Abstract
GRB031203 was a very low apparent luminosity gamma-ray burst (GRB). 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, however, also been claimed that GRB031203 was intrinsically sub-energetic, representative of a class of spectrally hard, low-energy bursts quite different from other GRBs. Reanalysis of the available data confirms our original finding that GRB031203 had a very large soft X-ray component, the time of which can be constrained to within a few minutes after the burst, implying that while GRB031203 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 the 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 lightcurve 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 GRB031203 was intrinsically under-energetic 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 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 GRB050502B.
Highlights
While -ray bursts (GRBs) are no longer as enigmatic as they were even a few years ago, the ability to use GRBs as a serious tool in cosmology and an understanding of their basic mechanisms still elude us
Given the size of these uncertainties and the fact that they are based on direct observation along this line of sight, we are forced to conclude that it is unlikely that the 1 keV fluence of the blast could have been substantially different
The analysis of the halo expansion was improved by allowing the time of the X-ray blast to be a free-fit parameter; Gaussian profiles were fit to the halo at different times to improve the radial size estimates, and the model fit was integrated over each time bin
Summary
While -ray bursts (GRBs) are no longer as enigmatic as they were even a few years ago, the ability to use GRBs as a serious tool in cosmology and an understanding of their basic mechanisms still elude us. Apart from GRB 031203, has extended this relation to very low luminosities and peak energies (i.e., the low-luminosity XRF 020903; Sakamoto et al 2004). It has been suggested that the total energy in -rays from a GRB is nearly constant at $1051 ergs ( Frail et al 2001), by correcting for the opening angle of the putative GRB jet. The determination of the opening angle is dependent on the time of the break in the light curve This measure has proved difficult to use or understand because of (1) the difficulty in deciding the jet break time in light curves that are often sparsely sampled, contaminated by supernova features, and subject to fluctuations caused by density variations, and (2) the (few) cases where the total apparent energy release (equivalent isotropic) is well below this value
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