ASCA

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	Authors: Sambruna, R.M.; Ghisellini, G.; Hooper, E; Kollgaard, R.I.; Pesce, J.E.; Urry, C.M.
	Journal-ref: ApJ 515 (1999) 140 [astro-ph/9810319 ]
	Title: ASCA and Contemporaneous Ground-based Observations of the BL Lacertae Objects 1749+096 and 2200+420 (BL Lac)
Abstract: We present ASCA observations of the radio-selected BL Lacertae objects 1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 September and November, respectively. The ASCA spectra of both sources can be described as a first approximation by a power law with photon index Γ ~ 2. This is flatter than for most X-ray-selected BL Lacs observed with ASCA, in agreement with the predictions of current blazar unification models. While 1749+096 exhibits tentative evidence for spectral flattening at low energies, a concave continuum is detected for 2200+420: the steep low-energy component is consistent with the high-energy tail of the synchrotron emission responsible for the longer wavelengths, while the harder tail at higher energies is the onset of the Compton component. The two BL Lacs were observed with ground-based telescopes from radio to TeV energies contemporaneously with ASCA. The spectral energy distributions are consistent with synchrotron self-Compton emission from a single homogeneous region shortward of the IR/optical wavelengths, with a second component in the radio domain related to a more extended emission region. For 2200+420, comparing the 1995 November state with the optical/GeV flare of 1997 July, we find that models requiring inverse Compton scattering of external photons provide a viable mechanism for the production of the highest (GeV) energies during the flare. In particular, an increase of the external radiation density and of the power injected in the jet can reproduce the flat gamma-ray continuum observed in 1997 July. A directly testable prediction of this model is that the line luminosity in 2200+420 should vary shortly after (~1 month) a nonthermal synchrotron flare. 1. Introduction References

Supernova Remnants

The unprecedented broad bandwidth and excellent spectral resolution of ASCA (Advanced Satellite for Cosmology and Astrophysics) has had a major impact on all areas of research into the nature of supernova remnants (SNRs). Recent results from ASCA have increased the number of neutron star/SNR associations, debunking the decade-long mystery of the paucity of such associations. Additionally, ASCA has uncovered localized regions of non-thermal X-ray emission inside of SNRs which are not associated with the synchrotron nebulae but are produced by previously unrecognized mechanisms. ASCA maps of SNRs in prominent X-ray emission lines and selected continuum bands show variation in temperature, ionization, chemical composition, and, indeed, even the nature of the underlying emission mechanism. ASCA's increased sensitivity has allowed for a systematic study of the remnants in the LMC leading to the discovery of new ejecta-dominated remnants and an independent measurement of the gas-phase abundances of the LMC. Highly absorbed Galactic SNRs, which were weak and nondescript in previous soft X-ray observations, turn out to be remarkable objects with booming emission lines when observed using ASCA. These and other discoveries are leading to new insights into the nature of the ejecta of young remnants, the physics of supernova-induced shock waves, and the discovery and study of pulsar-powered synchrotron nebulae.

The Connection between Neutron Stars and Supernova Remnants

Neutron Stars (NS) are thought to originate in SN explosions, however the relatively few known associations of NS with SNR had led some to doubt the underlying ideas of NS formation. ASCA has dramatically improved this situation by increasing the number of known associations. Among the current generation of X-ray instruments, only the ASCA instruments possess the capability to separate the soft, mostly thermal emission from the interstellar material swept up by the SN shock from the hard, non-thermal emission from the synchrotron nebula around a pulsar. If the compact object cannot be detected directly, ASCA allows us to infer its existence by detecting its interaction with the surrounding medium. Since this emission is not subject to the beaming effects of the pulsar emission, it is significantly more likely to be detected.

Perhaps, the best example of the contribution of ASCA to this field is the case of the SNR G11.2-0.3. Vasisht et al. (Fig. 1;1996 ApJ 456, L59) used ASCA to detect a plerion in this remnant, which was only hinted at by previous X-ray observations, and Torii et al. (1997 ApJ 489, L145) detected a 65 ms pulsar with ASCA, which had not been detected in the radio. G11.2-0.3 was suggested to be the remnant of the historical SN of A.D. 386 (Clark & Stephenson 1977); the rapid pulsation and the temperature of the shock derived from the ASCA data are both consistent with a remnant age of ~1600 yr. This is only the second association of a pulsar with a historical supernova after the Crab and its pulsar.

The detection of a synchrotron nebula around an as yet undetected pulsar also lends support to the standard picture of NS birth and SNR evolution. Harrus et al. (1996 ApJ 464L, 161) detected the X-ray synchrotron nebula around the known radio pulsar PSR B1853+01 in the SNR W44. In contrast, Slane et al. (1997 ApJ, 485, 221) report the detection of a plerion in CTA-1 which was previously unknown in the radio. Harrus, Hughes, & Slane (1998 ApJ, 499, 273) report the detection of the synchrotron nebula in MSH11-62 which was known in the radio. Other remnants for which ASCA has detected a localized region of non-thermal emission are Kes 75 (Helfand 1994 New Horizons meeting), G292.0+1.8 (Torii et al. IAU proceedings), G327.1-1.1 (Sun et al. in preparation), and MSH15-56 (Plucinsky et al. 1998 Elba Proceedings). ASCA also detected the plerionic component in the SNR N157B in the LMC which prompted a followup observation by RXTE, which detected a 16 ms pulsar !

ASCA G11

Figure 1. G11.2-0.3 in four energy bands in the adjacent panels. The upper left is the ASCA band from 0.5 to 3.3 keV, the upper right is the ASCA band from 3.3 to 9.0 keV, and the lower left is the ASCA band over the entire band from 0.5 to 9.0 keV. The lower right panel is the Einstein HRI. The ASCA data demonstrate clearly the existence of the X-ray plerion. The image is from Vasisht et al. 1996 (ApJ 456, L59).

Non-thermal Emission in SNRs

ASCA has detected localized regions of non-thermal X-ray emission which cannot be explained by synchrotron emission from a pulsar nebula. ASCA observations solved the long-standing mystery of the spectrum of SN1006 by localizing the non-thermal emission to the bright rims of the remnant (Koyama et al. 1995 Nature 378, 255), which has been interpreted as synchrotron emission from electrons with energies up to 100 TeV accelerated in the remnant blast wave (Reynolds 1996 ApJ 459, L13). ASCA discovered an extended region of hard X-ray emission in the SNR IC 443 (Keohane et al. 1997, ApJ 484, 350). It is coincident with a region of strong interaction between the remnant shock, and a dense molecular cloud. The authors speculate that the hard X-ray emission arises from TeV electrons whose population has been enhanced by virtue of shock-cloud collisions. If this is correct, then ASCA has unveiled a second means by which supernova remnants create high energy cosmic rays.

Perhaps the most revolutionary observations will be those of the enigmatic remnant G347.5-0.5. This remnant was first detected as a bright source in the ROSAT all-sky survey and was resolved into a shell-type SNR. The bright NW shell was caught serendipitously in an ASCA galactic plane survey pointing and the spectrum was revealed to be non-thermal (Koyama et al. 1997 PASJ 49, L7). ASCA observations of the entire remnant show that the outer shell and the interior of the remnant also have a non-thermal spectrum (Slane et al. 1998 ApJ in preparation). There is no evidence of any thermal X-ray emission from any part of this remnant; this is a puzzling yet exciting result! The shell-type X-ray morphology, completely non-thermal spectrum, and relatively large size are difficult to explain by the mechanisms observed in other remnants; nevertheless, these data confirm the power of ASCA's imaging and spectral capabilities.

SNR Surveys

Two different types of surveys have been initiated in the last several years to utilize ASCA's unique capabilities to provide moderate-resolution spectra of heavily absorbed objects. First, a followup of ROSAT all-sky survey sources which are believed to be extended and which are coincident with known radio SNR has been started. Three of the first five targets have been detected in the X-rays with ASCA. G337.2-0.7 exhibited booming lines of Si, S, Ar and Ca with supersolar abundances indicating that it is possibly a young, ejecta-dominated remnant. G309.2-0.6 also has strong Si and S lines in addition to a strong Fe line. G7.7-3.7 has a thermal spectrum with nearly solar abundances. The second survey project is aimed at detecting small diameter radio remnants with ASCA. The first three targets have been observed and two of the remnants have been detected. G340.6+0.3 is clearly detected and shows line emission. G328.4-0.2 is detected but shows a complex spectrum with a hard tail. Both of these surveys have produced promising results in their first years and should increase the number of known X-ray remnants.

LMC remnants

ASCA has also made a systematic study of the SNRs in the LMC. One of the early results of this project was the discovery of new ejecta-dominated remnants of Type Ia SNe (Hughes et al. 1995 ApJ 444, L81). This work showed how it was possible to determine the type of the SN explosion from a comparison of the ASCA X-ray spectra of the remnant with the nucleosynthetic yields expected from Type Ia and II SNe. One surprising conclusion was that roughly one-half of the SNRs produced in the LMC within the last ~1500 yrs came from Type Ia SNe. The fraction expected based on extragalactic patrols is more like 10%-20%. Hughes, Hayashi, & Koyama (1998 ApJ accepted) used the X-ray spectral information provided by ASCA in conjunction with a self-consistent nonequilibrium ionization model assuming a Sedov solution for the dynamical evolution, to deduce the ages, ambient interstellar densities, initial explosion energies, and metal abundances for seven middle-aged remnants. For the remnants for which the ionization timescale age and the Sedov dynamical age agree the derived mean explosion energy is 1.1+/-0.5x10⁵¹ ergs, in excellent agreement with the canonical value. For the remnants N63A, N132D, and N49B, the ionization timescale ages are significantly less than the Sedov dynamical ages and the explosion energies are rather large. Hughes, Hayashi, & Koyama suggest that both of these discrepancies can be resolved by invoking a scenario in which the progenitor was a massive star which had blown out a cavity. They have also provided a new and independent determination of the gas phase abundances in the LMC by using the X-ray spectra to determine the abundances of the astrophysically common elements O, Ne, Mg, Si, S, and Fe, to be 0.2-0.4 times solar. The X-ray-derived values are consistent with those from optical studies (e.g. Russell & Dopita 1992 ApJ 384, 508), but the X-ray data provide significantly more accurate measurements of the important species Mg and Si (for which few good emission lines in the optical band exist). Since the ISM contains the integrated sum of material lost by stars in winds and SNe over the galaxy's life, the chemical composition is one of the principle probes of the galaxy's star formation history.

Thermal X-ray Emission

The spectral capability of the SIS has been used to perform detailed modeling of the spectra of young Galactic remnants, and thereby learn new insights about the origin of the X-ray emission. Borkowski et al. (1996, ApJ 466, 866) performed a careful study of the Fe K lines from the core-collapse remnant Cas A, and concluded that its strength is accounted for only if a substantial amount of interstellar dust is present. In contrast, when Hwang, Hughes, & Petre (1998, ApJ 497, 833) performed the same analysis on the spectrum of the Type Ia remnant Tycho, they were able to place severe constraints on the amount of dust present. They also find that multiple emission components, presumably from ejecta and the blast wave, are required to explain the relative strengths of the Fe K and L lines. Hwang & Gotthelf (1997, ApJ 475, 665) produced a set of spatially filtered, narrow band maps of Tycho. Although each map has an overall morphology similar to the broad band map, each shows a set of distinctive features. Overall they find the emission morphology is consistent with a spherical shell, and not with a torus, and that some radial mixing of ejecta has occurred. Vink, Kastra & Bleeker (1997, A&A 328, 628) find a dramatic temperature gradient across RCW 86. They also find a relative lack of line emission which they suggest is the result of an electron distribution with a supra-thermal tail.

Discovery of Young X-ray Pulsars with ASCA

Zum Thema
1E 161348-5055 in RCW 103 PSR J1617-5055 nahe RCW 103	HESS J1616-508 (PSR J1617-5055)

PSR J1617-5055 — P = 69 ms
	Authors: Torii, K.; Kinugasa, K.; Toneri, T.; Asanuma, T.; Tsunemi, H.; Dotani, T.; Mitsuda, K.; Gotthelf, E. V.; Petre, R
	Journal-ref: ApJ 494 (1998) L207 [ ]
	Title: Discovery of a 69 Millisecond X-Ray Pulsar: A Compact Source in the Vicinity of the Supernova Remnant RCW 103
Abstract: We report a rare discovery of a fast (69 ms) pulsar using X-ray data acquired with the Advanced Satellite for Cosmology and Astrophysics (ASCA). The highly significant detection arises from the serendipitous ASCA X-ray source AXS J161730-505505, located near the young Galactic supernova remnant (SNR) RCW 103. The epoch-folded light curve displays a single asymmetric pulse profile with a pulse fraction of ~50%. Spectral fits to the high-energy (i.e., greater than 3.5 keV) source data using a simple absorbed power-law model, assuming the hydrogen column density to the SNR, gives a photon index of gamma =1.6 and an unabsorbed flux of ~6.4x10^-12 erg cm^-2 s^-1 . The extracted source spectrum below 3.5 keV is contaminated by mirror-scattered soft thermal emission from the ~9' diameter RCW 103, whose projected center is located just ~7' away. If the pulsar is associated with the remnant, the implied neutron star velocity is at the high end of the pulsar velocity distribution for the distance estimates to the remnant but still plausible. We suggest that AXS J161730-505505 is likely a young rotation-powered pulsar with a characteristic spin-down age of ~8000 yr. The physical association of the pulsar with RCW 103 and its central source, 1E 161348-5055, remains intriguing.

The study of pulsars in SNRs is critical to our understanding of the evolution of young neutron stars. It allows us to probe these fascinating objects for which only an astrophysical laboratory is available.

For example, ASCA has nearly doubled the number of known Crab-like pulsars with the discovery of a 65 ms pulsar in the young plerionic SNR G11.2-0.3 (Torii et al. 1997 ApJ 489, L145) and the 16 ms pulsar LMC SNR N157B (Marshall et al. 1998 ApJ 499, L179). The latter pulsar is located near the famous 50 ms LMC pulsar and is the most rapidly rotating pulsar associated with a SNR yet discovered. The properties of these pulsars are consistent with the canonical picture of a young pulsar born as a rapidly rotating (~10 ms) NS powered by the spin-down energy of a magnetized-dipole (~ 10¹² G).

Several other pulsars detected by ASCA are considered candidate SNR pulsars, due to their properties and proximity to young SNRs. These include the 69 ms pulsar discovered near the SNR RCW 103 (Torii et al. 1998 ApJ 494, L207) and the X-ray emission from the 63 ms radio pulsar PSR J1105-6107 (Gotthelf & Kaspi 1998 ApJ 497, L29).
The elusive NS candidate in the center of RCW 103, re-discovered by ASCA, may well be a pulsar with unseen pulses due to unfavorable beaming geometry (Fig. 2; Gotthelf et al. 1997 ApJ 497, L29).
The ASCA SIS image of the SNR RCW 103

The ASCA SIS image of the SNR RCW 103 in two spectral bands.
Below 1.5 keV (right) the flux from RCW 103 is predominately from the soft thermal emission of a shocked plasma, typical of a young SNR.
Above 3 keV (left) the intriguing central point source in RCW 103, 1E161348-505, is evident un-obscured by the nebula emission.
Due North of the central source is the serendipitous ASCA point source, the 69 ms pulsar PSR J1617-5055.
The images were produced using data from both SIS cameras and have been exposure corrected and smoothed.

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Authors: Kaspi, V.M.; Crawford, F.; Manchester, R.N.; Lyne, A.G.; Camilo, F.; D'Amico, N.; Gaensler, B.M.

Journal-ref: ApJ 503 (1998) L161 [astro-ph/9806328 ]

Title: The 69 Millisecond Radio Pulsar near the Supernova Remnant RCW 103

Abstract: We report the detection of the radio pulsar counterpart to the 69 ms X-ray pulsar discovered near the supernova remnant RCW 103 (G332.4-0.4).
Our detection confirms that the pulsations arise from a rotation-powered neutron star, which we name PSR J1617-5055.
The observed barycentric period derivative confirms that the pulsar has a characteristic age of only 8 kyr, the sixth youngest of all known pulsars. The unusual apparent youth of the pulsar and its proximity to a young remnant require that an association be considered.
Although the respective ages and distances are consistent within substantial uncertainties, the large inferred pulsar transverse velocity is difficult to explain given the observed pulsar velocity distribution, the absence of evidence for a pulsar wind nebula, and the symmetry of the remnant.
Rather, we argue that the objects are likely superposed on the sky; this is reasonable given the complex area.
Without an association, the question of where is the supernova remnant left behind following the birth of PSR J1617-5055 remains open.
We also discuss a possible association between PSR J1617-5055 and the gamma -ray source 2CG 333+01. Although an association is energetically plausible, it is unlikely given that EGRET did not detect 2CG 333+01.
1. Introduction
RCW 103 (G332.4-0.4) is a young shell supernova remnant with a complicated observational history. An X-ray point source near the center of the remnant was discovered by Tuohy & Garmire (1980), who suggested it is a “radio-quiet” thermally cooling neutron star, the stellar remnant of the supernova explosion.
This was supported by Tuohy et al. (1983) and later by Manchester, D’Amico & Tuohy (1985) and Kaspi et al. (1996) who failed to find evidence for pulsed emission in radio searches. Aoki et al. (1992) detected 69 ms pulsations from the direction of the remnant using Ginga, providing additional evidence for the neutron star identification. However, Ginga’s poor spatial resolution (~ 1° × 2°) precluded any firm association. Recently, Gotthelf, Petre & Hwang (1997) detected the central source using the ASCA X-ray observatory and argued that its spectrum is harder than that of a cooling neutron star. They found no evidence for pulsations, setting upper limits that were inconsistent with the Aoki et al. claim.
This history recently took a dramatic turn with the detection of the 69 ms periodicity in the ASCA data from a point source, AXS J161730-505505, located 7' north of the center of the remnant, outside the 5' remnant radius (Gotthelf, Petre & Hwang 1997, Torii et al. 1998).
Here we report the discovery of pulsed radio emission from this pulsar, which we name PSR J1617-5055.
References Gotthelf, E. V., Petre, R., & Hwang, U. 1997, ApJ 487, L175 Torii, K. et al. 1998, ApJ, 494, L207

Discovery of a new class of slowly rotating NSs

ASCA is also revealing a new class of slowly rotating NS candidates associated with SNR. These have profound implications for the theory of NS evolution. Perhaps the best example is the discovery using ASCA of 12 sec pulsations from the central object in the young SNR Kes 73 (Vasisht & Gotthelf 1997 ApJ 486, L129). Despite numerous previous observations, this pulsar had eluded detection by the Einstein and ROSAT observatories, which lacked the broad spectral band imaging capabilities of ASCA.

ASCA also discovered AX 1845-0258, a highly absorbed 7 sec pulsar in the distant Milky Way (Gotthelf & Vasisht 1998 NA 3, L293) and the 11 sec pulsar located in Scorpio (Sugizaki et al. 1998 PASJ 49, L25). The characteristics of these pulsars are similar to those of the ``anomalous X-ray pulsars'' (Mereghetti & Stella 1995 ApJ 442, L17; van Paradijs et al. 1995 A&A 299, L41), personified by the well studied 7 sec pulsar in CTB 109 (Gregory & Fahlman 1980; Corbet et al. 1995 ApJ 433, 786). The spin periods for these objects lie in the range of 5-12 sec and their ASCA spectra are unusually steep ( 0.6 keV or Gamma > 3) for an rotation- or accretion- powered pulsar. Their luminosities are typically around ~10³⁵ ergs/s and seem to be steady over many years. An accretion origin is unlikely as they lack an observed counterpart, show no indication of binary motion, or display flux variability as is typical of accreting systems.

ASCA is playing a key role in increasing our understanding of the evolution of young NSs. By the detection of new anomalous X-ray pulsars, and subsequent monitoring of their pulse and flux histories, ASCA has shown that the standard paradigms of young pulsar evolution may no longer be valid. For example, a follow-up ASCA observation of Kes 73 confirms the remarkable spin-down rate of its pulsar and that the measured luminosity cannot be simply powered by radiative losses due to spin-down (Gotthelf et al. 1998 in preparation). The inferred magnetic field for a rotating magnetic dipole is well above the quantum limit of 4x10¹⁴ G. The Kes 73 pulsar is likely the first example of a ``magnetar'' (Thompson & Duncan 1995 MNRAS 275, 255), a NS with an enormous magnetic field. The pulsar was likely spun down rapidly by magnetic field decay or possibly born as a slow pulsar. In either case, the ASCA data require us to consider alternative NS evolution scenarios in direct competition with the standard theory.

PSR J1811-1925 — P = 64.7 ms — t_c = 24 kyr

Authors: Torii, K., Tsunemi, H., Dotani, T., & Mitsuda, K.

Journal-ref: ApJ 489 (1997) L145 [ ]

Title: Discovery of a 65 Millisecond Pulsar in the Supernova Remnant G11.2-0.3 with ASCA

Abstract: We report here the discovery of a 65 ms X-ray pulsar in the supernova remnant G11.2--0.3 with ASCA.
The object is known to be a composite supernova remnant that shows both an extended shell component and a compact plerionic component. We have detected a sinusoidal pulsation with an amplitude of ~19% in the hard X-ray band (3--10 keV).
The object has been proposed to be the historical remnant of the supernova of A.D. 386, which makes it the second candidate for the secure association of a pulsar and a historical supernova after the Crab Nebula and its pulsar.

Crab-like supernova remnants (SNRs) (plerions; Weiler & Panagia 1978), distinguished by their center-filled morphology, flat radio spectrum, and high polarization, form a small and heterogeneous class of objects in a number of samples of SNRs.
Although the youngest (»1000 yr old) Crab-like SNRs will evolve into middle-aged SNRs for which both shell and synchrotron nebulae will be observable, the evolutionary sequence is not yet firmly established, mainly because of the limited number of samples.
Except for the Crab pulsar, only the 150 ms pulsar PSR 1509-58 (Seward & Harnden 1982) in the SNR MSH 15-52 and the 50 ms pulsar PSR 0540-69 (Seward, Harnden, & Helfand 1984) in the Large Magellanic Cloud are young pulsars whose characteristic age is less than 2000 yr.
In X-rays, some objects such as 3C 58 show center-filled morphologies and nonthermal power-law spectra (Torii et al. 1997), while extended shell and thin thermal spectra along with a synchrotron component can be observed for other objects, such as G11.220.3. The latter may be naturally interpreted as resulting from some evolution: shock heating of the circumstellar and interstellar medium at the outgoing blast wave and shock heating of the ejecta at the reverse shock, as a result of mass pickup from the surrounding material.
The source of G11.220.3 was proposed as a possible remnant of the supernova of a.d. 386, one of only eight historical supernovae from the last 2000 yr (Clark & Stephenson 1977).

Zum Thema

1E 161348-5055 in RCW 103 (XMM)

PSR J1811-1925 in G11.2-0.3 (Chandra) (INTEGRAL)

Literatur zu ""
Torii, K., Tsunemi, H., Dotani, T., & Mitsuda, K.	1997	ApJ 489, L145	"Discovery of a 65 Millisecond Pulsar in the Supernova Remnant G11.2-0.3 with ASCA"
Torii, K.; Kinugasa, K.; Toneri, T.; et al.	1998	ApJ 494, L207	"Discovery of a 69 Millisecond X-Ray Pulsar in the Vicinity of the SNR RCW 103"
Kaspi, V.M.; Crawford, F.; Manchester, R.N.; et al.	1998	ApJ 503, L161	"The 69 Millisecond Radio Pulsar near the Supernova Remnant RCW 103"
Sambruna, R.M.; Ghisellini, G.; Hooper, E; et al.	1999	ApJ 515, 140	"Observations of the BL Lacertae Objects 1749+096 and 2200+420 (BL Lac)"

H. Heintzmann

( Eintrag vom 17.5.2008)

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