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

GRB031203 was a very low apparent luminosity gamma-ray burst (GRB). It wasalso the first GRB with a dust-scattered X-ray halo. The observation of thehalo allowed us to infer the presence of a large soft X-ray fluence in thetotal burst output. It has, however, also been claimed that GRB031203 wasintrinsically sub-energetic, representative of a class of spectrally hard,low-energy bursts quite different from other GRBs. Reanalysis of the availabledata confirms our original finding that GRB031203 had a very large soft X-raycomponent, the time of which can be constrained to within a few minutes afterthe burst, implying that while GRB031203 did indeed have a very low apparentluminosity, it was also very soft. Notions propagated in the literatureregarding the uncertainties in the determination of the soft X-ray fluence fromthe halo data and on the available constraints from the hard X-ray data areaddressed: the properties of the scattering dust along the line of sight (grainsizes, precise location and the geometry) are determined directly from the highquality X-ray data so that there is little uncertainty about the scatterer;constraints on the X-ray lightcurve from the Integral spacecraft at the time ofthe soft X-ray blast are not complete because of a slew in the spacecraftpointing shortly after the burst. Claims that GRB031203 was intrinsicallyunder-energetic and that it represents a deviation from the luminosity-peakenergy relation do not appear to be substantiated by the data, regardless ofwhether the soft X-ray component is declared part of the prompt emission or theafterglow. We conclude that the difference between the soft and hard X-rayspectra from XMM-Newton and Integral indicate that a second soft pulse probablyoccurred in this burst as has been observed in other GRBs, notably GRB050502B.