Does the detection of X-ray emission from SN1998bw support its association with GRB980425?

Abstract

We show that the recent identification of X-ray emission from SN1998bw is naturally explained as synchrotron emission from a shock driven into the wind surrounding the progenitor by a mildly relativistic shell ejected by the supernova, the existence of which was inferred earlier from radio observations. X-ray observations imply a shell energy E~10^{49.7}erg, and constrain the initial shell velocity \beta*c and normalized wind mass loss rate, \dot{m}=(\dot{M}/10^{-5}M_sun/yr)/(v_w/10^3 km/s), to satisfy \beta^3*\dot{m}~10^{-1.5}. The inferred energy is consistent with energy estimates based on radio observations provided \dot{m}~0.04, in which case radio observations imply \beta~0.8, consistent with the X-ray constraint \beta^3*\dot{m}~10^{-1.5}. While X-ray observations allow to determine the parameters characterizing the pre-explosion wind and the mildly relativistic shell ejected by SN1998bw, they do not provide evidence for existence of an off-axis "standard" GRB jet associated with SN1998bw, that may have produced GRB980425. However, as recently pointed out in (astro-ph/0310320), the lack of observational signatures typically expected to be produced by such an off-axis jet on a 1yr time scale, may be due to a low \dot{m}<0.1, which implies that an off-axis jet will become observable only on >10yr time scale.