ar X iv : a st ro - p h / 03 04 52 1 v 1 2 9 A pr 2 00 3 XMM - NEWTON OBSERVATIONS OF THE FIELD OF GAMMA - RAY BURST 980425

The error box of GRB980425 has been observed by XMM-Newton in March 2002, with the aim of measuring the late epoch X-ray emission of the supernova 1998bw and of clarifying its supposed association with the GRB itself. We present here the preliminary results obtained with the EPIC PN camera. Our observations confirm the association between SN 1998bw and GRB980425. The EPIC PN measurement of the SN 1998bw flux is significantly below the extrapolation of the power-law temporal trend fitted to the BeppoSAX points and implies a faster temporal decay. We propose different physical interpretations of the SN X-ray light curve, according to whether it is produced by one or more radiation components. INTRODUCTION The hypothesis that supernovae are the progenitors of Gamma-Ray Bursts (GRB) dates back to the epoch of first GRB discovery (Colgate 1974) and has received support in recent years from the detection of supernova features in the optical afterglows of GRBs. These are re-brightenings at rest-frame intervals of 10-15 days after the GRB (e. The most tempting hint of association between GRBs and SNe is obviously the similarity of the intrinsic energy of these phenomena, when collimation and beaming are taken into account in GRBs (e.g., Frail et al. 2001). While these circumstances represent only possible evidences of a GRB-SN connection, on two occasions a clear association has been found. GRB980425 and SN 1998bw have been detected within very tight temporal and angular limits: they exploded simultaneously (with an uncertainty of ±1 day) and with a maximum separation in the sky of 8 ′ (Galama et al. 1998). More recently, prominent spectral features similar to those detected for SN 1998bw have been detected in the afterglow of GRB030329 (Stanek et al. 2003; Hjorth et al. 2003). To date, no other such compelling case of GRB-SN association has been detected, and SN 1998bw, whose brightness and kinematic conditions were exceptional, is considered a typical " hypernova " , the powerful SN explosion speculated to be at the