Late-time Radio Observations of 74 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-ray Bursts

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Abstract
We present late-time radio observations of 74 local Type Ibc supernovae, including six events with broad optical absorption lines (``hypernovae''). None of these objects show evidence for radio emission attributable to off-axis gamma-ray burst jets spreading into our line-of-sight. Comparison with our afterglow models reveals the following conclusions: (1) Less than ~ 1.4% of Type Ibc supernovae are associated with typical gamma-ray bursts initially directed away from our line-of-sight; this places an empirical constraint on the GRB beaming factor of f_b^{-1} < 1300 corresponding to an average jet opening angle, \theta_j > 2 degrees. (2) This holds in particular for the broad-lined supernovae (including SNe 1997dq, 1997ef, 1998ey, 2002ap, 2002bl and 2003jd) which have been argued to host GRB jets. Our observations reveal no evidence for typical (or even sub-energetic) GRBs, constraining the fraction of broad-lined events harboring relativistic jets to < 17%. Their large photospheric velocities and mildly asymmetric ejecta (inferred from spectropolarimetry and nebular spectroscopy) appear to be characteristic of the non-relativistic supernova explosion and do not necessarily imply the existence of associated GRB jets.

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