2004ApJ...605L..97W

We show that the recent identification of X-ray emission from SN 1998bw 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 ergs and constrain the initial shell velocity βc and normalized wind mass-loss rate, m{dot}≡(M{dot}/10^–5 M_☉/yr)/(v_w_/10³ km/s), to satisfy β³m{dot}~10^–1.5. The inferred energy is consistent with energy estimates based on radio observations provided m{dot}~0.04, in which case radio observations imply β~0.8, consistent with the X-ray constraint β³m{dot}~10^–1.5. While X-ray observations allow us to determine the parameters characterizing the preexplosion wind and the mildly relativistic shell ejected by SN 1998bw, they do not provide evidence for existence of an off-axis ``standard'' gamma-ray burst (GRB) jet associated with SN 1998bw, which may have produced GRB 980425. The lack of observational signatures, typically expected to be produced by such an off-axis jet on a 1 yr timescale, may be due to the low m{dot}≲0.1, which implies that an off-axis jet will become observable only on a ≳10 yr timescale.