The Soft X-Ray Blast in the Apparently Subluminous GRB 031203

Abstract

GRB 031203 was a very low apparent luminosity γ-ray burst (GRB). Coincidentally, it was also the first GRB with a dust-scattered X-ray halo. The observation of the halo allowed us to infer the presence of a large soft X-ray fluence in the total burst output. It has also been claimed, however, that GRB 031203 was intrinsically subenergetic, representative of a class of spectrally hard, low-energy bursts quite different from other GRBs. A careful reanalysis of the available data confirms our original finding that GRB 031203 had a very large soft X-ray component, the time of which can be constrained to within a few minutes after the burst, strongly suggesting that while GRB 031203 did indeed have a very low apparent luminosity, it was also very soft. Notions propagated in the literature regarding the uncertainties in the determination of the soft X-ray fluence from the halo data and on the available constraints from the hard X-ray data are addressed: the properties of the scattering dust along the line of sight (grain sizes, precise location, and geometry) are determined directly from the high-quality X-ray data so that there is little uncertainty about the scatterer; constraints on the X-ray light curve from the INTEGRAL spacecraft at the time of the soft X-ray blast are not complete because of a slew in the spacecraft pointing shortly after the burst. Claims that GRB 031203 was intrinsically underenergetic and that it represents a deviation from the luminosity-peak-energy relation do not appear to be substantiated by the data, regardless of whether the soft X-ray component is (arbitrarily) declared part of the prompt emission or the afterglow. We conclude that the difference between the soft and hard X-ray spectra from XMM-Newton and INTEGRAL indicate that a second soft pulse probably occurred in this burst, as has been observed in other GRBs, notably GRB 050502B.