1 Introduction

GRB 980425 was located inside the BeppoSAX Wide Field Camera (WFC) error circle, along with two weak X-ray sources, one of which faded away 2-3 days after the gamma-ray burst, as observed by the BeppoSAX Narrow Field Instruments ([Pian et al. 1998a]). An unusual supernova (SN), apparently of type Ic, was also detected within the WFC error circle ([Galama et al. 1998]). This supernova, SN1998bw, was not positionally coincident with the fading X-ray source - a candidate for the typical GRB X-ray afterglow.

A radio source, detected by [Wieringa (1998)] three days after the burst, brightened to become approximately two orders of magnitude more luminous than any previously studied radio source associated with a type Ic SN and was inferred to be expanding relativistically ([Kulkarni et al. 1998]). Modeling of the optical light curve implies an explosion energy of 2-5 10⁵² ergs, and core collapse within about 1 day of GRB 980425 ([Iwamoto et al. 1998]). These observations and deductions have led many investigators to examine the possibility that GRB 980425 and onset of SN 1998bw are one and the same event: The combined positional and temporal chance probability has been estimated as 10^-4 ([Galama et al. 1998]), and the explosion dynamics and energetics could possibly power an observable gamma-ray at the distance to SN 1998bw (38 Mpc). This interpretation requires that the fading X-ray source is not associated with the GRB.

We have further examined the hypothesis discussed by [Bloom et al. (1998)] that GRBs with time profiles resembling that of GRB 980425 (BATSE trigger 6707) may be associated in general with SNe type Ib or Ic. Bloom et al. detail the criteria for inclusion in a supernova-GRB (S-GRB) class, including: prompt radio emission with high brightness temperature; no long-lived X-ray afterglow; broad line emission and high optical luminosity; and a gamma-ray profile consisting of a single, relatively smooth, broad pulse. The single shock expected in a SN core collapse would be expected to give rise to a single gamma-ray pulse.

  

Figure 1: Time profiles in the energy range 50-300 keV for the eight bursts categorized as single-pulse, NHE events. The abscissa range for all plots is 120 s. BATSE trigger numbers are indicated