A very low luminosity X-ray flash:: XMM-NEWTON observations of GRB 031203

GRB 031203 was observed by XMM-Newton twice, first with an observation beginning 6 hr after the burst and again after 3 days. The afterglow had average 0.2-10.0 keV fluxes for the first and second observations of (4.2 +/- 0.1) x 10(-13) and (1.8 +/- 0.1) x 10-(13) ergs cm(-2) s(-1), respectively, decaying very slowly according to a power law with an index of -0.55 +/- 0.05. The prompt soft X-ray flux, inferred from a detection of the dust echo of the prompt emission, strongly implies that this burst is very soft and should be classified as an X-ray flash (XRF) and further, implies a steep temporal slope (less than or similar to - 1.7) between the prompt and afterglow phases or in the early afterglow, very different from the later afterglow decay slope. A power law (Gamma = 1.90 +/- 0.05) with absorption at a level consistent with the Galactic foreground absorption fits the afterglow spectrum well. A bright low- redshift (z = 0.105) galaxy lies within 0.5 of the X-ray position and is likely to be the gamma-ray burst (GRB) host. At this redshift, GRB 031203 is the closest GRB or XRF known after GRB 980425. It has a very low equivalent isotropic gamma-ray energy in the burst (similar to3 x 10(49) ergs) and X-ray luminosity in the afterglow (9 x 10(42) ergs s(-1) at 10 hr), 3-4 orders of magnitude less than typical bursts, though higher than either the faint XRF 020903 or GRB 980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB 980425, indicating that GRB 031203 may be representative of low-luminosity bursts.