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

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  • 13 January 2004
Abstract
GRB031203 was observed by XMM-Newton twice, first with an observation beginning six hours after the burst, and again after three days. The afterglow had an average 0.2-10.0keV flux for the first observation of 4.1e-13erg/cm^2/s, decaying exceptionally slowly according to a power-law with an index of -0.4. The average flux of the afterglow during the second observation was 1.2e-13erg/cm^2/s, implying a steepening in the lightcurve to a slope of -0.9 about one day after the burst. 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 (<~-1.7) between the prompt and afterglow phases or in the early afterglow, inconsistent with the later afterglow decay slopes. A power-law (Gamma=1.90) 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 GRB host. At this redshift, GRB 031203 is the closest GRB or XRF known after GRB980425. It has a very low equivalent isotropic gamma-ray energy in the burst (~3e49erg) and X-ray luminosity in the afterglow (9e42erg/s at ten hours), three to four orders of magnitude less than typical bursts, though higher than either the faint XRF020903 or GRB980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB980425, indicating that GRB031203 may be representative of low-luminosity bursts.

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