evenly distributed. Equally importantly, Integral found evidence that a population of binary stars is also significantly off-centre, corresponding in extent to the cloud of antimatter. That powerfully suggests these objects, known as hard (because they emit at high energies) low mass X-ray binaries, are responsible for a major amount of antimatter. | |
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A low mass X-ray binary (LMXB) is a celestial system in which a relatively normal star is being eaten alive by a nearby stellar corpse, either a neutron star or a black hole. The gravitational field of the stellar corpse is so strong that it rips gas from the normal star. As this gas spirals down towards that object, it is heated so much that positron-electron pairs can be spontaneously generated in the intense radiation field, although the 511 keV emission is probably too weak to be detected from individual LMXBs by Integral. “Simple estimates suggest that about half and possibly all of the antimatter is coming from the X-ray binaries,” says Weidenspointner. The other half could be coming from a similar process around the galaxy’s central black hole and the various exploding stars there. He points out that the lopsided distribution of hard LMXBs is unexpected, as stars are distributed more or less evenly around the galaxy. More investigations are needed to determine whether the observed distribution is real. Integral is currently the only mission that can see both the 511 keV radiation and the hard LMXBs. Weidenspointner and colleagues will be watching keenly to see whether it discovers more LMXBs and, if so, where they are located. “The link between LMXBs and the antimatter is not yet proven but it is a consistent story,” says Weidenspointner. It has real astrophysical importance because it decreases the need for dark matter at the centre of our galaxy. | ||
K
3 Ein Schwarzes Loch mit greller Vergangenheit
Zum Thema | |
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Internationales Forscherteam gelingt mit ESA-Gammasatellit "Integral" Blick in stürmische Vergangenheit der Gravitationsfalle im Zentrum der Milchstrasse.
Seit einigen Jahren ist nachgewiesen, dass sich im Zentrum unserer Milchstrasse ein supermassereiches Schwarzes Loch befindet, das sich zur Zeit allerdings in einer Art "Ruhezustand" befindet. Doch neueste Beobachtungen mit "Integral", dem Gammastrahlen-Observatorium der ESA, haben jetzt gezeigt, dass dieses Schwarze Loch noch vor etwa 350 Jahren wesentlich aktiver gewesen sein muss und im Vergleich zu heute Millionen mal mehr Energie abgegeben hat. Die Messungen wurden von einem internationalen Forscherteam durchgeführt, an dem auch Wissenschaftler des Max-Planck-Instituts für Astrophysik beteiligt waren. Die Forscher rechnen damit, dass die Materiefalle auch in Zukunft wieder aktiv werden könnte
Image credit: ESA, M. Revnivtsev (IKI/MPI Astrophysik)
Abb.: Die Falschfarben-Abbildung zeigt die Region um das Zentrum unserer Milchstraße,
wie sie von "Integral" im Gammalicht gesehen wird. Die Positionen des supermassereichen
Schwarzen Loches Sgr A* und der Molekülwolke Sgr B2 sind markiert.
Die Entfernung zwischen Sgr A* und Sgr B2 beträgt etwa 350 Lichtjahre.
Daher wird Sgr B2 erst jetzt von der Gammastrahlung getroffen,
die Sgr A* vor 350 Jahren während seiner letzten aktiven Phase abgestrahlt hat.
Diese intensive Strahlung wird von dem molekularen Wasserstoffgas in Sgr B2 absorbiert
und auf charakteristische Weise wieder abgegeben, wobei das Gas im Gammalicht zu
fluoreszieren beginnt.
Die anderen hellen Objekte auf der rechten Bildseite sind bereits bekannte Quellen von Gammastrahlung.
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Die meisten Galaxien beherbergen in ihren Zentrum ein supermassereiches Schwarzes Loch, welches Millionen, ja sogar Milliarden mal massereicher als unsere Sonne sein kann. Auch unsere eigene Galaxie, die Milchstrasse, enthält eine solche supermassereiche Gravitationsfalle. Astronomen nennen sie Sgr A* (gesprochen: "Sagittarius A star") - wegen ihrer Position im südlichen Sternenbild Schütze (lat. Sagittarius).
Trotz seiner enormen Masse von mehr als einer Million Sonnen erscheint Sgr A* heute als eine eher ruhige und harmlose Materiefalle. Doch nun haben neue Untersuchungen mit dem ESA-Gammastrahlen-Observatorium "Integral" gezeigt, dass Sgr A* in der Vergangenheit wesentlich aktiver gewesen sein muss. Die Daten belegen eindeutig, dass es mit seiner Umgebung in heftige Wechselwirkungen getreten ist und damals Millionen mal mehr Energie freigesetzt hat als heute.
Zu diesem Ergebnis ist ein internationales Team von Wissenschaftlern unter der Leitung von Dr. Mikhail Revnivtsev, Space Research Institute, Moskau, Russland, und Max-Planck-Institut für Astrophysik, Garching, gekommen. Wie Revnivtsev es formuliert: "Vor etwa 350 Jahren ging die Region um Sgr A* buchstäblich in einer Flut von Gammablitzen unter."
Diese Gammastrahlung ist unmittelbare Folge der früheren Aktivität von Sgr A* in einer Phase, in welcher Gas und Staub, eingefangen von der Gravitation des Schwarzen Lochs, verdichtet und so lange aufgeheizt wurden, bis sie sehr intensiv Röntgen- und Gammastrahlung abgaben, um dann schließlich hinter dem Ereignishorizont - dem "point of no return", von welchem nicht einmal mehr Licht zu entrinnen vermag - zu verschwinden.
Animation [Bild: ESA]:
Die Animation zeigt, wie sich Astronomen vorstellen, was in der Vergangenheit
nahe dem Zentrum unserer Milchstraße passiert ist. Ein starker Gammablitz,
den Sgr A* vor etwa 350 Jahren abgegeben hat und der mehrere Jahre
andauerte, hat den interstellaren Raum durchquert und jetzt Sgr B2, eine
Wolke aus molekularem Wasserstoffgas, erreicht. Die starke Gammastrahlung
wird vom Gas absorbiert und bringt es zum Leuchten. In dem Astronomen die
von Sgr B2 reflektierte und wieder abgegebene Strahlung studieren, können
sie erstmals die stürmische Vergangenheit des supermassereichen Schwarzen Lochs Sgr A* genau rekonstruieren.
Auf Grund seiner Distanz zu dem Schwarzen Loch im Zentrum der Milchstraße wird Sgr B2 erst heute von den vor 350 Jahren von Sgr A* emittierten Gammastrahlen erreicht, Strahlung also aus einer Zeit hoher Aktivität der Materiefalle. Diese hochenergetische Strahlung wird von dem Gas in Sgr B2 absorbiert und anschließend mit einer eindeutigen Signatur erneut emittiert.
"Wir sehen heute gewissermaßen das Echo des Ausbruchs in einem natürlichen Spiegel nahe dem galaktischen Zentrum - die riesige Wolke Sgr B2 reflektiert also jene Gammastrahlen, die von Sgr A* vor 350 Jahren emittiert wurden", sagt Revnivtsev. Der Blitz war offenbar so stark, dass die davon beleuchtete Wolke selbst im Röntgenbereich fluoresziert hat, so dass diese Strahlung schon vor "Integral" von anderen Röntgenteleskopen gesehen wurde. Allerdings gab es bisher auch andere Vorschläge, die Herkunft der Röntgenstrahlung zu erklären, beispielsweise als Wechselwirkung der Wolke mit kosmischer Strahlung. Doch die neuen Messungen mit "Integral" zeigen eindeutig, dass es hochenergetische Strahlung vom Schwarzen Loch selbst sein muss, die von der Wolke reflektiert und reprozessiert wurde. Damit ist es den Wissenschaftlern erstmals gelungen, eine stürmische Periode in der Geschichte von Sgr A* zu rekonstruieren.
Das Stadium hoher Aktivität Schwarzer Löcher steht in unmittelbarem Zusammenhang mit der Art und Weise ihres Wachstums: Denn supermassereiche Schwarze Löcher kommen nicht mit diesem Geburtsgewicht zur Welt, sondern wachsen schrittweise auf Grund ihrer gewaltigen Schwerkraft, indem sie mit der Zeit mehr und mehr Materie aus ihrer Umgebung verschlingen. Beim Verschlucken dieser Materie entstehen dann gewaltige Blitze im Röntgen- und Gammalicht. Dabei gilt: Je gieriger das Schwarze Loch, desto stärker ist seine freigesetzte Strahlung.
Die Entdeckung mit "Integral" löst endlich das Geheimnis, welche Eigenschaften die Emission solcher "stillen" supermassereichen Schwarzen Löcher wie Sgr A* besitzt. Schon länger hatten Wissenschaftler vermutet, dass es viele solcher "stillen" Schwarzen Löcher im Universum geben muss. Doch bisher war man nicht in der Lage zu sagen, wie viel Energie und in welcher Weise sie diese abstrahlen. "Noch vor einigen Jahren hätten wir nicht zu träumen gewagt, dieses Rätsel so bald zu lösen", sagt Revnivtsev. "Dank "Integral" ist es uns nun gelungen!"
Die Messergebnisse von Revnivtsev und seinem Team belegen, dass das letzte Stadium hoher Aktivität von Sgr A* vor etwa 350 Jahren mindestens zehn Jahre gedauert haben muss.
K
4Millisecond pulsar IGR J00291+5934 (i)
Zum Thema | |
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Der schnellste Röntgen-msec-Pulsar ist vom Satelliten INTEGRAL entdeckt worden -
er schafft 599 Umdrehungen pro Sekunde.
Daten: R.A. = 00h29m03'', decl. = +59°34'19'' (l=120°.1, b=-3°.2)
D = 3 kpc, z = 160 pc
Prot = 1.67 ms; nrot = 598.89 Hz;
Torbit = 8844 s (2.46 hr)
F = 8 × 10-10 erg cm-2 s-1
L = 0.9 × 1036 erg s-1(d/3kpc)2
| IGR J00291+5934 — nrot = 598.88 Hz | |
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Authors: D.K. Galloway, C.B. Markwardt, E.H. Morgan, D. Chakrabarty, T.E. Strohmayer |
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Journal-ref: ApJ 622 (2005) L45-L48 [astro-ph/0501064 ] |
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Discovery of the accretion-powered millisecond X-ray pulsar IGR J00291+5934
We report on observations of the sixth accretion-powered millisecond pulsar, IGR J00291+5934, with the Rossi X-Ray Timing Explorer. The source is a faint, recurrent X-ray transient initially identified by INTEGRAL. The 599 Hz (1.67 ms) pulsation had a fractional rms amplitude of 8% in the 2-20 keV range, and its shape was approximately sinusoidal. The pulses show an energy-dependent phase delay, with the 6-9 keV pulses arriving up to 85 us earlier than those at lower energies. No X-ray bursts, dips, or eclipses were detected. The neutron star is in a circular 2.46 hr orbit with a very low-mass donor, most likely a brown dwarf. The binary parameters of the system are similar to those of the first known accreting millisecond pulsar, SAX J1808.4-3658. Assuming that the mass transfer is driven by gravitational radiation and that the 2004 outburst fluence is typical, the 3-yr recurrence time implies a distance of at least 3kpc. |
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)
divides naturally into two groups. Three of the five sources known to date
(XTE J1751-305, XTE J0929-314, and XTE J1807-294)
are in ultracompact binaries with orbital periods of » 40 min. The Roche lobes in such tiny binaries cannot contain a main-sequence companion, indicating that the mass donors are highly evolved and H-poor.
The other two
(SAX J1808.4-3658, and XTE J1814-338)
have orbital periods of 2.01 and 4.28 hr, respectively, and have H-rich donors (likely a brown dwarf in the case of SAX J1808.4-3658). The latter two sources have also both exhibited thermonuclear (type-I) X-ray bursts, whereas the three ultracompact binaries have not.
All five pulsars are soft X-ray transients, with outburst durations of order weeks and recurrence times of order years. It remains unclear why millisecond pulsations are easily detectable in these five sources, but not in the over 50 other neutron stars in low-mass X-ray binaries.
The 2002 October launch of the INTEGRAL gamma-ray mission, with its wide field of view and good sensitivity to hard X-ray sources, has provided a new avenue for detections of X-ray transients.
INTEGRAL discovered the new X-ray transient (l=120°.1, b=-3°.2) on 2004 Dec. 2. Followup Rossi X-ray Timing Explorer observations revealed pulsations at a frequency of 598.88 Hz, and exhibiting a sinusoidal frequency modulation indicative of a 147.4 min (2.46 hr) orbit.
The detection of pulsations motivated extensive multiwavelength followup. An R» 17.4m candidate optical counterpart was identified within the INTEGRAL error circle, at R.A. = 00h29m03'', decl. = +59°34'19''.
Optical spectroscopy of the candidate revealed weak He II and Ha emission, supporting its association with IGR J00291+5934. Radio observations also revealed evidence for variable emission consistent with the counterpart position.
| IGR J00291+5934 — an accreting X-ray ms pulsar — LX ~ 1036 erg s-1 | |
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Authors: S.E. Shaw, N. Mowlavi, J. Rodriguez, P. Ubertini, F. Capitanio, K. Ebisawa, D. Eckert, T. J.-L. Courvoisier, N. Produit, R. Walter, M. Falanga |
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Journal-ref: A&A 432 (2005) L13 [astro-ph/0501507 ] |
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Discovery of the INTEGRAL X/Gamma-ray transient IGR J00291+5934: a Comptonised accreting ms pulsar?
We report the discovery of a high-energy transient with the IBIS/ISGRI detector on board the INTEGRAL observatory. The source, namely IGR J00291+5934, was first detected on 2nd December 2004 in the routine monitoring of the IBIS/ISGRI 20--60 keV images. The observations were conducted during Galactic Plane Scans, which are a key part of the INTEGRAL Core Programme observations. After verifying the basic source behaviour, the discovery was announced on 3rd December. The transient shows a hard Comptonised spectrum, with peak energy release at about 20 keV (flux ~ 8 × 10-10 erg cm-2 s-1) and a total luminosity of L5-100 keV ~ 0.9 × 1036 erg s-1, assuming a distance of 3 kpc. Following the INTEGRAL announcement of the discovery of IGR J00291+5934, a number of observations were made by other instruments. We summarise the results of those observations and, together with the INTEGRAL data, identifiy IGR J00291+5934 as the 6th member of a class of accreting X-ray millisecond pulsars. |
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Star eats companion
[6 September 2005] ESA's Integral space observatory, together with NASA's Rossi X-ray Timing Explorer
spacecraft, has found a fast-spinning pulsar in the process of devouring its companion.
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Image credit: NASA/Dana Berry
Fig. — This is an artist's impression of a spinning neutron star (pulsar) approximately 20 kilometres
in diameter, accreting material from a companion star. The strong gravity from the dense pulsar attracts
material from the companion. The flow of gas from the companion to the pulsar is energetic and glows in
X-ray light.
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Rossi observations revealed that the companion is already a fraction the size of our Sun, perhaps as small as 40 Jupiter masses. The binary orbit is 2.5 hours long (as opposed to the year long Earth-Sun orbit). The full system is very tight; both stars are so close that they will fit into the radius of the Sun. These details support the theory that the two stars are close enough for accretion to take place and that the companion star is being cannibalised.
INTEGRAL / RXTE (spin-up in outburst)
Einem Millisekundenpulsar beim »Aufziehen« zugeschaut
haben die Hochenergiesatelliten INTEGRAL und RXTE während eines Strahlungsausbruchs des 599-mal
in der Sekunde rotierenden Röntgenpulsars IGR J00291+5934 im Dezember 2004:
Der Neutronenstern wird immer schneller! Zwar nimmt seine Periode nur um 8.4 x 10-13 Hertz/Sekunde
(oder ein 22-Millionstel pro Jahr) zu, aber das ist nichtsdestotrotz der Effekt, den man durch den
Massenzufluß von einem Begleiter erwarten sollte.
Und die fortwährende Beschleunigung der Rotation durch solche Akkretion gilt als führende Theorie der
Entstehung von Millisekundenpulsaren aus normalen, alten Pulsaren, die anfangs höchstens ein paar Mal
pro Sekunde rotierten.
| IGR J00291+5934 — 2004 December outburst | |
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Authors: M. Falanga, L. Kuiper, J. Poutanen, E. W. Bonning, W. Hermsen, T. Di Salvo, P. Goldoni, A. Goldwurm, S. E. Shaw, L. Stella |
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Journal-ref: A&A 444 (2005) 15 [astro-ph/0508613 ] |
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Title: INTEGRAL and RXTE observations of accreting millisecond pulsar IGR J00291+5934 in outburst |
| Abstract: Simultaneous observations of the accretion-powered
millisecond pulsar IGR J00291+5934 by International Gamma-Ray Astrophysics Laboratory and
Rossi X-ray Timing Explorer during the 2004 December outburst are analysed.
The average spectrum is well described by thermal Comptonization with an electron temperature
of 50 keV and Thomson optical depth tT ~ 1 in a slab geometry.
The spectral shape is almost constant during the outburst.
We detect a spin-up of the pulsar with n' = 8.4x10-13 Hz s-1. The ISGRI data reveal the pulsation of X-rays at a period of 1.67 milliseconds up to ~ 150 keV. The pulsed fraction is shown to increase from 6 per cent at 6 keV to 12--20 per cent at 100 keV. This is naturally explained by the action of the Doppler effect the exponentially cutoff Comptonization spectrum from the hot spot. The nearly sinusoidal pulses show soft lags with complex energy dependence, increasing up to 7 keV, then decreasing to 15 keV, and seemingly saturating at higher energies. | |
Accretion Torque in IGR J00291+5934 | |
| IGR J00291+5934 — Timing | |
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Authors: Authors: L. Burderi, T. Di Salvo, A. Riggio, M.T. Menna, G. Lavagetto, A. Papitto, R. Iaria, N. R. Robba, L. Stella |
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ref: Multifrequency Behaviour of High Energy Cosmic Sources
(Frascati Workshop 2005) [astro-ph/0509224 ] |
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Title: Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934 |
| Abstract: We present here a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of December 2004. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we find a clear parabolic trend of the pulse phase delays showing that the pulsar is spinning up as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar. | |
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INTRODUCTION - The recycling scenario
The so-called recycling scenario links two different classes of astronomical objects, namely the millisecond radio pulsars (usually found in binary systems) and the Low Mass X-ray Binaries (hereafter LMXBs), or, at least, a subgroup of them. The leading idea of this scenario is the recycling process itself, during which an old, weakly magnetized, slowly spinning neutron star is accelerated by the accretion of matter and angular momentum from a (Keplerian) accretion disk down to spin periods in the millisecond range. In this way, at the end of the accretion phase, the neutron star rotates so fast that it is resurrected from the radio pulsar graveyard, allowing the radio pulsar phenomenon to occur again despite the weakness of the magnetic field. Although this scenario was firstly proposed long time ago (see e.g. Bhattacharya & van den Heuvel 1991 for a review), the most embarassing problem was the absence of coherent pulsations in LMXBs. Only recently, the long seeked for millisecond coherent os- cillations in LMXBs have been found, thanks to the capabilities (the right combination of high temporal resolution and large collecting area) of the RXTE satellite. In April 1998, a transient LMXB, SAX J1808.4–3658, was discovered to harbour a millisecond pulsar (Pspin ~ 2.5 ms) in a compact (Porb ~ 2 h) binary system (Wijnands & van der Klis 1998). We now know seven accreting millisecond pulsars (Wijnands 2005); all of them are X-ray transients in very compact systems (orbital period between 40 min and 4 h), the fastest of which (Pspin ~ 1.7 ms), IGR J00291+5934, has been discovered in December 2004 (Galloway et al. 2005) Timing techniques applied to data of various accreting millisecond pulsars, spanning the first few days of their outbursts, allowed an accurate determination of their main orbital parameters. However, only a few attemps have been made to determine the spin period derivative. We apply an accurate timing technique to the fastest currently known accreting millisecond pulsar, IGR J00291+5934, in the hope of constraining the pre- dictions of different torque models with good quality experimental data. Our results indicate quite clearly that a net spin up occurs during the December 2004 outburst of IGR J00291+5934 and that the derived torque is in good agreement with that expected from matter accreting from a Keplerian disk. IGR J00291+5934 —
a fast-spinning pulsar in the process of devouring its companion [September 2005]
ESA's Integral space observatory, together with NASA's Rossi X-ray Timing Explorer spacecraft, has found a fast-spinning pulsar in the process of devouring its companion. This finding supports the theory that the fastest-spinning isolated pulsars get that fast by cannibalising a nearby star. Gas ripped from the companion fuels the pulsar's acceleration. This is the sixth pulsar known in such an arrangement, and it represents a 'stepping stone' in the evolution of slower-spinning binary pulsars into faster-spinning isolated pulsars. "We're getting to the point where we can look at any fast-spinning, isolated pulsar and say, 'That guy used to have a companion'," said Dr Maurizio Falanga, who led the Integral observations. 'Pulsars' are rotating neutron stars, which are created in stellar explosions. They are the remnants of stars that were once at least eight times more massive than the Sun. These stars still contain about the mass of our Sun compactified into a sphere of only about 20 kilometres across. This pulsar, called IGR J00291+5934, belongs to a category of 'X-ray millisecond pulsars', which pulse with the X-ray light several hundred times a second, one of the fastest known. It has a period of 1.67 milliseconds which is much smaller that most other pulsars that rotate once every few seconds. Neutron stars are born rapidly spinning in collapses of massive stars. They gradually slow down after a few hundred thousand years. Neutron stars in binary star systems, however, can reverse this trend and speed up with the help from the companion star. For the first time ever, this speeding-up has been observed in the act. "We now have direct evidence for the star spinning faster whilst cannibalising its companion, something which no one had ever seen before for such a system," said Dr Lucien Kuiper from the Netherlands Institute for Space Research (SRON), in Utrecht. A neutron star can remove gas from its companion star in a process called 'accretion'. The flow of gas onto the neutron star makes the star spin faster and faster. Both the flow of gas and its crashing upon the neutron star surface releases much energy in the form of X-ray and gamma radiation. Neutron stars have such a strong gravitational field that light passing by the star changes its direction by almost 100 degrees (in comparison light passing by the Sun is deflected by an angle which is 200 thousands times smaller). "This 'gravitational bending' allows us to see the back side of the star," points out Prof. Juri Poutanen from the University of Oulu, Finland. "This object was about ten times more energetic than what is usually observed for similar sources," said Falanga. "Only some kind of monster emits at these energies, which corresponds to a temperature of almost a billion degrees." From a previous Integral result, scientists deduced that because the neutron star has a strong magnetic field, charged particles from its companion are channeled along the magnetic field lines until they slam into the neutron star surface at one of its magnetic poles, forming 'hot spots'. The very high temperatures seen by Integral arise from this very hot plasma over the accretion spots. IGR J00291+5934 was discovered by Integral during a routine scan of the sky on 2 December 2004, in the outer reaches of our Milky Way galaxy, when it suddenly flared. On the day after, scientists accurately clocked the neutron star with the Rossi X-ray Timing Explorer. Rossi observations revealed that the companion is already a fraction the size of our Sun, perhaps as small as 40 Jupiter masses. The binary orbit is 2.5 hours long (as opposed to the year long Earth-Sun orbit). The full system is very tight; both stars are so close that they will fit into the radius of the Sun. These details support the theory that the two stars are close enough for accretion to take place and that the companion star is being cannibalised. "Accretion is expected to cease after a billion of years or so," said Dr Duncan Galloway of the Massachusetts Institute of Technology, USA, responsible for the Rossi observations. "This Integral-Rossi discovery provides more evidence of how pulsars evolve from one phase to another - from an initially slowly spinning binary neutron star emitting high energies, to a rapidly spinning isolated pulsar emitting in radio wavelengths." The discovery is the first of its kind for Integral (four of the first five rapidly spinning X-ray pulsars were discovered by Rossi). This bodes well in the combined search for these rare objects. Integrals's sensitive detectors can identify relatively dim and distant sources and so, knowing where to look, Rossi can provide timing information through a dedicated observation extending over the entire two-week period of the typical outburst. |
Fast-spinning star could test gravitational waves
One of the fastest-spinning stars ever seen has been found by the INTEGRAL spacecraft. But researchers say the star's speed could be limited by gravitational wave radiation - theoretical ripples in space-time. The idea could be tested by upgraded detectors within the next few years.
The European Space Agency's INTEGRAL spacecraft, launched in 2002 to study high-energy phenomena in space, detected the star on 2 December 2004. Called IGR J00291+5934, the object appears to lie about 9800 light years away and emits a periodic signal every 1.67 milliseconds (frequency ~ 598.88 Hz). That is the telltale signature of a type of neutron star called a "millisecond pulsar" - one that spins at least 100 times per second.
About 150 millisecond pulsars are known, with some emitting X-rays and others radio waves. All appear to spin so rapidly due to interactions with a companion star.
The companion bloats up when it reaches a certain age and dumps gas onto its neutron star partner, causing the star to spin faster. The neutron star's magnetic fields can funnel the gas onto certain spots, causing them to heat up and emit X-rays. This type of neutron star is called an accreting X-ray millisecond pulsar. With a spin rate of 599 revolutions per second, this star is the fastest of the six known pulsars of this type and it glows in the X-ray wavelengths.
The pulsar was discovered by an international team led by Simon Shaw at the University of Southampton, UK.
Elite class
The vast majority of millisecond pulsars - including the fastest overall, which rotates 641 times per second - emit at radio wavelengths and have stopped actively accreting material from their companions.
X/gamma-ray spectrum of IGR~J00291+5934, an average of ISGRI observations
(squares). The line shows a simultaneous fit to the two sets of data with a model.
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"This is the fastest pulsar found in this process of getting spun up," says Deepto Chakrabarty, not a member of Shaw's team.
Its speed puts it in an elite class with just a handful of other superfast objects. "The bulk of the millisecond pulsars have been detected spinning at up to 300 times per second. It's much rarer to find objects spinning 600 times per second," says David Nice.
"What's interesting is that spin rate is actually much slower than what we think the maximum is," Chakrabarty told New Scientist. Pulsars are expected to spin as fast as 3000 times per second before they split apart, he says. "Nature is applying some kind of brake, but we don't know what that is."
Space-time ripples
One possible mechanism could be gravitational waves - hypothesised ripples in space-time that are yet to be detected. Fast-spinning objects that are not perfectly symmetrical are predicted to radiate away energy in gravitational waves, with faster objects unleashing much more energy than slower ones.
It is possible, says Chakrabarty, that spin rates of 600 times per second allow gravitational waves to siphon away enough energy to prevent pulsars from spinning any faster.
If future finds continue to show this "pile-up" at about 600 rotations per second, that would suggest gravitational waves are at play, he says. But if this "natural brake" on the pulsars' spinning is due to another mechanism, the number of pulsars spinning above 600 revolutions per second may drop off more gradually.
An upgraded version of the Laser Interferometer Gravitational Wave Observatory (LIGO), now operating from two sites in the US, could settle the issue. With an improved detector system recently included in the US National Science Foundation's budget plans for 2008, LIGO should be sensitive enough to pick up gravitational waves from pulsars like IGR J00291+5934.
"There is controversy over the gravitational wave idea [as a brake on pulsars], but it is eminently testable," says Chakrabarty. "If it's right, Advanced LIGO will see gravitational waves at a certain level. If they don't see it, the theory is dead."
cont.: X-ray Transient IGR J00291+5934 in quiescence
K5.1
IGR J18483-0311: an accreting X-ray pulsar
| IGR J18483-0311 — Porb = 18.52 d — Prot = 21 s | ||
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Authors: V. Sguera, A. B. Hill, A. J. Bird, A. J. Dean, A. Bazzano, P. Ubertini, N. Masetti, R. Landi, A. Malizia, D. J. Clark, M. Molina | |
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Journal-ref: A&A 467 (2007) 249 [astro-ph/0702477 ] | |
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Title: IGR J18483-0311: an accreting X-ray pulsar observed by INTEGRAL | |
Abstract:
Context: IGR J18483-0311 is a poorly known transient hard X-ray source discovered by INTEGRAL during observations of the Galactic Center region performed between 23--28 April 2003. Aims: To detect new outbursts from IGR J18483-0311 using INTEGRAL and archival Swift XRT observations and finally to characterize the nature of this source using the optical/near-infrared (NIR) information available through catalogue searches. Methods. We performed an analysis of light curves and spectra of INTEGRAL and archival Swift XRT data as well as of optical/NIR catalogues. Results: We report on 5 newly discovered outbursts from IGR J18483-0311 detected by INTEGRAL. For two of them it was possible to constrain a duration of the order of a few days. The strongest outburst reached a peak flux of 120 mCrab (20--100 keV): its broad band JEM--X/ISGRI spectrum (3--50 keV) is best fitted by an absorbed cutoff power law with photon index=1.4+/-0.3, cutoff energy of ~22 keV and NH = 6 × 1022 cm-2. Timing analysis of INTEGRAL data allowed us to identify periodicities of 18.52 days and 21.0526 seconds which are likely the orbital period of the system and the spin period of the X-ray pulsar respectively. Swift XRT observations of IGR J18483-0311 provided a very accurate source position which strongly indicates a highly reddened star in the USNO--B1.0 and 2MASS catalogues as its possible optical/NIR counterpart. Conclusions: The X-ray spectral shape, the periods of 18.52 days and 21.0526 seconds, the high intrinsic absorption, the location in the direction of the Scutum spiral arm and the highly reddened optical object as possible counterpart, all favour the hypothesis that IGR J18483-0311 is a HMXB with a neutron star as compact companion. The system is most likely a Be X-ray binary, but a Supergiant Fast X-ray Transient nature can not be entirely excluded. 1. Introduction
Since its launch in 2002, the INTEGRAL satellite has discovered more than one hundred new hard X-ray sources,
mainly located toward the inner regions of the Galaxy which continue to be extensively monitored. Many of them
are characterized by absorbed hard spectra, with little or no detectable emission in the soft X-rays since they
are heavily absorbed by the interposing material. This, together with their often transient nature,
explains why they have not been detected by any previous X-ray mission.
As discussed by Dean et al. (2006), most of the newly discovered INTEGRAL sources should be high mass X-ray binaries (HMXBs), although Masetti et al. (2006) found that several of them are actually Active Galactic Nuclei. This picture has been supported by various identifications with transient Be HMXBs or bright persistent highly absorbed supergiant HMXBs (SGXBs), either based on their secure identification at optical/infrared wavebands or on their X-ray characteristics and discovery of periodic pulsations (Walter et al. 2006). As well as the highly absorbed persistent SGXBs, INTEGRAL is also finding new members of a newly discovered class of SGXBs which escaped detection by previous X-ray missions mainly because of their fast X-ray transient behaviour. They have been labeled as Supergiant Fast X-ray Transients SFXTs (Negueruela et al. 2005, Sguera et al. 2005, Sguera et al. 2006). References Bird, A. J., Barlow, E. J., Bassani, L., et al., 2006, ApJ, 636, 765 Dean, A. J., Bazzano, A., Hill, A. B., et al. 2005, A&A, 443, 485 Sguera, V., Bazzano, A., Bird, A.J., et al., 2006, ApJ, 646, 452 Sguera, V., Barlow, E. J., Bird, A. J., et al., 2005, A&A, 444, 221 | ||
K5.2
IGR J11215-5952: Supergiant Fast X-ray Transient
Zum Thema:
Eine neue Klasse von Röntgendoppelsternen | |
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| IGR J11215-5952 — Prot = 186.78 ± 0.3 s — Porb ~ 329 d — d = 6.2 kpc — LX ~ 1036 erg s-1 | |
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Authors: P. Romano, L. Sidoli, V. Mangano, S. Mereghetti, G. Cusumano |
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Journal-ref: A&A 469 (2007) L5 [arxiv.org/0704.0543 ] |
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Title: Swift/XRT observes the fifth outburst of the periodic Supergiant Fast X-ray Transient IGR J11215-5952 |
| Abstract:
Context. IGR J11215-5952 is a hard X-ray transient source discovered in April 2005 with INTEGRAL and a confirmed member of the new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs). Archival INTEGRAL data and RXTE observations showed that the outbursts occur with a periodicity of ~330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic outbursts, possibly related to the orbital period. Aims. We performed a Target of Opportunity observation with Swift with the main aim of monitoring the source behaviour around the time of the fifth outburst, expected on 2007 Feb 9. Methods. The source field was observed with Swift twice a day (2ks/day) starting from 4th February, 2007, until the fifth outburst, and then for ~5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of ~73 ks. This is the most complete monitoring campaign of an outburst from a SFXT. Results. The spectrum during the brightest flares is well described by an absorbed power law with a photon index of 1 and NH = 9 × 1022 cm-2. A peak luminosity of LX(1-10 keV) ~ 1036 erg s-1 was derived (assuming 6.2 kpc, the distance of the optical counterpart). These Swift observations are a unique data-set for an outburst of a SFXT, thanks to the combination of sensitivity and time coverage, and they allowed a study of IGR J11215-5952 from outburst onset to almost quiescence. We find that the accretion phase lasts longer than previously thought on the basis of lower sensitivity instruments observing only the brightest flares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit with many flares superimposed, possibly due to episodic or inhomogeneous accretion. 1. Introduction
The hard X–ray transient IGR J11215–5952 was discovered with the INTEGRAL satellite during an outburst in
April 2005 (Lubinski et al., 2005) and was associated with HD 306414
(Negueruela et al., 2005), a B1Ia supergiant located at a distance of 6.2 kpc (Masetti et al., 2006).
The short duration of the outburst together with the likely optical counterpart suggested that IGR J11215–5952 could be a new member of the class of Supergiant Fast X-ray Transients (SFXTs; Negueruela et al. 2006). Analysing archival INTEGRAL observations of the source field, Sidoli, Paizis, & Mereghetti (2006, hereafter Paper I) discovered two previously unnoticed outbursts (in July 2003 and in May 2004) which demonstrate the recurrent nature of this transient and suggest a possible periodicity of ~330 days. This periodicity was confirmed by the detection of the fourth outburst from IGR J11215–5952 with Rossi XTE/PCA on 2006 March 16–17, 329 days after the third outburst (Smith et al., 2006b). The RXTE/PCA observations showed strong flux variability and a hard spectrum (power-law photon index of 1.7 ± 0.2 in the range 2.5–15 keV) as well as a possible pulse period of ~195 s (Smith et al., 2006a). The periodicity was confirmed with RXTE observations of the latest outburst, yielding P = 186.78 ± 0.3 s (Swank et al., 2007). Follow-up observations with Swift/XRT refined the source position and confirmed the association with HD 306414 (Steeghs et al., 2006). A hard power-law with a high energy cut-off around 15 keV is a good fit to the spectra observed with INTEGRAL (Paper I). For the distance of 6.2 kpc, the peak fluxes of the outbursts correspond to a luminosity of LX(5–100 keV) ~ 3 × 1036 erg s-1. All these findings confirmed IGR J11215–5952 as a member of the class of the SFXTs, and the first object of this class of High Mass X–ray Binaries displaying periodic outbursts. References
Masetti, N., Pretorius, M. L., Palazzi, E., et al. 2006, A&A, 449, 1139
Negueruela, I., Smith, et al. 2006, in “The X-ray Universe 2005”, ESA SP-604, 165–170
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| IGR J11215-5952 — d = 6.2 kpc — LX, burst ~ 3 × 1036 erg s-1 — LX, quiescent ~ 1032 erg s-1 — Porb ~ 165 d | |
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Authors: L. Sidoli, P. Romano, S. Mereghetti, A. Paizis, S. Vercellone, V. Mangano, D. Gotz |
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Journal-ref: A&A 476 (2007) 1307 [0710.1175 ] |
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Title: An alternative hypothesis for the outburst mechanism in Supergiant Fast X-ray Transients: the case of IGR J11215-5952 |
| Abstract:
• Context. The physical mechanism responsible for the short outbursts in a recently recognized class of High Mass X–ray Binaries, the Supergiant Fast X–ray Transients (SFXTs), is still unknown. Two main hypotheses have been proposed to date: the sudden accretion by the compact object of small ejections originating in a clumpy wind from the supergiant donor, or outbursts produced at (or near) the periastron passage in wide and eccentric orbits, in order to explain the low (LX, quiescent ~ 1032 erg s-1) quiescent emission. Neither proposed mechanisms seem to explain the whole phenomenology of these sources. • Aims. Here we propose a new explanation for the outburst mechanism, based on the X–ray observations of the unique SFXT known to display periodic outbursts, IGR J11215-5952. • Methods. We performed three Target of Opportunity observations with Swift, XMM–Newton and INT EGRAL at the time of the fifth outburst, expected on 2007 February 9. Swift observations of the February 2007 outburst have been reported elsewhere. Another ToO with Swift was performed in July 2007, in order to monitor the supposed “apastron” passage. • Results. XMM–Newton observed the source on 2007 February 9, for 23 ks, at the peak of the outburst, while INT EGRAL started the observation two days later, failing to detect the source, which had already undergone the decaying phase of the fast outburst. The Swift campaign performed in July 2007 reveals a second outburst occurring on 2007 July 24, as bright as that observed about 165 days before. • Conclusions. The new X–ray observations allow us to propose an alternative hypothesis for the outburst mechanism in SFXTs, linked to the possible presence of a second wind component, in the form of an equatorial disk from the supergiant donor. We discuss the applicability of the model to the short outburst durations of all other Supergiant Fast X–ray Transients, where a clear periodicity in the outbursts has not been found yet. The new outburst from IGR J11215–5952 observed in July suggests that the true orbital period is ~165 days, instead of 329 days, as previously thought. 1. Introduction
The Galactic plane monitoring performed by the INTEGRAL satellite in the last 5 years has allowed the discovery
of a number of new High Mass X–ray Binaries (HMXBs). Several of these new sources are intrinsically highly
absorbed and were difficult to discover with previous missions (e.g. IGR J16318–4848, Walter et al. 2003).
Others are transient HMXBs (associated with OB supergiant) displaying short outbursts (few hours, typically less
than a day; Sguera et al. 2005), and form the recently recognized new class of Supergiant Fast X–ray Transients
(SFXTs).
IGR J11215–5952 is a hard X–ray transient discovered by INTEGRAL during a fast outburst in April 2005 (Lubinski et al. 2005). The short duration of this outburst led Negueruela et al. (2006) to propose that IGR J11215–5952 could be a new member of the class of Supergiant Fast X-ray Transients. The optical counterpart is indeed a B-type supergiant, HD 306414 located at a distance of 6.2 kpc (Masetti et al. 2006). From the analysis of archival INTEGRAL observations and the discovery of two previously unnoticed outbursts, a recurrence period in the X–ray activity of ~330 days has been found (Sidoli, Paizis, & Mereghetti 2006), likely linked to the orbital period of the binary system. This periodicity was later confirmed by the fourth outburst from IGR J11215–5952 observed with RXTE/PCA on 2006 March 16–17, 329 days after the previous one (Smith et al. 2006b). The INTEGRAL spectrum was well fitted by a hard power-law with a high energy cut-off around 15 keV (Paper I). Assuming a distance of 6.2 kpc, the peak fluxes of the outbursts correspond to a luminosity of ~ 3 × 1036 erg s-1 (5–100 keV; Paper I). The RXTE/PCA observations showed a possible pulse period of ~195 s (Smith et al. 2006a), later confirmed during the February 2007 outburst, yielding P=186.78 ± 0.3 s (Swank et al. 2007). References Masetti, N., et al. 2006, A&A 449, 1139 | |
| IGR J11215-5952 — d = 6 kpc — Prot = 186 s — Porb ~ 164.6 d — NH = 1022 cm-2 — LX(1-10 keV) ~ 1036 erg s-1 | |
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Authors: P. Romano, L. Sidoli, G. Cusumano, S. Vercellone, V. Mangano |
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Journal-ref: ApJ (2009) [0902.1985 ] |
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Title: Disentangling the system geometry of the Supergiant Fast X-ray Transient IGR J11215-5952 with Swift |
| Abstract:
IGR J11215-5952 is a hard X-ray transient discovered in 2005 April by INTEGRAL and a member of the new class
of HMXB, the Supergiant Fast X-ray Transients (SFXTs).
While INTEGRAL and RXTE observations have shown that the outbursts occur with a periodicity of ~330 days, Swift data have recently demonstrated that the true outburst period is ~165 days. IGR J11215-5952 is the first discovered SFXT displaying periodic outbursts, which are possibly related to the orbital period. We performed a Guest Investigator observation with Swift that lasted 20ks and several follow-up Target of Opportunity (ToO) observations, for a total of ~32ks, during the expected "apastron" passage (defined assuming an orbital period of ~330 days), between 2008 June 16 and July 4. The characteristics of this "apastron'' outburst are quite similar to those previously observed during the "periastron'' outburst of 2007 February 9. The mean spectrum of the bright peaks can be fit with an absorbed power law model with a photon index of 1 and an absorbing column of NH = 1022 cm-2. This outburst reached luminosities of LX(1-10 keV) ~ 1036 erg s-1, comparable with the ones measured in 2007. The light curve can be modelled with the parameters obtained by Sidoli et al. (2007) for the 2007 February 9 outburst, although some differences can be observed in its shape. The properties of the rise to this new outburst and the comparison with the previous outbursts allow us to suggest that the true orbital period of IGR J11215-5952 is very likely 164.6 days, and that the orbit is eccentric, with the different outbursts produced at the periastron passage, when the neutron star crosses the inclined equatorial wind from the supergiant companion. Based on a ToO observation performed on 2008 March 25-27, we can exclude that the period is 165/2 days. 1. Introduction
References Romano, P., et al. 2007, A&A 469, L5 | |
K5.3
Transient Black Hole Candidates
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Authors: F. Capitanio, A. Bazzano, A. J. Bird, P. Ubertini, M. Federici |
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Journal-ref: ESA SP 622 (2007) [0710.0298 ] |
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Title: Monitoring of Transient Black Hole Candidates observed in the INTEGRAL survey |
| Abstract:
The INTEGRAL/IBIS survey was performed collecting all the GPS and GCDE data
together with all the available public data. The second catalogue, published
in 2006 by Bird et al., is dominated by detection of 113 X-ray binaries, with
38 being high-mass and 67 low-mass.
In most systems the compact object is a neutron star, but the sample also contains 4 confirmed Black Holes and 6 LMXB black hole candidates (BHC). There are also, in additional, 6 tentative associations as BHCs based simply on spectral and timing properties. In the sample of 12 sources (BHC and tentatively associated BHC) there are 7 transient sources that went into outbursts during the INTEGRAL survey observations. We present here the monitoring of the time and spectral evolution of these 7 outbursts. is associated with H1743-322, a bright black hole candidate (BHC) observed by HEAO1 in 1977. After the start of the outburst in 2003, the source remained bright in soft X-rays (E < 15 keV) for ~ 8 months and was regularly detected. • IGR J17091–3624 was discovered in 2003 April by INTEGRAL/IBIS. Its flux reached 40 mCrab and 25 mCrab in the 15–40 keV and 40–100 keV bands respectively. RXTE observed the source simultaneously. • XTE J1908+094 has been detected for the first time during a RXTE/PCA scanning of the soft-gamma ray repeater SGR 1900+14. The source spectrum (2-30 keV) can be best fit with a power-law function including photoelectric absorption (column density Nh=2.3 × 1022, photon index = 1.55). An iron line emission is present, but may be due to the Galactic ridge. The maximum source flux (2-10 keV) reached was from 64 mCrab, no coherent pulsations are seen between 0.001 and 1024 Hz. XTE J1908+094 is classified as a possible black hole candidate. • IGR J18539+0727, this faint transient source was detected during a routine scan of the Galactic Plane and deep observations of GRS1915+105 field on April 17-18 2003. It is classified as a probably BHC because of its hard spectrum. • XTE J1720-318 was discovered on 2003 January 9 with the ASM monitor onboard RXTE as a transient source in outburst. The source flux increased to the maximum value of 430 mCrab in 2 days, and then started to decay slowly. • 4U 1630-47 is one of the most active transient BHC. Its last outburst was very long covering more then two years (2002-2004). Moreover the source went again in outburst at the end of 2005. • IGR J17285-2922 had a single period of activity in September 2003 when it was detected by INTEGRAL. References [1] Bird, A., Barlow, E., J., Bassani, L. et al. 2006, ApJ 636, 765 | |
| Literatur zu "INTEGRAL (i)" | |||
| G. Weidenspointner et al. | 2004 | ESA SP-552, 133 |
"SPI Observations of Positron Annihilation Radiation: Sky Distribution"
|
| S. Sazonov, E. Churazov, M. Revnivtsev, et al. | 2005 | A&A 444, L37 |
"Identification of 8 INTEGRAL hard X-ray sources with Chandra"
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| J. Rodriguez, C. Cabanac, D.C. Hannikainen, et al. | 2005 | A&A 432, 235 |
"Unveiling the Nature of the High Energy Source IGR J19140+0951"
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| J.B. Stephen, L. Bassani, M. Molina, et al. | 2005 | A&A 432, L49 |
"Using the ROSAT Bright Source Catalogue to find Counterparts for IBIS/ISGRI Survey Sources "
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| A.B. Hill, R. Walter, C. Knigge, et al. | 2005 | A&A 439, 255 |
"The 1 - 50 keV spectral and timing analysis of IGR J18027-2016: an eclipsing, high mass X-ray binary"
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| A. Rau, A.v. Kienlin, K. Hurley, G.G. Lichti | 2005 | A&A 438, 1175-83 |
"The 1st INTEGRAL SPI-ACS Gamma-Ray Burst Catalogue"
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| D.K. Galloway, C.B. Markwardt, E.H. Morgan, D. Chakrabarty, T.E. Strohmayer | 2005 | ApJ 622, L45-8 |
"Discovery of the accretion-powered millisecond X-ray pulsar IGR J00291+5934 "
|
| Sguera, V., Hill, A.B., Bird, A.J., et al. | 2007 | A&A 467, 249 |
"IGR J18483-0311: an accreting X-ray pulsar observed by INTEGRAL"
|
| P. Romano, L. Sidoli, et al. | 2007 | A&A 469, L5 |
"Swift/XRT observes the fifth outburst of the periodic SFXT IGR J11215-5952"
|
| Sidoli, L., Romano, P., et al. | 2007 | A&A 476, 1307 |
"An alternative hypothesis for the outburst mechanism in SFXTs: the case of IGR J11215-5952"
|
| Piranomonte, S., D’Avanzo, P., Covino, S., et al. | 2007 | A&A, 491, 183 |
"The short GRB 070707 afterglow and its very faint host galaxy"
|
| Weidenspointner, G; Skinner, G; Jean, P; et al. | 2008 | Nature 451, 159 |
"An asymmetric distribution of positrons in the galactic disk revealed by gamma rays"
|
| Foley, S.; McGlynn, S.; Hanlon, L.; McBreen, S.; McBreen, B. | 2008 | A&A 484, 143 |
"Global characteristics of GRBs & the inferred large population of low-luminosity GRBs"
|
 | H. Heintzmann | ( Eintrag vom 24.12.2008) |
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