Time Variable Xray Halo

Small-angle scattering of X-rays by dust grains results in an X-ray "halo" around a distant X-ray source where the radial intensity distribution depends on the dust properties. Because of the longer path length of scattered X-rays, photons in the halo arrive later than unscattered source photons. If the X-ray source emits a timevariable signal with a unique profile which can be identified in the halo emission, then a distance estimate of the source [91] and/or the location and properties of the dust can be derived [47].

While X-ray halos have been detected from bright galactic X-ray binaries since the Einstein era, a detection of a X-ray halo due to the emission from a GRB has been achieved first for GRB 031203 using the XMM-Newton satellite [95]. GRB 031203 was located at a galactic latitude of only BII = —4°6 behind an optical extinction of E (B — V) = 1.0, and no optical/IR afterglow was found. The candidate host galaxy at the position of the radio afterglow is at z = 0.105, making this GRB the second-closest GRB. The XMM-Newton observation started about 6 h after the GRB and lasted 58 ks. Splitting the observation into contiguous time intervals revealed a X-ray halo with two distinct rings with increasing radius (Fig. 24.10). The expansion of both rings was proportional to t1/2, consistent with small-angle scattering. A second XMM-Newton observation at 3 days after the GRB did not detect the halo anymore. The temporal profile of the evolution of the two rings implies the dust slabs to be at distances of 1388 ± 32 and 882 ± 20 pc, respectively [95]. The halo brightness implies an initial soft X-ray pulse consistent with the X-ray intensity during the GRB emission, while the afterglow contributes only about 2% to the halo emission.

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