The Many Facets of Cosmic Explosions

Over the past few years long-duration gamma-ray bursts (GRBs)including the subclass of X-ray flashes (XRFs) have been revealed tobe a rare variety of Type Ibc supernova (SN Ibc). While all theseevents result from the death of massive stars the electromagneticluminosities of GRBs and XRFs exceed those of ordinary Type Ibc SNe bymany orders of magnitude. The observed diversity of stellar deathcorresponds to large variations in the energy velocity and geometryof the explosion ejecta. Using multi-wavelength (radio opticalX-ray) observations of the nearest GRBs XRFs and SNe Ibc I showthat GRBs and XRFs couple at least 1048 erg to relativisticmaterial while SNe Ibc typically couple less than 1048 erg to theirfastest (albeit non-relativistic) outflows. Specifically I find thatless than 3 percent of local SNe Ibc show any evidence for association witha GRB or XRF. Interestingly this dichotomy is not echoed by theproperties of their optical SN emission dominated by the radioactivedecay of Nickel-56; I find that GRBs XRFs and SNe Ibc showsignificant overlap in their optical peak luminosity and photosphericvelocities. Recently I identified a new class of GRBs and XRFs thatare under-luminous in comparison with the statistical sample of GRBs.Owing to their faint high-energy emission these sub-energetic burstsare only detectable nearby (z < 0.1) and are likely 10 times morecommon than cosmological GRBs. In comparison with local SNe Ibc andtypical GRBs/XRFs these explosions are intermediate in terms of both volumetric rate and energetics. Yet the essential physical processthat causes a dying star to produce a GRB XRF or sub-energeticburst and not just a SN remains a crucial open question. Progressrequires a detailed understanding of ordinary SNe Ibc which will befacilitated with the launch of wide-field optical surveys in the near future.