Dust-scattered X-ray halos around gamma-ray bursts: GRB 031203 revisited and the new case of GRB 050713A

Scattering by interstellar dust grains can produce time variable X-ray halos around gamma-ray bursts (GRB). In particular, an X-ray expanding ring is expected when a short pulse of X-ray radiation is scattered by a narrow layer of dust in our Galaxy. We present a new method for detecting and analyzing dust-scattering expanding rings around gamma-ray bursts, using as an example the XMM-Newton data of GRB 031203. Consistent with previous reports, we found that the two expanding rings observed in this burst are due to dust unevenly distributed in two layers at distances of 870 and 1384 pc, with the more distant one responsible for 70% of the total optical depth. Our modelling of the rings indicates that the prompt X-ray emission of GRB 031203 was consistent with a power law spectrum with photon index Γ=2.1±0.2 and 1-2 keV fluence of (3.6±0.2)×10^-7 erg cm^-2. Thanks to the sensitivity of our technique, we discovered an expanding ring around another burst recently observed with XMM-Newton, GRB 050713A. In this case the dust layer is located at a distance of 364 pc, and we derived a GRB fluence of (1.2±0.3)×10^-7 erg cm^-2 (1-2 keV). A search for similar halos in twelve other bursts observed with XMM-Newton gave negative results.