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 (SNe Ib/c) as gamma-ray burst (GRB) progenitors is motivated by the association of GRB 980425 with SN 1998bw and of GRB 030329 with SN 2003dh. While the gamma-ray luminosity of GRB 030329 was typical to cosmological GRBs, the luminosity of the nearby (40 Mpc) GRB 980425 was ~5 orders of magnitude lower. The large luminosity difference is commonly explained by hypothesizing either that SNe Ib/c produce two different classes of GRBs or that GRB 980425 was a typical cosmological GRB jet viewed off-axis. In the latter scenario, strong radio emission, L_ν ~ 10³⁰ν ergs s^-1 Hz^-1, is expected at ~1 yr delay as a result of jet deceleration to subrelativistic speed, as observed from GRB 970508. The radio luminosity of SN 1998bw was 3 orders of magnitude lower than this value. We show that 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 typically expected, ≡ (/10^-5 M_☉ yr^-1)/(v_w/10³ km s^-1) ≃ 0.1 instead of ≳ 1. The lower value of is consistent with the observed radio emission from the SN shock driven into the wind. This interpretation predicts transition to subrelativistic expansion at ~10 yr delay, with current ≈1 mJy 10 GHz flux and m_V ≈ 23 optical flux, and with ≈10 mas angular source size. It also implies that in order to search for the signature of off-axis GRBs associated with nearby SNe Ib/c, follow-up observations should be carried out on a multiyear timescale.