August 21
Revelations from EP260321a
Speaker: Dr. Christopher Irwin, Tohoku University
Abstract: The recently detected X-ray transient EP260321a associated with a broad-lined Type Ic supernova (SN) suggests a tentative link between previous well-known shock breakout (SBO) candidates: the Type Ib SN 2008D and certain low-luminosity gamma-ray bursts like GRB 060218. Surprisingly, although these systems vary by three orders of magnitude in X-ray luminosity, L_X, their durations, t_X, are comparable to within a factor of 10. We consider whether SBO can explain this spread in luminosities for several scenarios including SBO from the edge of the star, from deep within the circumstellar medium (CSM), and from the edge of the CSM. We find that L_X spans a sufficiently wide range only if bright events have flatter energy-velocity distributions than faint ones, i.e. E(v) ~ v^{-k} with k ranging from >4 to <2 with increasing L_X. Variations in the progenitor star's mass or radius, the CSM's mass or radius, or the total explosion energy cannot generate a broad enough luminosity distribution. We prefer a CSM edge breakout scenario because it naturally leads to a UV/optical bump after a few hours, without producing flash ionization features or overluminous shock interaction emission. The need for small k in bright events offers strong support for the presence of a central engine which couples significant energy to high-velocity (v ~ 0.1 c) gas. These results agree with multiwavelength observations of GRB-SNe and numerical simulations of choked jets, but this is the first supporting evidence gleaned directly from the SBO event population.
Host: Robert Stein & Jillian Rastinejad