A FAILED GAMMA-RAY BURST WITH DIRTY ENERGETIC JETS SPIRITED AWAY? NEW IMPLICATIONS FOR THE GAMMA-RAY BURST SUPERNOVA CONNECTION FROM SN 2002ap

摘要

The Type Ic supernova (SN) 2002ap is an interesting event with very broad spectral features like the famous energetic SN 1998bw associated with a gamma-ray burst (GRB) 980425. Here we examine the jet hypothesis from SN 2002ap recently proposed based on the redshifted polarized continuum found in a spectropolarimetric observation. We show that jets should be moving at about 0.23c to a direction roughly perpendicular to us, and the degree of polarization requires a jet kinetic energy of at least 5 x 10~(50) ergs, a similar energy scale to the GRB jets. The weak radio emission from SN 2002ap has been used to argue against the jet hypothesis, but we argue that this is not a problem because the jet is expected to be freely expanding and unshocked. However, the jet cannot be kept ionized because of adiabatic cooling without external photoionization or a heating source. We explored various ionization possibilities and found that only the radioactivity of ~(56)Ni is a plausible source, indicating that the jet is formed and ejected from the central region of the core collapse, not from the outer envelope of the exploding star. Then we point out that, if the jet hypothesis is true, the jet will eventually sweep up enough interstellar medium and generate shocks in a few to 10 yr, producing strong radio emission that can be spatially resolved, giving us a clear test for the jet hypothesis. Discussions are also given on what the jet would imply for the GRB-SN connection, when it is confirmed. We suggest the existence of two distinct classes of GRBs from similar core-collapse events but by completely different mechanisms. Cosmologically distant GRBs having an energy scale of ～10~(50)-10~(51) ergs are collimated jets generated by the central activity of core collapses, associated with ~(56)Ni ejection along with the jets. SN 2002ap can be considered as a failed GRB of this type with large baryon contamination. On the other hand, much less energetic ones including GRB 980425 are rather isotropic, which may be produced by hydrodynamical shock acceleration at the outer envelope. We propose that the radioactive ionization for the SN 2002ap jet may give a new explanation also for the X-ray line features often observed in GRB afterglows.