Supernova 1998bw in ESO 184-G82

Abstract

S. R. Kulkarni and J. S. Bloom, California Institute of Technology; D. A. Frail, National Radio Astronomy Observatory; and R. Ekers, M. Wieringa, R. Wark, and J. L. Higdon, Australian Telescope National Facility, report: ``Within the localization of GRB 980425 (IAUC 6884), Galama et al. (IAUC 6895) reported a possible supernova candidate for which Wieringa et al. (IAUC 6896) saw a brightening radio source. The object appears to be a young supernova, possibly of type-Ib (IAUC 6901), which exploded on or around Apr. 24, the epoch of GRB 980425. For an assumed expansion speed of 20 000 km/s and a distance of 44 Mpc to the host galaxy of the supernova (from the redshift given on IAUC 6896), we derive a brightness temperature of 3 x 10E14 K from the observed 39 mJy at 6 cm on May 5 (IAUC 6896). This is in excess of the usual Compton limit of 10E12 K. Despite this, no x-ray emission is seen (GCN 69). Thus we are forced to invoke a relativistic expansion speed that results in a larger source size and correspondingly smaller brightness temperature. We suggest that the radio emission arises in a relativistic shock and the optical emission in a standard low-velocity shock. We urge observers to carry out higher- frequency radio observations and infrared observations, as the radio spectrum may rapidly evolve. The model predicts that the radio source should not exhibit diffractive scintillation. VLBI observations would be of great value.’’ Visual magnitude estimates by B. Monard, Pretoria, South Africa: May 10.14 UT, 13.5; 13.15, 13.5.

Publication
IAU Circulars