XMM-Newton observation of a dust echo and X-ray flash in GRB 031203 Original Research Article

We present XMM-Newton observations of the X-ray afterglow of GRB 031203, approximately 6 h after the initial burst detection by Integral. At z = 0.105, GRB 031203 is one of the least luminous γ-ray bursts, with an equivalent isotropic γ-ray energy of ∼3 × 1049 ergs. The XMM-Newton images reveal an expanding X-ray halo around GRB 031203, in the form of two concentric rings centred on the afterglow position. The rings expand radially outwards with time as t0.5, consistent with small angle scattering of burst X-rays off dust in our own galaxy. The total X-ray fluence derived from the dust echo of 2 × 10−6 ergs cm−2 keV−1 (at 1 keV) implies that a strong X-ray flash is likely to have occurred from GRB 031203. The estimate of the X-ray fluence is robust to the determination of the total dust extinction in the scatterer. However the classification of GRB 031203 as an X-ray Flash appears at odds with the prompt (20–400 keV) Integral emission from this burst, which has a hard photon index of Γ = 1.6 and a peak energy of >190 keV. Thus either GRB 031203 is highly unusual and has a double peaked spectral energy distribution, or a second flash occurred later in soft X-rays, at least equal in total energy to that of the initial γ-ray burst.