The Nature of GRB 980425 and the Search for Off‐Axis Gamma‐Ray Burst Signatures in Nearby Type Ib/c Supernova Emission

The identification of type Ib/c supernovae as GRB progenitors is motivated bythe association of GRB980425 with SN1998bw and of GRB030329 with SN2003dh.While the gamma-ray luminosity of GRB030329 was typical to cosmological GRBs,the luminosity of the nearby (40 Mpc) GRB980425 was ~5 orders of magnitudelower. The large luminosity difference is commonly explained by hypothesizingthat either SNe Ib/c produce two different classes of GRBs, or that GRB980425was a typical cosmological GRB jet viewed off-axis. In the latter scenario,strong radio emission, ~10^{30} erg/s Hz, is expected at ~1 yr delay due to jetdeceleration to sub-relativistic speed, as observed from GRB970508. The radioluminosity of SN1998bw was 3 orders of magnitude lower than this value. We showthat the low radio flux may be consistent with the off-axis jet interpretation,if the density of the wind surrounding the progenitor is lower than typicallyexpected, \dot{m}=(\dot{M}/10^{-5} M_sun/yr)/(v/10^3 km/s)~0.1 instead of\dot{m}>=1. The lower value of \dot{m} is consistent with the observed radioemission from the supernova shock driven into the wind. This interpretationpredicts transition to sub-relativistic expansion at ~10 yr delay, with current\~1 mJy 10GHz flux and m_V~23 optical flux, and with ~10 mas angular sourcesize. It also implies that in order to search for the signature of off-axisGRBs associated with nearby Ib/c supernovae, follow up observations should becarried on a multi-yr time scale.Comment: 13 pages, 1 figure; 2 references added; ApJ, in pres