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Register a new userThe dust-scattered X-ray halo around Swift GRB 050724
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S. Vaughan
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This paper discusses the X-ray halo around the Swift gamma-ray burst GRB 050724 (z=0.258), detected by the Swift X-Ray Telescope. The halo, which forms a ring around the fading X-ray source, expands to a radius of 200 within 8 ks of the burst exactly as expected for small-angle X-ray scattering by Galactic dust along the line of sight to a cosmologically distant GRB. The expansion curve and radial profile of the halo constrain the scattering dust to be concentrated at a distance of D = 139 +/- 9 pc (from Earth) in a cloud/sheet of thickness delta-D < 22 pc. The halo was observed only out to scattering angles of 200, for which the scattering is dominated by the largest grains, with a maximum size estimated to be a_max ~ 0.4-0.5 um. The scattering-to-extinction ratio was estimated to be tau_scat/A_V > 0.022; this is a lower limit to the true value because contribution from smaller grains, which scatter to larger angles, was not directly observed. The line-of-sight to the GRB passes close to the Ophiuchus molecular cloud complex, which provides a plausible site for the scattering dust.
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Two new expanding X-ray rings were detected by the Swift XRT instrument during early follow-up observations of GRB 061019 and GRB 070129, increasing to 5 the number of dust scattering X-ray halos observed around GRBs. Although these two halos were particularly faint, a sensitive analysis can be performed that optimizes the method originally developed by Tiengo & Mereghetti (2006) to analyze dust scattering rings observed with XMM-Newton for the Swift satellite. In the case of GRB 061019, a known giant molecular cloud is identified as the one responsible for the scattering process, and its distance is accurately measured (d=940$pm$40 pc) through the dynamics of the expanding ring. In the second case, XRT observed both the main peak of the prompt emission of GRB 070129 and the scattering halo, but the small number of detected halo photons prevents us from distinguish between different dust models.
X-ray photons coming from an X-ray point source not only arrive at the detector directly, but also can be strongly forward-scattered by the interstellar dust along the line of sight (LOS), leading to a detectable diffuse halo around the X-ray point source. The geometry of small angle X-ray scattering is straightforward, namely, the scattered photons travel longer paths and thus arrive later than the unscattered ones; thus the delay time of X-ray scattered halo photons can reveal information of the distances of the interstellar dust and the point source. Here we present a study of the X-ray scattered around IGR J17544-2619, which is one of the so-called supergiant fast X-ray transients. IGR J17544-2619 underwent a striking outburst when observed with Chandra on 2004 July 3, providing a near delta-function lightcurve. We find that the X-ray scattered halo around IGR J17544-2619 is produced by two interstellar dust clouds along the LOS. The one which is closer to the observer gives the X-ray scattered at larger observational angles; whereas the farther one, which is in the vicinity of the point source, explains the halo with a smaller angular size. By comparing the observational angle of the scattered halo photons with that predicted by different dust grain models, we are able to determine the normalized dust distance. With the delay times of the scattered halo photons, we can determine the point source distance, given a dust grain model. Alternatively we can discriminate between the dust grain models, given the point source distance.
The exquisite angular resolution available with Chandra should allow precision measurements of faint diffuse emission surrounding bright sources, such as the X-ray scattering halos created by interstellar dust. However, the ACIS CCDs suffer from pileup when observing bright sources, and this creates difficulties when trying to extract the scattered halo near the source. The initial study of the X-ray halo around GX13+1 using only the ACIS-I detector done by Smith, Edgar & Shafer (2002) suffered from a lack of sensitivity within 50 of the source, limiting what conclusions could be drawn. To address this problem, observations of GX13+1 were obtained with the Chandra HRC-I and simultaneously with the RXTE PCA. Combined with the existing ACIS-I data, this allowed measurements of the X-ray halo between 2-1000. After considering a range of dust models, each assumed to be smoothly distributed with or without a dense cloud along the line of sight, the results show that there is no evidence in this data for a dense cloud near the source, as suggested by Xiang et al. (2005). Finally, although no model leads to formally acceptable results, the Weingartner & Draine (2001) and nearly all of the composite grain models from Zubko, Dwek & Arendt (2004) give poor fits.
This paper discusses Swift observations of the gamma-ray burst GRB 050315 (z=1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t^-5) for ~200 s and several breaks. However, both the prompt hard X-ray/gamma-ray emission (observed by the BAT) and the first ~ 300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt gamma-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray rich GRB. The prompt (BAT) spectrum was a steep down to 15 keV, and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline the X-ray afterglow did not fade for ~1.2*10^4 s, after which time it decayed with a temporal index of alpha ~ 0.7, followed by a second break at ~2.5*10^5 s to a slope of alpha ~ 2. The apparent `plateau in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break then the jet opening angle was theta_0 ~ 5 deg, and implying E_gamma > 10^50 erg.
The unique capability of the Swift satellite to perform a prompt and autonomous slew to a newly detected Gamma-Ray Burst (GRB) has yielded the discovery of interesting new properties of GRB X-ray afterglows, such as the steep early lightcurve decay and the frequent presence of flares detected up to a few hours after the GRB trigger. We present observations of GRB 050607, the fourth case of a GRB discovered by Swift with flares superimposed on the overall fading X-ray afterglow. The flares of GRB 050607 were not symmetric as in previously reported cases, showing a very steep rise and a shallower decay, similar to the Fast Rise, Exponential Decay that are frequently observed in the gamma-ray prompt emission. The brighter flare had a flux increase by a factor of approximately 25,peaking for 30 seconds at a count rate of approximately 30 counts s-1, and it presented hints of addition short time scale activity during the decay phase. There is evidence of spectral evolution during the flares. In particular, at the onset of the flares the observed emission was harder, with a gradual softening as each flare decayed. The very short time scale and the spectral variability during the flaring activity are indicators of possible extended periods of energy emission by the GRB central engine. The flares were followed by a phase of shallow decay, during which the forward shock was being refreshed by a long-lived central engine or by shells of lower Lorentz factors, and by a steepening after approximately 12 ks to a decay slope considered typical of X-ray afterglows.
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