"Millisekunden Pulsare (2)" — (1)
Pulsare — I:
Millisekunden Pulsare (2)
Die Recycling Hypothese
K1
Accretion-driven millisecond X-ray pulsars
extra Link:
An observational review of accretion-driven millisecond X-ray pulsars
Rudy Wijnands
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Author: Rudy Wijnands |
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Journal-ref: NuPhS 132 (2004) 496 [astro-ph/0309347 ] |
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Title: An observational review of accretion-driven millisecond X-ray pulsars |
| Abstract:
I present an observational review of the five currently known accretion-driven millisecond
X-ray pulsars. A prominent place in this review is given to SAX J1808.4-3658; it was the first
such system discovered and currently four outbursts have been observed from this source.
This makes SAX J1808.4-3658 the best studied example of the group. Its most recent outburst
in October 2002 is of particular interest because of the discovery of two simultaneous kilohertz
quasi-periodic oscillations and nearly coherent oscillations during type-I X-ray bursts.
This is the first time that such phenomena are observed in a system for which the neutron star
spin frequency is exactly known. The other four systems were discovered within the last two
years and only limited results have been published.
Since new exiting results are to be expected in the future for all five sources,
this review will only represent a snap-shot of the current observational knowledge
of accretion-driven millisecond X-ray pulsars.
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| SAX J1808.4-3658 — n = 401 Hz | |
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Author: Rudy Wijnands |
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Ref: Trends in Pulsar Research (Lowry, J.A., Ed.), Nova Science Publishers, NY, pp. 53–78
[astro-ph/0501264  ] |
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Title: Accretion-driven millisecond X-ray pulsars |
| Abstract: I present an overview of our current observational knowledge of the six known accretion-driven millisecond X-ray pulsars. A prominent place in this review is given to SAX J1808.4-3658; it was the first such system discovered and currently four outbursts have been observed from this source, three of which have been studied in detail using the Rossi X-ray Timing Explorer satellite. This makes SAX J1808.4-3658 the best studied example of an accretion-driven millisecond pulsar. Its most recent outburst in October 2002 is of particular interest because of the discovery of two simultaneous kilohertz quasi-periodic oscillations and nearly coherent oscillations during type-I X-ray bursts. This is the first (and so far only) time that such phenomena are observed in a system for which the neutron star spin frequency is exactly known. The other five systems were discovered within the last three years (with IGR J00291+5934 only discovered in December 2004) and only limited results have been published. | |
Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars —
D. Chakrabarty et al. — Nature 424 (2003) 42
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Author: G S Bisnovatyi-Kogan |
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Journal-ref: Physics-Uspekhi 49 (2006) 53-61 [astro-ph/0611398 ] |
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Title: Binary and recycled pulsars: 30 years after observational discovery |
| Abstract:
Binary radio pulsars, first discovered by Hulse and Taylor in 1974, are a
unique tool for experimentally testing general relativity (GR), whose validity
has been confirmed with a precision unavailable in laboratory experiments.
In particular, indirect evidence of the existence of gravitational waves has been obtained. Radio pulsars in binary systems (which have come to be known as recycled) have completed the accretion stage, during which neutron star spins reach millisecond periods and their magnetic fields decay 2 to 4 orders of magnitude more weakly than ordinary radio pulsars. Among about a hundred known recycled pulsars, many have turned out to be single neutron stars. The high concentration of single recycled pulsars in globular clusters suggests that close stellar encounters are highly instrumental in the loss of the companion. A system of one recycled pulsar and one 'normal' one discovered in 2004 is the most compact among binaries containing recycled pulsars. Together with the presence of two pulsars in one system, this suggests new prospects for further essential improvements in testing GR. This paper considers theoretical predictions of binary pulsars, their evolutionary formation, and mechanisms by which their companions may be lost. The use of recycled pulsars in testing GR is discussed and their possible relation to the most intriguing objects in the universe, cosmic gamma-ray bursts, is examined. 1. Introduction
The discovery of pulsars in 1967, along with quasars and cosmic microwave background radiation discovered several
years earlier, was the greatest milestone in physics and astronomy. Unlike the quasars and the cosmic microwave
background, which had been dedicatedly studied before their discovery, pulsars were discovered quite accidentally
while investigating interplanetary radio scintillations, which unexpectedly turned out to be strictly periodic.
Figure 1. A meeting of theoreticians at the Landau Institute
Figure 2. The location of pulsars on the P — P' diagram. [28.]
9. Two pulsars in a binary system: the most compact neutron star binary
as the best laboratory for testing GR
The discovery of PSR J0737-3039A with the period 23 ms around another compact object with the orbital period
2.4 h in a low-eccentricity orbit (e = 0.0878) was reported in Ref. [68].
9.1 Pulse arrival time in the binary pulsar
The binary parameters suggested another neutron star as the companion. In Ref. [2], this companion was
reported to be the radio pulsar J0737f-3039B with the spin period 2.8 s. Thus, for the first time, a binary
system consisting of two pulsars, a recycled one and a normal one,
was discovered. This binary has the shortest orbital period known among all RPs with two neutron stars.
Two pulsars with quite narrow beams in a short-period binary yield
unprecedented opportunities to test fundamental laws of
gravitational physics and make this system a remarkable
laboratory for relativistic astrophysics research.
9.4 The past and future of pulsars in a binary system: PSR J0737-3039A,B
The emission of gravitational waves results in the ultimate coalescence of the components after about
85 Myr. The simultaneous timing of both pulsars in their motion in the common gravity field increases the
accuracy of GR tests and measurements of gravitational wave emission. The properties of pulsars in this binary
system perfectly fit the evolutionary formation scheme for binary pulsars suggested in Ref. [6].
According to this scenario, the 23 ms pulsar was recycled by accretion and its magnetic field was simultaneously screened by the accreted plasma. The second pulsar with the period 2.8 s resulted later from the second supernova explosion that did not destroy the binary system. Therefore, the millisecond pulsar in this binary should be older and have a weaker magnetic field. Indeed, the magnetic field of the 23 ms pulsar was estimated to be Bms ~ 6.3 × 109 G and its characteristic age is t ~ 210 Myr. The ordinary 2.8 s pulsar has the field Bn ~ 1.6 × 1012 G and its age is t ~ 50 Myr [2]. The masses of neutron stars in this system are 1.34M  and 1.25M for the
23 ms and 2.8 s pulsars, respectively.
Knowing the current parameters of this system allows tracing its past and future evolution, because gravitational radiation is the only physical process changing the binary system parameters. Magnetic stellar wind and tidal interaction acting in ordinary stars are not important in the binary neutron star system. Changes to the binary major semiaxis a and eccentricity e in a system with masses m1 and m2, averaged over the orbital period, are given by the formulas [88]: 
The current values of the parameters of the double pulsar system are [2]
m1 = 1.34M (millisecond pulsar);
m2 = 1.25M;
e = 0.0878 ; Pb = 0.102 d = 8.83 × 103 s; a = 8.8 × 1010 cm : Integration of system yields the functions a(t), e(t), and Pb(t). The lifetime of the system before the coalescence is 84 Myr in accordance with the estimate in Ref. [2]. Integration into the past shows that over the characteristic lifetime of the second (50 Myr) and millisecond (210Myr) pulsars, the binary parameters change insignificantly. It is however very difficult to imagine why the orbital eccentricity e did not increase after the collapse and supernova explosion of the second companion, as is observed in other binary neutron star systems, which have e > 0.18, for example, e = 0.617 (PSR 1913+16) and e = 0.68 (PSR 2121+11C). References 2. Lyne A G et al. Science 303 1153 (2004) 6. Bisnovatyi-Kogan G S, Komberg B V Astron. Zh. 51 373 (1974) 28. Lorimer D R Living Rev. Relativity 4 (6) 5 (2001); astro-ph/0104388 68. Burgay M et al. Nature 426 531 (2003) 88. Lightman A P et al. Problem Book in Relativity and Gravitation (Princeton Univ. Press, 1975) | |
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Author: Watts , A.L. |
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Journal-ref: 'A decade of accreting ms x-ray pulsars' (2008) [0808.2536 ] |
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Title: Lighthouses with two lights: burst oscillations from the accretion-powered millisecond pulsars |
| Abstract:
The key contribution of the discovery of nuclear-powered pulsations from the accretion-powered millisecond
pulsars (AMPs) has been the establishment of burst oscillation frequency as a reliable proxy for stellar
spin rate. This has doubled the sample of rapidly-rotating accreting neutron stars and revealed the unexpected
absence of any stars rotating near the break-up limit.
The resulting `braking problem' is now a major concern for theorists, particularly given the possible role of gravitational wave emission in limiting spin. This, however, is not the only area where burst oscillations from the AMPs are having an impact. Burst oscillation timing is developing into a promising technique for verifying the level of spin variability in the AMPs (a topic of considerable debate). These sources also provide unique input to our efforts to understand the still-elusive burst oscillation mechanism. This is because they are the only stars where we can reliably gauge the role of uneven fuel deposition and, of course, the magnetic field. 1. Introduction
The first AMXP was not SAX J1808.4-3658 [2], but rather the far less well-known 4U 1728-34 [3]. How on earth,
you might ask, could such a slip go unnoticed? The trick, of course, lies in the terminology.
Most astronomers (the author included) tend to think of the AMXPs as comprising only
the accretion-powered millisecond pulsars (AMPs), forgetting the equally large class of
nuclear-powered millisecond pulsars (NMPs) - the burst oscillation sources.
Most of this volume focuses on the AMPs, where persistent pulsations are generated as accreting material is channeled by the magnetic field onto magnetic polar caps that are offset from the rotational poles. The NMPs, by contrast, show pulsations during Type I X-ray bursts (thermonuclear explosions on the stellar surface caused by rapid unstable burning of accreted material). The cause of the brightness asymmetry in the NMPs remains an open question [4, 5], and to do full justice to NMP phenomenology would merit a much longer discussion. In this article, however, I will focus on the small set of NMPs that are also AMPs. These rare objects provide a unique insight into many current problems in neutron star astrophysics because, as suggested by my title, they are lighthouses with two different light sources. The accretion-powered pulsations tell us how the material arrives on the stellar surface, while the nuclear-powered pulsations tell us what happens once it gets there. References 2. R. Wijnands, and M. van der Klis, Nature 394, 344–345 (1998). | |
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Author: M.A. Alpar |
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Journal-ref: 'A decade of accreting ms x-ray pulsars' (2008) [0808.3485 ] |
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Title: A New Class of Radio Pulsars - Back in 1982 |
| Abstract:
Basic ideas about the torques on the neutron star and the existence of an equilibrium rotation period followed
from the recognition that most X-ray binaries contain accretion powered neutron stars.
The evolution of binaries through a phase of accretion onto the neutron star, eventually leading to a post-accretion radio pulsar phase, was initially discussed as a way to understand the scarcity of binaries among the radio pulsars and the relatively short rotation periods of the first discovered binary radio pulsars in terms of magnetic fields that would be smaller than the familiar 1012 G range. The discovery of the millisecond pulsars made us realize that the fields can be much lower in a new class of radio pulsars that have been spun up by accretion in LMXBs. The predicted spin-down rates of the millisecond pulsar was soon confirmed. The observers' search for millisecond X-ray periods was on, leading first to the discovery of QPOs, and eventually to the discovery of the X-ray millisecond pulsars. The theorists' quest for explanations of why X-ray millisecond pulsations are not observed from LMXBs also started right away. 1. Introduction
The time line of pulsar and neutron star observations starts from the discovery of the radio
pulsars and proceeds with discoveries and important new timing observations every
year or every few years, until a break between 1985 and 1998, ending with the discovery
of the accreting millisecond pulsar. The discovery of radio pulsars (Hewish et al 1968)
and the understanding of their basic nature (Pacini 1968, Gold 1968), the proposal of
accretion powered neutron stars in X-ray binaries (Zeldovich & Guseynov 1966) and
the discovery of Sco X-1 (Giacconi et al. 1964), of pulsations from Cen X-3 (Giacconi
et al 1971) and of Doppler shifts of the pulse period confirming the binary nature of
this source (Schreier et al 1972) all followed one after the other, culminating with the
introduction of the concepts of Alfven radius, corotation radius and rotational equilibrium(
Pringle & Rees 1972, Davidson & Ostriker 1973, Lamb, Pethick & Pines 1973).
Enquiring the future evolution of Her X-1, which was then, as it still is, one of the most interesting and most studied sources, together with the absence or relative scarcity of radio pulsars in binaries, led Bisnovatyi-Kogan & Komberg to propose that accreting neutron stars in binaries have weak magnetic fields, giving rise, at the end of the accretion phase, to low radio luminosity pulsars which are difficult to detect (Bisnovatyi-Kogan & Komberg 1974). According to these authors accretion would lead not only to spin up, but also to suppression of the magnetic field. Accreting throughout the long lifetime of a Her X-1 type X-ray progenitor system, the neutron star would end up with B ~ 108-9 G, small enough for lack of radio pulsar activity post accretion. Many of the important ideas concerning the evolution of LMXBs to yield low magnetic field, short rotation period neutron stars are already present in the work of Bisnovatyi-Kogan & Komberg, except that the endpoint is not a radio pulsar because they do not consider the possibility that the neutron star rotation period could become short enough to make an active radio pulsar in spite of the low B. References 1. Hewish, A., Bell, S.J., Pilkington, J.D., Scott, P.F. & Collins, R.A., Nature 217, 709 (1968). 2. Pacini, F., Nature 219, 145 (1968). 3. Gold, T., Nature 218, 731 (1968). 4. Zeldovich, Ya.B. & Guseynov,O.H. ApJ 144, 840 (1966). 5. Giacconi, R., Gursky, H., Waters, C.R., Clark, G. & Rossi, B., Nature 204, 981 (1964). 6. Giacconi, R., Gursky, H., Kellogg, E.,Schreier, E. & Tananbaum, H., ApJ 167, L67 (1971) 7. Schreier, E., Levinson, R., Gursky, H., Kellogg, E., Tananbaum, H. Giacconi, R., ApJ 172, L79 (1972). 8. Pringle, J.P.E. & Rees, M.J., A&A 21, 1 (1972). 9. Davidson, K. & Ostriker, J.P., ApJ 179, 585 (1973). 10. Lamb, F.K., Pethick, C.J. & Pines, D., ApJ 184, 271 (1973). 11. Bisnovatyi-Kogan, G.S. & Komberg, B.V., Sov. Astron. 18, 217B (1974). 12. Hulse, R.A. & Taylor, J.H., ApJ 195, L51(1975). | |
K2
Seven AMSPs
Zum Thema:
Accretion-Powered Millisecond Pulsars (II) | |
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SAX J1808.4-3658: The first pulsar (1998)
Zum Thema | |
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| Discovery of SAX J1808.4-3658 — P = 2.49 ms, n = 401 Hz | |
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Authors: Rudy Wijnands, Michiel van der Klis |
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Journal-ref: Nature 394 (1998) 344 [astro-ph/9804216
] |
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Title: Discovery of the first accretion-powered millisecond X-ray pulsar |
| Abstract:
The precise origins of the millisecond radio pulsars, discovered in the early 1980s,
remain uncertain until this day. They plausibly evolve from accreting low magnetic-field
neutron stars in X-ray binary systems. If so, these stars should spin at millisecond rates.
In accordance with this idea, quasi-periodic oscillations discovered in X-ray binaries
around 50 Hz and 1 kHz, and drifting oscillations at several 100 Hz in X-ray bursts have all
been interpreted in terms of millisecond spins of weakly magnetized neutron stars.
However, in 15 years of searching, the expected coherent millisecond signals from X-ray binaries
remained elusive.
In this Letter, we report the discovery of the first example of such a signal. Using the Rossi X-ray Timing Explorer we find persistent 2.49 millisecond X-ray pulsations in an X-ray binary, which we interpret to come from an accretion-powered millisecond X-ray pulsar in the system. This is the first known object of its kind. It is likely to switch on as a millisecond radio pulsar when the accretion turns off completely. The source is positionally coincident with the known transient X-ray burster SAX J1808.4-3658, which also makes this the first X-ray pulsar which exhibits thermonuclear X-ray bursts. | |
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Authors: Chakrabarty, Deepto; Morgan, Edward H. |
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Journal-ref: Nature 394 (1998) 346-348 [astro-ph/9804248 ] |
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Title: The two-hour orbit of a binary millisecond X-ray pulsar |
| Abstract: Typical radio pulsars are magnetized neutron stars that are born rapidly rotating and slow down as they age on time scales of 10 to 100 million years. In contrast, millisecond radio pulsars spin very rapidly even though many are billions of years old. The most compelling explanation is that they have been `spun up' by the transfer of angular momentum during the accretion of material from a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs consist of a neutron star or black hole accreting matter from a companion with mass less than one solar mass.) The recent detection of coherent X-ray pulsations with a millisecond period from a suspected low-mass X-ray binary system appears to confirm this link. Here we report observations showing that the orbital period of this binary system is two hours, which establishes it as an LMXB. We also find an apparent modulation of the X-ray flux at the orbital period (at the two per cent level), with a broad minimum when the pulsar is behind the low-mass companion star. This system seems closely related to the `black-widow' millisecond radio pulsars, which are evaporating their companions through irradiation. It may appear as an eclipsing radio pulsar during periods of X-ray quiescence. | |
K2.2
XTE J1751-305: The second pulsar (2002)
| pulsar XTE J1751-305 — n = 435 Hz | |
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Authors: C. B. Markwardt, J. H. Swank, T. E. Strohmayer, J. J. M. in 't Zand, F. E. Marshall |
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Journal-ref: ApJ 575 (2002) L21-L24 [astro-ph/0206491] |
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Title: Discovery of a Second Millisecond Accreting Pulsar: XTE J1751-305 |
| Abstract:
We report the discovery by the RXTE PCA of a second transient accreting
millisecond pulsar, XTE J1751-305, during regular monitoring observations of
the galactic bulge region.
The pulsar has a spin frequency of 435 Hz, making it one of the fastest pulsars. The pulsations contain the signature of orbital Doppler modulation, which implies an orbital period of 42 minutes, the shortest orbital period of any known radio or X-ray millisecond pulsar. The mass function, fx = 1.27 × 10-6M ,
yields a minimum mass for
the companion of between 0.013 and 0.017 M, depending
on the mass of the neutron star.
No eclipses were detected. A previous X-ray outburst in June, 1998, was discovered in archival All-Sky Monitor data. Assuming mass transfer in this binary system is driven by gravitational radiation, we constrain the orbital inclination to be in the range 30-85 deg, and the companion mass to be 0.013-0.035 M . The companion is most
likely a heated helium dwarf.
We also present results from the Chandra HRC-S observations which provide the best known position of XTE J1751-305. | |
| XTE J1751-305 — n = 435 Hz — < n' > = (3.7 ± 1.0) × 10-13 Hz -1 — d >~ 6.7 kpc | |
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Authors: A. Papitto, M.T. Menna, L. Burderi, T. Di Salvo, A. Riggio |
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Journal-ref: MNRAS 375 (2007) 971 [0710.1215 ] |
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Title: Measuring the spin up of the Accreting Millisecond Pulsar XTE J1751-305 |
| Abstract:
We perform a timing analysis on RXTE data of the accreting millisecond pulsar XTE J1751-305 observed during
the April 2002 outburst. After having corrected for Doppler effects on the pulse phases due to the orbital
motion of the source, we performed a timing analysis on the phase delays, which gives, for the first
time for this source, an estimate of the average spin frequency derivative
< n' > = (3.7 ± 1.0) × 10-13 Hz -1.
We discuss the torque resulting from the spin-up of the neutron star deriving a dynamical estimate of the mass accretion rate and comparing it with the one obtained from X-ray flux. Constraints on the distance to the source are discussed, leading to a lower limit of ~ 6.7 kpc. 1. Introduction
Accreting millisecond pulsars (AMSP in the following) are the long sought connection between low mass X-ray
binaries (LMXBs) and millisecond radio pulsars.
In fact, although it was hypothesised soon after their discovery that fast spinning radio pulsars were “recycled” by an accretion phase in a LMXB system, during which the neutron star (NS) is spun–up, evidence has been elusive since SAX J1808.4-3658, the first accretion-driven millisecond X-ray pulsar, was discovered (Wijnands & van der Klis 1998). SAX J1808.4-3658, with a spin period of 2.5 ms, exhibiting both X-ray bursts and coherent pulsations, proved to be the missing link between the two classes of sources. Since then, six more millisecond X-ray pulsars were discovered (see Wijnands 2006 for an observational review). All of these sources are transients with usually low duty cycles. Except for the case of HETE J1900.1-2455 which remained active for more than a year after its discovery in June 2005 (Galloway et al. 2007), the outbursts of AMSP last for no more than a couple of months, with recurrence times usually larger than 2 yr (Galloway 2006). Although the sample is still small, monitoring of future outbursts exhibited by the known sources is extremely important for our understanding of LMXBs and their evolution. The high temporal resolution satellite Rossi X-Ray Timing Explorer (RXTE), allowed the measurement of the spin frequency derivative in the case of • IGR J00291+5934 (Falanga et al. 2005; Burderi et al. 2007), • XTE J0929-314 (Galloway et al. 2002), • XTE J1814-338 (Papitto et al. 2007), • XTE J1807–294 (Riggio et al. 2007) and of one of the oubursts shown by • SAX J1808.4–3658 (Burderi et al. 2006). See Di Salvo et al. (2007) and references therein for a review. References Burderi, L., Di Salvo, T. et al. 2006 ApJ 653, L133 | |
K2.3
XTE J0929-314: The third pulsar (2002)
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XTE J0929-314 — n = 185 Hz — mass function
fp = 2.7 × 10-7M
— Porb = 43.6 min | |
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Authors: Duncan K. Galloway, Deepto Chakrabarty, Edward H. Morgan, and Ronald A. Remillard |
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Journal-ref: ApJ 576 (2002) L137-L140 [astro-ph/0206493] |
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Title: Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary |
| Abstract:
We have identified the third known accretion-powered millisecond pulsar, XTE J0929-314, with the
Rossi X-Ray Timing Explorer. The source is a faint,
high-Galactic-latitude X-ray transient (d >~ 5 kpc) that was in outburst during
2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude
of 3-7% and was generally broad and sinusoidal, although occasionally
double-peaked. The hard X-ray pulses arrived up to 770 microseconds earlier
than the soft X-ray pulses. The pulsar was spinning down at an average rate of
-9.2 × 10-14 Hz/s; the spin-down torque may arise from magnetic
coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational
radiation from the rapidly spinning pulsar. The pulsations were modulated by a
43.6 min ultracompact binary orbit, yielding the smallest measured mass
function (fp = 2.7 × 10-7M)
of any stellar binary. The binary parameters imply
an approximately 0.01 M white dwarf donor
and a moderately high inclination. We note that all three known accreting millisecond pulsars are
X-ray transients in very close binaries with extremely low mass transfer rates.
This is an important clue to the physics governing whether or not persistent
millisecond pulsations are detected in low-mass X-ray binaries.
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K2.4 XTE J1807-294
Zum Thema | |
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| XTE J1807-294 — QPOs — n = 191 Hz — Porb = 40 min | |
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Authors: Rudy Wijnands, Michiel van der Klis, Jeroen Homan, Deepto Chakrabarty, Craig B. Markwardt, Ed H. Morgan |
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Journal-ref: Nature 424 (2003) 44-47 [astro-ph/0307123 &
] |
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Title: Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar |
| Abstract: The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supranuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin. | |
| XTE J1807-294 — Timing Features | |
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Authors: F. Zhang, J.L. Qu, C.M. Zhang, W. Chen, T.P. Li |
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Journal-ref: ApJ (2006) [astro-ph/0602529 ] |
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Title: Timing Features of the Accretion-driven Millisecond X-Ray Pulsar XTE J1807-294 in 2003 March Outburst |
| Abstract: In order to probe the activity of the inner disk flow and its
effect on the neutron star surface emissions, we carried out the timing
analysis of the Rossi X-Ray Timing Explorer (RXTE) observations of the
millisecond X-ray pulsar XTE J1807—294, focusing on its correlated behaviors
in X-ray intensities, hardness ratios, pulse profiles and power density spectra.
The source was observed to have a serial of broad "puny" flares on a timescale of hours to days on the top of a decaying outburst in March 2003. In the flares, the spectra are softened and the pulse profiles become more sinusoidal. The frequency of kilohertz quasi-periodic oscillation (kHz QPO) is found to be positively related to the X-ray count rate in the flares. These features observed in the flares could be due to the accreting flow inhomogeneities. It is noticed that the fractional pulse amplitude increases with the flare intensities in a range of ~ 2%-14%, comparable to those observed in the thermonuclear bursts of the millisecond X-ray pulsar XTE J1814—338, whereas it remains at about 6.5% in the normal state. Such a significant variation of the pulse profile in the "puny" flares may reflect the changes of physical parameters in the inner disk accretion region. Furthermore, we noticed an overall positive correlation between the kHz QPO frequency and the fractional pulse amplitude, which could be the first evidence representing that the neutron-star surface emission properties are very sensitive to the disk flow inhomogeneities. This effect should be cautiously considered in the burst oscillation studies. 1 INTRODUCTION
Neutron stars in low-mass X-ray binaries (LMXBs) accrete matter from a companion with a mass of less than 1
M via an accretion disk. In many models, the Keplerian
accretion disk is supposed to be terminated at an inner radius rin of a few Schwarzschild radii by
e.g. relativistic effects, radiation drag, or neutron-star magnetosphere-disk interactions.
The motion of the inner disk flow and the geometry of the innermost disk are still uncertain. The observations of the correlated behaviors in X-ray spectral and timing variabilities in LMXBs have provided the important probes of the accretion-flow dynamics and the stellar-disk interactions. Kilohertz quasi-periodic oscillations (kHz QPOs, in the frequency range of 200 Hz — 1300 Hz) have been observed in more than 20 accreting neutron star LMXBs. Their frequencies are usually regarded to be associated with the Keplerian orbital frequency at some preferred radius related to rin. An important evidence for the movement of the inner edge of the disk under different mass accretion rate comes from the observation of a positive correlation of frequency vs. count rate on timescales of hours to days in some low-luminosity LMXBs, i.e. atoll sources. A similar correlation (or in some cases an anti-correlation) has been also found in some high-luminosity LMXBs, i.e. Z sources, in the form of the correlations with curve length S_{z} along the track traced in an X-ray color-color diagram. The effect of stellar radiations on the disk movement has also been observed, e.g. in 4U 1608-52. Nearly coherent brightness oscillations have been discovered during thermonuclear X-ray bursts in some LMXBs. Recent observations of the accreting millisecond pulsars, SAX J1808.4-3658 and XTE J1814-338, confirm that the burst oscillation frequency is extremely close to the spin frequency. It supports the interpretation of the burst oscillation in terms of a hot spot on the stellar surface, which triggers the efforts to investigate X-ray emission properties on the neutron star surface by folded pulse profiles. Among the discovered 7 millisecond X-ray pulsars, XTE J1807—294 is the best candidate for us to investigate the impact of disk flow activities on the neutron-star surface emission for four reasons:
With the knowledge mentioned above, many basic features of XTE J1807-294 have been available as well, such as the shortest orbital period of » 40 minutes and a relatively slow spin frequency of » 191 Hz. In addition, this source locates at 5°.7 away from the Galactic center, with the best known position based on a Chandra observation. Assumed a distance of 8 kpc, the source luminosity dropped from 1.3 × 1037 erg s-1 on February 28, 2003, to 3.6 × 1036 erg s-1 on March 22, 2003. Literatur Bhattacharyya S., Strohmayer T. E., Miller M. C., Markwardt C. B., 2005, ApJ 619, 483 [astro-ph/0402534] Constraints on Neutron Star Parameters from Burst Oscillation Light Curves of the Accreting Millisecond Pulsar XTE J1814-338 Chakrabarty, D., Morgan, E. H., Muno, M. P., et al. 2003, nat, 424, 42 | |
| XTE J1807-294 — | |
|
Authors: Y. Chou, Y. Chung, C.P. Hu, T.C. Yang |
|
Journal-ref: ApJ (2008) [0801.0909 ] |
|
Title: Precise Orbital Parameters and Anomalous Phase Variations of the Accretion-powered Millisecond Pulsar XTE J1807-294 |
| Abstract:
This study reports pulse variation analysis results for the forth discovered
accretion-powered millisecond pulsar XTE J1807-294 during its 2003 outburst
observed by Rossi X-ray Timing Explorer.
The pulsation is significantly detected only in the first ~90d out of ~150d observations. The pulse phase variation is too complex to be described as an orbital motion plus a simple polynomial model. The precise orbital parameters with Porb = 40.073601 min and ax sin(i) = 4.823 lt-ms were obtained after applying the trend removal to the daily observed 150s segments pulse phases folded with a constant spin frequency without Keplerian orbit included. The binary barycenter corrected pulse phases show smooth evolution and clear negative phase shifts coincident with the flares seen on the light curve and the enhancements of fractional pulse amplitude. The non-flare pulse phases for the first ~60d data are well described as a fourth order polynomial implying that the neutron star was spun-up during the first ~60d with a rate n' = (1.7 ± 0.3) × 10-13 Hz/s at the beginning of the outburst. Significant soft phase lags up to ~500 µs (~10% cycle) between 2 to 20 keV were detected for the nonflare pulse phases. We conclude that the anomalous phase shifts are unlikely due to the accretion torque but could result from the “hot spot” moving on the surface of neutron star. 1. Introduction
Low Mass X-ray Binaries (LMXBs) are considered to be the progenitor of the millisecond pulsars detected in the
radio band. It is widely believed that the weakly magnetized, slowly rotating neutron star is gradually spun-up
through the transfer of angular momentum carried by the matter from an accretion disk. Millisecond-time-scale
variations have been observed in many LMXB systems, including kilohertz Quasi-Periodic Oscillation (kHz QPO
and burst oscillation; however, the attempts to measure
coherent millisecond pulsation from an LMXB, which provides a direct evolutionary link
between radio millisecond pulsar and LMXB, were unsuccessful until the discovery of the
first accretion-powered millisecond pulsar SAX 1808.4-3658 with a spin period Pspin ~ 2.5
ms (Wijnands & van der Klis 1998; Chakrabarty & Morgan 1998), in 1998. To date, coherent
millisecond pulsations have been detected in ten LMXBs with spin periods ranging from
1.67 ms to 5.5 ms (Wijnands 2007; Markwardt et al. 2007; Gavriil et al. 2007; Casella et al.
2007; Altamirano et al. 2007).
References Altamirano, D., Casella, P., Patruno, A., Wijnands, R., van der Klis, M., ApJL | |
K2.5
Accretion-Powered Millisecond Pulsar XTE J1814-338
| XTE J1814-338 — n = 314 Hz — fX = 2.64 x 10-8 erg cm-2 s-1 | |
|
Authors: T.E. Strohmayer, C.B. Markwardt, J.H. Swank, J. in’t Zand |
|
Journal-ref: ApJ 596 (2003) L67–L70 [astro-ph/0308353 ] |
|
Title: X-Ray Bursts from the Accreting Millisecond Pulsar XTE J1814-338 |
| Abstract:
Since the discovery of the accreting millisecond pulsar XTE J1814-338 a total
of 27 thermonuclear bursts have been observed from the source with the
Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE).
Spectroscopy of the bursts, as well as the presence of continuous burst oscillations, suggests that all but one of the bursts are sub-Eddington. The remaining burst has the largest peak bolometric flux of fX = 2.64 x 10-8 erg cm-2 s-1, as well as a gap in the burst oscillations, similar to that seen in Eddington limited bursts from other sources. Assuming this burst was Eddington limited we obtain a source distance of about 8 kpc. All the bursts show coherent oscillations at the 314.4 Hz spin frequency. The burst oscillations are strongly frequency and phase locked to the persistent pulsations. Only two bursts show evidence for frequency drift in the first few seconds following burst onset. In both cases the initial drift corresponds to a spin down of a few tenths of a Hz. The large oscillation amplitude during the bursts confirms that the burst flux is modulated at the spin frequency. We detect, for the first time, a significant first harmonic component in burst oscillations. The ratio of count rate in the first harmonic to that in the fundamental can be > 0.25 and is, on average, less than that of the persistent pulsations. If the pulsations result from a single bright region on the surface, the harmonic strength suggests the burst emission is beamed, perhaps due to a stronger magnetic field than in non-pulsing LMXBs. Alternatively, the harmonic content could result from a geometry with two bright regions. | |
| XTE J1814-338 — n = 314 Hz — d ~ 8.0 kpc | |
|
Authors: M.I. Krauss, Z. Wang, A. Dullighan, A.M. Juett, D.L. Kaplan, D. Chakrabarty, M.H. van Kerkwijk, D. Steeghs, P.G. Jonker, C.B. Markwardt |
|
Journal-ref: ApJ 627 (2005) 910-914 [astro-ph/0503671] |
|
Title: The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338 |
| Abstract:
We report the precise optical and X-ray localization of the 3.2 ms
accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray
Observatory as well as optical observations conducted during the 2003 June
discovery outburst.
Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power-law (with photon index = 1.4) plus blackbody (with kT = 0.95 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broad-band spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star, and therefore very unlikely to be responsible for the soft excess, making synchroton emission a more reasonable candidate. 1. INTRODUCTION
The soft X-ray transient XTE J1814-314 (l = 358.°7, b = -7.°6) was discovered in outburst on 2003 June 5
during scans of the central Galactic plane with the Rossi X-Ray Timing Explorer (RXTE; Markwardt & Swank
2003). The outburst lasted for approximately 55 days, and had a peak 2–10 keV flux of around
13 mCrab.
RXTE observations also established the source as a 314 Hz (3.2 ms) accretion-powered X-ray pulsar in a 4.3 hour binary with a main sequence companion of at least 0.17 M (using the mass function of
0.002016 M, assuming a
neutron star mass of 1.4 M). Over two dozen thermonuclear X-ray bursts with millisecond oscillations at
the spin frequency were detected from XTE J1814-338
during the 2003 June outburst.
These burst oscillations had the particularly interesting characteristic of containing significant harmonic content, which allowed Bhattacharyya et al. (2005) to constrain the neutron star as well as orbital parameters. One of these bursts showed evidence for photospheric radius expansion, allowing Strohmayer et al. (2003) to infer a source distance of 8.0 ± 1.6 kpc. | |
| XTE J1814-338 — Porb = 4.275 h | |
|
Authors: A. Papitto, T. Di Salvo, L. Burderi, M.T. Menna, G. Lavagetto, A. Riggio |
|
Journal-ref: MNRAS 375 (2007) 971 [astro-ph/0611942 ] |
|
Title: Timing of the Accreting Millisecond Pulsar XTE J1814-338 |
| Abstract:
We present a precise timing analysis of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst,
observed by RXTE. A full orbital solution is given for the first time; Doppler effects induced by the motion of
the source in the binary system were corrected, leading to a refined estimate of
the orbital period, Porb = 15388.7229 s, and of the projected semimajor axis, a sini/c= 390.633(9) lt-ms. We could then investigate the spin behaviour of the accreting compact object during the outburst. We report here a refined value of the spin frequency (n = 314.35610879 Hz) and the first estimate of the spin frequency derivative of this source while accreting (n' = (-6.7 +/- 0.7) × 10-14 Hz/s). This spin down behaviour arises when both the fundamental frequency and the second harmonic are taken into consideration. We discuss this in the context of the interaction between the disc and the quickly rotating magnetosphere, at accretion rates sufficiently low to allow a threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. We also present indications of a jitter of the pulse phases around the mean trend, which we argue results from movements of the accreting hotspots in response to variations of the accretion rate. These two classes of objects are linked by the recycling scenario, that argues how an old and weakly magnetised neutron star can be effectively spun up to spin periods of few milliseconds by accretion of matter and angular momentum through a (Keplerian) accretion disc. Despite the low magnetic fields involved, these neutron stars are sufficiently fast at the end of the accretion phase to switch on again the mechanism that drives the radio pulsar phenomenon. The X-ray transient XTE J1814–338 was discovered in 2003 during scans of the central Galactic plane with RXTE (Markwardt & Swank 2003, hereafter MS03). This accreting pulsar has a 3.14 ms spin period and resides in a binary system, whose orbital period (4.275 hr) and minimum companion mass (~ 0.15 M ) make it the widest and most massive
among all the seven systems discovered so far, that harbour an AMSP.
| |
| XTE J1814-338 — n = 314 Hz — Porb = 4.275 h — fX = 2.64 x 10-8 erg cm-2 s-1 — d ~ 8.0 kpc | |
|
Authors: A.L. Watts, A. Patruno, M. van der Klis |
|
Journal-ref: ApJ (2008) L [0805.4610 ] |
|
Title: Coherence of burst oscillations and accretion-powered pulsations in the accreting millisecond pulsar XTE J1814-338 |
| Abstract:
X-ray timing of the accretion-powered pulsations during the 2003 outburst of the
accreting millisecond pulsar XTE J1814-338 reveals variation in the pulse time of arrival residuals.
These can be interpreted in several ways, including spin-down and wandering of the fuel impact point around the magnetic pole. In this Letter we show that the burst oscillations of this source are coherent with the persistent pulsations, to the level where they track all of the observed variation in the residuals. Only one burst, which occurs at the lowest accretion rates, shows a significant phase offset. We discuss what might lead to such rigid phase-locking between the modulations in the accretion and thermonuclear burst emission, and consider the implications for spin variation and the burst oscillation mechanism. Premature ignition due to higher local temperatures at the fuel stream impact point may play a role in generating the highly unusual burst oscillations of this source. 1. Introduction
XTE J1814-338 was discovered in 2003 in the Rossi X-ray Timing Explorer (RXTE) Galactic bulge monitoring campaign
(Markwardt & Swank 2003), and remained in outburst for nearly 2 months. The pulsar has a spin frequency
of 314.4 Hz and resides in a binary with an orbit of 4.3 hours (Markwardt et al. 2003). During the
outburst over 425 ks of high time resolution data were taken with RXTE’s Proportional Counter Array (PCA,
Jahoda et al. 2006).
SAX J1808.4-3658 (J1808) and XTE J1814-338 (J1814) also show thermonuclear-powered pulsations, or burst oscillations. These are high frequency variations seen during Type I X-ray bursts, powered by unstable burning of accreted fuel. In both systems the burst oscillation frequency is at, or very close to, the known spin frequency (Chakrabarty et al. 2003; Strohmayer et al. 2003). J1814’s accretion-powered pulsations show significant pulse time of arrival (TOA) residuals even after correction for orbital Doppler shifts (Papitto et al. 2007, hereafter P07). References Chakrabarty, D. et al., 2003, Nature 424, 42 | |
| XTE J1814-338 — | |
|
Authors: D.A. Leahy, S.M. Morsink, Yi-Ying Chung, Yi Chou |
|
Journal-ref: ApJ (2008) [0806.0824 ] |
|
Title: Constraints on the Properties of the Neutron Star XTE J1814-338 from Pulse Shape Models |
| Abstract:
The accretion-powered (non-X-ray burst) pulsations of XTE J1814-338 are modeled to
determine neutron star parameters and their uncertainties. The model is a
rotating circular hot spot and includes:
• (1) an isotropic blackbody spectral component; • (2) an anisotropic Comptonized spectral component; • (3) relativistic time-delays and light-bending; and (4) the oblate shape of the star due to rotation. This model is the simplest possible model that is consistent with the data. The resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3 σ allowed regions in the mass-radius diagram. We analyzed all data combined from a 23 day period of the 2003 outburst, and separately analyzed data from 2 days of the outburst. The allowed mass-radius regions for both cases only allow equations of state (EOS) that are stiffer than EOS APR (Akmal et al. 1998), consistent with the large mass that has been inferred for the pulsar NGC 6440B (Freire et al. 2008). 1. Introduction
References Akmal, A., Pandharipande, V.R., & Ravenhall, D.G. 1998, Phys. Rev. C 58, 1804 Freire, P.C.C., et al. 2008, ApJ 675, 670 | |
IGR J00291+5934 (ii)
Zum Thema:
The millisecond X-ray pulsar IGR J00291+5934.
Der schnellste Röntgen-msec-Pulsar ist vom Satelliten INTEGRAL entdeckt worden - er schafft 599 Umdrehungen pro Sekunde. | |
|
|
| IGR J00291+5934 — Prot = 1.67 ms — nrot = 598 Hz; — Torbit = 8844 s (2.46 hr) — D = 3 kpc, z = 160 pc — L ~ 1036 erg s-1(d/3kpc)2 | |
|
Authors: P. D'Avanzo, S. Campana, S. Covino, G. L. Israel, L. Stella, G. Andreuzzi |
|
Journal-ref: A&A (2007) [0707.3037 ] |
|
Title: The optical counterpart of IGR J00291+5934 in quiescence |
| Abstract:
The recent (December 2004) discovery of the sixth accretion-powered millisecond X-ray
pulsar IGR J00291+5934 provides a very good chance to deepen our knowledge of such systems.
Although these systems are well studied at high energies, poor informations are available for their optical/NIR counterparts during quiescence. Up to now, only for SAX J1808.4-3658, the first discovered system of this type, we have a secure multiband detection of its optical counterpart in quiescence. Among the seven known system IGR J00291+5934 is the one that resembles SAX J1808.4-3658 more closely. With the Italian 3.6 m TNG telescope, we have performed deep optical and NIR photometry of the field of IGR J00291+5934 during quiescence in order to look for the presence of a variable counterpart. We present here the first multiband (VRIJH) detection of the optical and NIR counterpart of IGR J00291+5934 in quiescence as well as a deep upper limit in the K-band. We obtain an optical light curve that shows variability consistent with a sinusoidal modulation at the known 2.46 hr orbital period and present evidence for a strongly irradiated companion. 1. Introduction
In 1998, the discovery of the first accretion-powered millisecond X-ray pulsar SAX J1808.4-3658, confirmed the
evolutionary link between Low Mass X-Ray Binaries (LMXBs) and millisecond radio pulsars supporting the idea that
the formers are the progenitors of the latters.
In the following years, six more accretion-powered millisecond X-ray pulsars have been discovered. All these systems are transients of the Soft X-Ray Transients class (SXRTs), have orbital periods in the range between 40 min and 4.5 hr and spin frequencies from 1.7 to 5.4 ms. These seven accreting millisecond pulsars are well studied at high energies, especially in the X-rays, both in outburst and in quiescence (see Wijnands 2005 for a review). On the other hand, with the significant exception of SAX J1808.4-3658, their optical/NIR quiescent counterparts are only poorly known. The optical light curve of SAX J1808.4-3658 in outburst and quiescence shows variability modulated at the orbital period, in antiphase with the X-ray light curve. This is unlike other quiescent transient that normally show a double-humped morphology, due to an ellipsoidal modulation, and indicates that the companion star is subject to some irradiation. Burderi et al. (2003) proposed that the irradiation is due to the release of rotational energy by the fast spinning neutron star, switched on, as a radio pulsar, during quiescence. Following this idea, Campana et al. (2004) measured the required irradiating luminosity needed to match the optical flux and found that it is a factor of about 100 larger than the quiescent X-ray luminosity of the system. Neither accretion-driven X-rays nor the intrinsic luminosity of the secondary star or the disc can account for it. So, these authors conclude that the only source of energy available within the system is the rotational energy of the neutron star, reactivated as a millisecond radio pulsar. Optical and NIR observations performed in the past by different groups for the other systems of this class only led to deep upper limits for the counterparts of XTE J1751-305 and XTE J1814-314 or to the detection of very faint candidates, if any. The intrinsic faintness of the targets, in combination with the high interstellar absorption and high stellar crowding of the relevant fields are among the main reasons for the lack of detections at optical wavelengths. References Burderi, L. Di Salvo, T., D’Antona, F., Robba, N. R., Testa, V. 2003, A&A 404, L43 Burderi, L., Di Salvo, T., Lavagetto, G., et al. 2006, ApJ, | |
HETE J1900.1-2455 (2005)
Zum Thema | |
|
|
| HETE J1900.1-2455 — d = 5 kpc — brown dwarf companion — LX(0.1-200 keV) = 5 × 1036 erg s-1 | |
|
Authors: M. Falanga, J. Poutanen, E.W. Bonning, L. Kuiper, J.M. Bonnet-Bidaud, A. Goldwurm, W. Hermsen, L. Stella |
|
Journal-ref: A&A 464 (2006) 1069 [astro-ph/0609776 ] |
|
Title: Simultaneous INTEGRAL and RXTE observations of the accreting millisecond pulsar HETE J1900.1-2455 |
Abstract:
 Aims:
Hete J1900.1-2455 is the seventh known X-ray transient accreting
millisecond pulsar and has been in outburst for more than one year. We compared
the data on Hete J1900.1-2455 with other similar objects and made an attempt at
deriving constraints on the physical processes responsible for a spectral formation.
Methods:
The broad-band spectrum of the persistent emission in the
2-300 keV energy band and the timing properties were studied using simultaneous
INTEGRAL and the publicly available RXTE data obtained in October 2005. The
properties of the X-ray bursts observed from Hete J1900.1-2455 were also investigated.
Results:
The spectrum is well described by a two-component model
consisting of a blackbody-like soft X-ray emission at 0.8 keV temperature and a
thermal Comptonized spectrum with the electron temperature of 30 keV and
Thomson optical depth tT ~ 2 for the slab geometry.
The source is detected by INTEGRAL up to 200 keV at the luminosity of LX(0.1-200 keV)= 5 × 1036 erg s-1 (assuming a distance of 5 kpc). We have also detected one type I X-ray burst which shows the photospheric radius expansion. The burst occurred at an inferred persistent emission level of ~ 3-4% of the Eddington luminosity. Using the data for all X-ray bursts observed to date from Hete J1900.1-2455, the burst recurrence time is estimated to be about 2 days. No pulsations have been detected either in the RXTE or in the INTEGRAL data which puts interesting constraints on theories of the magnetic field evolution in neutron star low-mass X-ray binaries. 1. Introduction
The detection of X-ray millisecond pulsation in persistent emission from low-mass X-ray binaries (LMXBs)
remained elusive for many years until the discovery of the first accreting millisecond pulsar (MSP) by
Wijnands & van der Klis (1998). Since that time, a total of seven accreting MSP transients have been detected.
They are weakly magnetized (~ 108-9 G) neutron stars (NS) with spin frequencies in the 180–600 Hz
range and orbital periods between 40 min and 5 hr (see reviews by Wijnands, 2005; Poutanen, 2006).
Their companion stars have been found to be either highly evolved white or brown dwarfs. For the first time, the predicted decrease of the NS spin period during accretion was measured in the accreting MSP IGR J00291+5934 (Falanga et al., 2005b). This provided a strong confirmation of the theory of ’recycled’ pulsars in which the old neutron stars in LMXBs become millisecond radio pulsars through spin-up by transfer of angular momentum by the accreting material. MSP energy spectra are successfully fitted by a two-component model consisting of a multicolor blackbody soft X-ray emission and a Comptonized spectrum, for the hard X-ray emission. The soft thermal component could be associated with the radiation from the accretion disc and/or the heated NS surface around the shock. The hard emission is likely to be produced by thermal Comptonization in the hot accretion shock on the NS surface. HETE J1900.1–2455 was discovered during a bright X-ray burst by the High Energy Transient Explorer 2 (HETE-2) on 14 June 2005 (Vanderspek et al., 2005). Follow-up observations with the Rossi X-ray Timing Explorer (RXTE) identified the source as the seventh X-ray accreting millisecond pulsar, with a pulse frequency of 377.3 Hz, an orbital period of 83 min, and most likely a 0.016–0.07 M brown dwarf companion
(Kaaret et al., 2006).
The detected burst was consistent with a type I X-ray burst with the photospheric radius expansion. Assuming that the bolometric burst peak luminosity during the photospheric radius expansion saturated at the Eddington limit, Kawai & Suzuki (2005) estimated the distance to the source to be ~ 5 kpc assuming helium burst burning and canonical NS values. | |
|
HETE J1900.1–2455 — Mc ~ 0.085 M | |
|
Authors: P. Elebert, P.J. Callanan, A.V. Filippenko, P.M. Garnavich, G. Mackie, J.M. Hill, V. Burwitz |
|
Journal-ref: MNRAS (2007) [0711.0297 ] |
|
Title: Optical Photometry and Spectroscopy of the Accretion-Powered Millisecond Pulsar HETE J1900.1-2455 |
| Abstract:
We present phase resolved optical photometry and spectroscopy of the accreting
millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit a
sinusoidal modulation, at close to the orbital period, which we initially
attributed to X-ray heating of the irradiated face of the secondary star.
However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Ha emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km/s, placing a lower limit of Mc = 0.05 M
on the secondary mass.
For a 1.4 M
primary, this implies that the orbital inclination is low, < 20 degrees.
Utilising the observed relationship between the secondary mass and orbital period in short period cataclysmic variables, we estimate the secondary mass to be ~0.085 M , which implies an upper limit of
~2.4 M for the primary mass.
) secondary, via Roche lobe overflow.
X-ray transients (XRT) are LMXBs which undergo periodic outbursts, explained by the disc instability model (e.g. Dubus, Hameury & Lasota 2001), while persistently bright systems are permanently in the outbursting state. One subclass of LMXBs are the accretion-powered millisecond pulsars (AMSPs), the first of which, SAX J1808.4-3658, was discovered in 1998 (Wijnands & Van der Klis 1998). Seven more of these systems have been discovered since then (Poutanen 2006; Wijnands 2006; Markwardt, Krimm & Swank 2007). In general, the outburst duration for AMSPs ranges from a few weeks to months. AMSPs generally contain weakly magnetised (~108-9 G) neutron stars with spin frequencies between 180 and 600 Hz. The orbital periods range between 40 min and 5 h. The secondary star in these systems is either a white or brown dwarf (Falanga et al. 2007, and references therein). Falanga et al. (2005) have measured the predicted decrease in the NS spin period for the AMSP system IGR J00291+5934, supporting the idea that these AMSPs are in fact old NS, which have over time been spun up to millisecond periods by acquiring angular momentum from the accretion of material from the secondary. As such, they provide the missing link between LMXBs and old, isolated millisecond radio pulsars. In comparison to brighter LMXBs, relatively little is known about the optical properties of AMSPs. Even in outburst, the eight so far discovered are comparatively faint, the brightest being SAX J1808.4-3658 at R ~ 16.2 mag. HETE J1900.1-2455 is the seventh AMSP, discovered by Vanderspek et al. (2005) on 2005 June 14 by the High Energy Transient Explorer II (HETE II ). Several type-I bursts have since been observed, and assuming that the peak flux observed during the brightest of these is Eddington-limited, HETE J1900.1-2455 is at a distance of ~4.3 kpc (Suzuki et al. 2007). The pulsation frequency was determined with the Rossi X-ray Transient Explorer to be 377.3 Hz (Morgan, Kaaret & Vanderspek 2005). Pulse timing analysis by Kaaret et al. (2006) revealed a circular orbit with a period (Porb) of ~83.3 minutes (4995.258 ± 0.005 s), and a projected primary semimajor axis, a1sini, of 18.41 ± 0.01 × 10-3 lt-sec. These parameters combine to provide a mass function f(M) = 2.004 ± 0.003 × 10-6 M .
Galloway et al.
(2006) report that pulses were detected intermittently in
the first two months after discovery. No pulses have been
reported since then.
Unusually for an XRT, this system has remained active for more than 2 years, at a level of ~2 × 1036 erg s-1. Several times during 2007 March/February, the X-ray flux fell by an order of magnitude, but each time returned to outburst levels within ~1 week (Galloway et al. 2007). In 2007 May, the source was observed to decline to its lowest level since discovery (by a factor of > 103), lasting for several weeks, prompting speculation that HETE J1900.1-2455 was returning to quiescence (Degenaar et al. 2007a; Galloway et al. 2007; Torres et al. 2007). However, the system returned to its outburst state within ~2–3 weeks (Degenaar et al. 2007b; Garnavich et al. 2007). References Dubus G., Hameury J.-M., Lasota J.-P., 2001, A&A, 373, 251 Falanga M., Kuiper L., Poutanen J., et al. 2005, A&A, 444, 15 | |
SWIFT J1756.9-2508
Zum Thema | |
| |
| SWIFT J1756.9-2508 — nrot = 182.06 Hz; | |
|
Authors: Markwardt, C.B., Krimm, H.A., Swank, J.H. |
|
Journal-ref: ATel, 1108 (2007) [ ] |
|
Title: SWIFT J1756.9-2508 is a 182 Hz Millisecond X-ray Pulsar |
| Abstract:
We report that the new transient SWIFT J1756.9-2508 is the eighth known millisecond X-ray pulsar.
RXTE PCA observed the sources starting on 13 June 2007 for about 1700 seconds.
X-ray pulsations were detected at a (barycentered) frequency of 182.06 Hz, with an r.m.s. pulsed amplitude
of a few percent.
In a crude analysis, small changes in the pulse frequency are seen (of order mHz), which might
be suggestive of orbital period modulation.
However the current data span is far too short to capture a full orbital cycle, if such a cycle exists.
RXTE observations will continue for at least the next few days. Observations at other wavelengths are
encouraged.
| |
K3
Nuclear-Powered Millisecond Pulsars
Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars
by Chakrabarty et al.
| EXO 0748-676 — nrot = 45 Hz | |
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Authors: A.R. Villarreal, T.E. Strohmayer |
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Journal-ref: ApJ 614 (2004) L121 [astro-ph/0409384] |
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Title: Discovery of the Neutron Star Spin Frequency in EXO 0748-676 |
| Abstract: We report the results of a search for burst oscillations during thermonuclear X-ray bursts from the low mass X-ray binary (LMXB) EXO 0748-676. With the proportional counter array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) we detected a 45 Hz oscillation in the average power spectrum of 38 thermonuclear X-ray bursts from this source. We computed power spectra with 1 Hz frequency resolution for both the rising and decaying portions of 38 X-ray bursts from the public RXTE archive. We averaged the 1 Hz power spectra and detected a significant signal at 45 Hz in the decaying phases of the bursts. The signal is detected at a significance level of 4 x 10^-8. No similar signal was detected in the rising intervals. A fit to the oscillation peak at 0.25 Hz resolution gives a frequency of 44.7 ± 0.06 Hz and an oscillation quality factor of Q = 80 ± 18. The average signal amplitude is 3% (rms). The detection of 45 Hz burst oscillations from EXO 0748-676 provides compelling evidence that this is the neutron star spin frequency in this system. We use the inferred spin frequency to model the widths of absorption lines from the neutron star surface and show that the widths of the absorption lines from EXO 0748-676 recently reported by Cottam et al. are consistent with a 45 Hz spin frequency as long as the neutron star radius is in the range from about 9.5-15 km. With a known spin frequency, precise modelling of the line profiles from EXO 0748-676 holds great promise for constraining the dense matter equation of state. | |
K4
Accretion-driven millisecond X-ray pulsars in quiescence
| — | |
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Authors: S. Campana, P. D'Avanzo, J. Casares, S. Covino, G.L. Israel, G. Marconi, Rob Hynes, P. Charles, L. Stella |
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Journal-ref: ApJ 614 (2004) L49-L52 [astro-ph/0408584 ] |
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Title: Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658 |
| Abstract:
Millisecond radio pulsars are neutron stars that have been spun-up
by the transfer of angular momentum during the low-mass X-ray binary phase.
The transition from an accretion-powered to a rotation-powered pulsar takes
place on evolutionary timescales at the end of the accretion process, however
it may also occur sporadically in systems undergoing transient X-ray activity.
We have obtained the first optical spectrum of the low mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence. 1. Introduction
SAX J1808.4–3658 was the first-discovered low mass X–ray binary (LMXB) transient showing coherent pulsations.
This confirmed unambiguously the long-sought connection between LMXBs and millisecond radio pulsars.
The detection of coherent X–ray pulsations during outbursts testifies that the neutron star possesses a magnetic field of B ~ 108-9 G, sufficient for a small magnetosphere to form. Unlike persistent LMXBs, SAX J1808.4–3658 is a transient system, i.e. it is active in X–rays only for short intervals lasting a few months (outbursts) followed by quiescent periods of years. During quiescence LMXB transients are very faint in X–rays (5-6 orders of magnitude less than in outburst) usually with luminosities of 1032-33 erg s-1. Transient systems, therefore, represent a unique laboratory for the study of compact objects in accretion regimes that are inaccessible to persistent sources. In the optical, SAX J1808.4–3658 is dim during quiescence (mean R ~ 20.9±0.1; Homer et al. 2001) and brightens considerably in outbursts (R ~ 16.2±0.2; Wang et al. 2001). The optical light curve in outburst and quiescence is modulated at the orbital period and it is in anti-phase with the X–ray light curve, likely indicating that irradiation of the companion star plays a crucial role in spite of the low X–ray luminosity. This is unlike other quiescent transients. The mass function derived from X–ray data (4×10-5M , Chakrabarty & Morgan 1998)
and the requirement that the companion fills its Roche lobe led to the conclusion that it must be a rather low
mass star, possibly a brown dwarf (Bildsten & Chakabarty 2001).
| |
| Chandra — XTE J0929-314 and XTE J1751-305 — quiescent state | ||
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Authors: : Rudy Wijnands, Jeroen Homan, Craig O. Heinke, Jon M. Miller, Walter H. G. Lewin | |
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Journal-ref: ApJ 619 (2005) 492-502 [astro-ph/0406057 ] | |
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Title: Chandra observations of the accretion-driven millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in quiescence | |
From XTE J0929-314 we detected 22 photons (0.3-8 keV) in 24.4 ksec, resulting in a count rate of 9 x 10-4 c/s. The small number of photons detected did not allow for a detailed spectral analysis, but we can demonstrate that the spectrum is harder than simple thermal emission which is what is usually presumed to arise from a cooling neutron star that has been heated during the outbursts. Assuming a power-law model for the spectrum, we obtain a power-law index of ~1.8 and an unabsorbed flux of
fX(0.5-10 keV) = 6 x 10-15 erg cm-2 s-1,
resulting in a luminosity of
LX(0.5-10 keV) = 7 x 1031(d/10 kpc)2 erg s-1, with d in kpc.
No thermal component could be detected; such a component contributed at most 30% to the 0.5-10 keV flux. Variability in the count rate of XTE J0929-314 was observed at the 95% confidence level. We did not conclusively detect XTE J1751-305 in our 43 ksec observation, with 0.5-10 keV flux upper limits between fX(0.5-10 keV) = 0.2 and 2.7 x
10-14 erg cm-2 s-1 depending on assumed spectral shape,
resulting in luminosity upper limits of
LX(0.5-10 keV) =
0.2 - 2 x 1032 (d/8 kpc)2 ergs/s.
We compare our results with those obtained for other neutron-star X-ray transients in their quiescent state. Using simple accretion disk physics in combination with our measured quiescent luminosity of XTE J0929-314 and the luminosity upper limits of XTE J1751-305, and the known spin frequency of the neutron stars, we could constrain the magnetic field of the neutron stars in XTE J0929-314 and XTE J1751-305 to be less than 3 x 109 (d/10 kpc) and 3 - 7 x 108 (d/8 kpc) Gauss (depending on assumed spectral shape of the quiescent spectrum), respectively. Added spectral variability search for the data of XTE J0929-314 and added the non-detection with Chandra of XTE J1751-305. | ||
K4.1
The limit on neutron star spin rate
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Author: Duncan K Galloway | |||||||||
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ref: ASP Conference Series (2006) [astro-ph/0608540 ] | |||||||||
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Title: Pushing the limit on neutron star spin rates | |||||||||
| Abstract:
Millisecond X-ray pulsars consist of a rapidly-spinning neutron star
accreting from a low-mass stellar companion, and are the long-sought
evolutionary progenitors of millisecond radio pulsars, as well as promising
candidate sources for gravitational radiation. The population of these sources
has grown significantly over the last three years, with the discovery of six
new examples to bring the total sample to seven. Three sources are ultracompact
binaries with H-depleted donors and orbital periods of approx. 40 min, like the
185 Hz pulsar XTE J0929-314. Three more have orbital periods of 2 hr or longer,
similar to IGR J00291+5934, first detected in outburst by INTEGRAL in December
2004. The neutron star in this 2.46 hr binary has the most rapid spin of the
accreting pulsars at 599 Hz. The most recently-discovered pulsar, HETE
J1900.1-2455 (377 Hz), has an intermediate orbital period of 83.3 min, and has
been active for more than 1 yr, much longer than the typical transient
outburst. Pulsations were detected only in the first few months of the
outburst; this source has since resembled a faint, persistent non-pulsing
low-mass X-ray binary, typical of the broader low-mass X-ray binary population.
Introduction
HETE J1900.1-2455 — n = 377.3 Hz ; Porbit = 83.3 min
Discovery of the Millisecond X-Ray Pulsar HETE J1900.1-2455
The most recently-discovered accretion-powered pulsar, HETE J1900-2455 exhibits distinctly different behaviour in several respects to the rest of the population. The source was discovered in 2005 June when a strong thermonuclear (type-I) burst was detected by HETE-II (Vanderspek et al. 2005). HETE-II was designed to study gamma-ray bursts, but thanks to sensitivity extending into the X-ray band can also detect thermonuclear bursts from accreting neutron stars. Subsequent RXTE observations of the field revealed pulsations at 377.3 Hz (Morgan et al. 2005) as well as Doppler shifts of the apparent pulsar frequency, originating from orbital motion in the 83.25 min binary (Kaaret et al. 2006). In this case the Roche lobe can accommodate a brown dwarf with no need for extra heating. The rms amplitude of the pulsations was unusually low compared to the other pulsars, at best 2%. During the initial RXTE observations, the source underwent a short-lived flare after which the pulsations became undetectable. Such behaviour has not been observed in the other six pulsars, in which pulsations are consistently detected while the sources are X-ray bright. However, the pulsar outburst has also lasted much longer than in the other sources (Galloway et al. 2005b), and in fact at the time of writing (2006 June) the source is still active and being monitored with regular RXTE observations. Assuming that activity continues at the present level, the time-averaged accretion rate (at the estimated distance of 5 kpc; Kawai et al. 2005) is the highest amongst all of the millisecond pulsars (Galloway 2006). Kaaret, P., Morgan, E. H., Vanderspek, R., & Tomsick, J. A. 2006, ApJ, 638, 963 | ||||||||||
the burst oscillation sources (dotted line, including the sources
without a detected spin frequency) and
K4.2
Accretion-powered Millisecond Pulsar Outbursts
| HETE J1900.1-2455 — | ||
|
Author: Galloway, D.K. | |
|
AIPC 840, 50 (2006) [astro-ph/0604345 ] | |
|
Title: Accretion-powered Millisecond Pulsar Outbursts | |
TABLE 1. Outburst properties and distance limits for the millisecond X-ray pulsars
Bolometric fluence FX, in units of 10-3 erg cm-2
Estimated time-averaged bolometric flux FX in units of 10-11 erg cm-2 s-1 | ||
K4.3
Accreting X-ray millisecond pulsars observed with INTEGRAL
| XTE J1708-294 — IGR J00291+5934 — HETE J1900.1-2455 | ||
|
Author: Maurizio Falanga | |
|
ref: ESA SP-622 (2007) [astro-ph/0702454 ] | |
|
Title: Accreting X-ray millisecond pulsars observed with INTEGRAL | |
Out of seven recently discovered
accretion-powered pulsars (one discovered by INTEGRAL), three were observed
with the INTEGRAL satellite up to 300 keV.
Detailed timing and spectral results will be presented, including data obtained during the most recent outburst of the pulsar HETE J1900.1-2455 (28 October 2005, 10:25:12 UTC). Accreting X-ray millisecond pulsars are key systems to understand the spin and accretion history of neutron stars. They are also a good laboratory in which to study the source spectra, pulse profile, and phase shift between X-ray pulses in different energy ranges which give additional information of the X-ray production processes and emission environment. 1. Introduction
All of the accreting MSPs are X-ray transients; they spend most of the time in a quiescent phase,
with X-ray luminosities of order of 1031-33 erg s-1. They sometimes show X-ray outbursts reaching X-ray luminosities of LX = 1036-37 erg s-1, during which coherent pulsations are observed with frequencies in the range between 180 and 600 Hz. This frequency is interpreted as the NS rotation frequency given by a hot spot (or spots) in an atmospheric layer of the rotating NS. MSPs represent a new class of objects connecting accretion powered X-ray pulsars with rotation powered millisecond radio pulsars. | ||
|
Literatur zu "Millisecond Pulsars" | |||
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|
| Bildsten L. & Chakrabarty, D. | 2001 | ApJ 557, 292-296 |
A Brown Dwarf Companion for the Accreting Ms Pulsar SAX J1808.4-3658 [astro-ph/0104153 ]
|
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|
| Wijnands, R., van der Klis, M., Homan, et al. | 2003 | Nature 424, 44 |
Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
|
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|
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|
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|
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|
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|
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|
| Strohmayer, T. & Bildsten, L., in: Compact Stellar X-Ray Sources | 2006 | CUP |
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|
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|
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|
 | H. Heintzmann | ( Eintrag vom 28.3.2009) |
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