A Radio Survey of Type Ib and Ic Supernovae: Searching for Engine‐driven Supernovae

The association of gamma-ray bursts (GRBs) and core-collapse supernovae (SNe)of Type Ib and Ic was motivated by the detection of SN 1998bw in the error boxof GRB 980425 and the now-secure identification of a SN 1998bw-like event inthe cosmological GRB 030329. The bright radio emission from SN 1998bw indicatedthat it possessed some of the unique attributes expected of GRBs, namely alarge reservoir of energy in (mildly) relativistic ejecta and variable energyinput. The two popular scenarios for the origin of SN 1998bw are a typicalcosmological burst observed off-axis or a member of a new distinct class ofsupernova explosions (gSNe). In the former, about 0.5% of local Type Ib/c SNeare expected to be similar to SN1998bw; for the latter no such constraintexists. Motivated thus, we began a systematic program of radio observations ofmost reported Type Ib/c SNe accessible to the Very Large Array. Of the 33 SNeobserved from late 1999 to the end of 2002 at most one is as bright as SN1998bw. From this we conclude that the incidence of such events is <3%.Furthermore, analysis of the radio emission indicates that none of the observedSNe exhibit clear engine signatures. Finally, a comparison of the SN radioemission to that of GRB afterglows indicates that none of the SNe could haveresulted from a typical GRB, independent of the initial jet orientation. Thus,while the nature of SN 1998bw remains an open question, there appears to be aclear dichotomy between the majority of hydrodynamic and engine-drivenexplosions.Comment: 18 pages, 6 figures, 3 table