Dust Scattering X-ray Expanding Rings around Gamma-ray Bursts

— Scattering by dust grains in our Galaxy can produce X–ray halos, visible as expanding rings, around GRBs. This has been observed in three GRBs to date, allowing to derive accurate distances for the dust clouds as well as some constraints on the prompt GRB X–ray emission that was not directly observed. We developed a new analysis method to study dust scattering expanding rings and have applied it to all the XMM-Newton and Swift/XRT follow-up observations of GRBs. Soon after the discovery of celestial X–ray sources it was realized that scattering from interstellar dust grains could lead to the formation of detectable X–ray halos surrounding the source images [1]. The usefulness of this phenomenon to study the properties of the dust was pointed out by several authors, but observations had to await the development of imaging X–ray telescopes. The first dust halos were discovered with the Einstein Observatory around bright galactic sources at the beginning of the eighties [2, 3]. Currently, the modelling of the energy-dependent radial profiles of X–ray halos is a well established tool in the study of the interstellar dust. Owing to the longer path lengths of scattered compared to unscattered photons, variability in the source can lead to time-dependent changes in the halo radial profile. If the spatial distribution of the dust along the line of sight is known, it is possible to constrain the distance of variable sources by studying their halos [4]. This method was also proposed [5] as a way to distinguish between the galactic and cosmological origin of GRBs, but it could not be applied due to the lack of sensitive imaging detectors. Only recently dust halos have been detected around three bursts: GRB 031203 [6], GRB 050724 [7] and GRB 050713A [8]. Due to the short duration of the bursts and the relatively small thickness of the dust layers, such halos appear as expanding rings. Since the scattering dust is in our Galaxy, at a distance d s much smaller than that of the GRB, the ring radius Θ and the time delay ∆t = t–t GRB are simply related by ∆t = (d s /2c) Θ 2. Thus it is possible to accurately measure the dust distance d s by fitting the time expansion of the ring.