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

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 × 10 ⁴⁹ 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 t^0.5, 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.