The discovery of the radio afterglow from the optically dim gamma-ray burst of 1998 March 29

We report on the discovery of a variable radio source, VLA J070238.0+385044, associated with the proposed X-ray counterpart, 1SAX J0702.6+3850, of GRB 980329. The source was monitored from 1 day after the burst to 1 month later at centimeter wavelengths (1.4, 4.9, 8.3, and 15 GHz; Very Large Array) and in the 3 mm band (90 GHz; Owen Valley Radio Observatory). Based on its variability, compactness, and spectrum, we identify VLA J070238.0+385044 as the afterglow from GRB 980329. We interpret the rapid flux density variations as interstellar scintillation, and we interpret the sharp turnover in the radio spectrum below 13 GHz as arising from synchrotron self-absorption. This suggests that the angular diameter of the fireball from GRB 980329 was on the order of a few microarcseconds in the first 2 weeks. The absence of a readily detectable afterglow in the optical, but clear detections in both the radio and infrared, can be understood as the result of extinction by dust. In this context, it is interesting to note that half of all well-localized gamma-ray bursts have no optical counterpart. Based on our study of GRB 980329, we suggest that the progenitors of a sizable fraction of gamma-ray bursts are associated with regions of moderate to high gas density.