Illuminating the pair-instability supernova mass gap with super-kilonovae

Abstract

The core collapse of rapidly rotating massive 10Msun stars (“collapsars”), and resulting formation of hyper-accreting black holes, are a leading model for the central engines of long-duration gamma-ray bursts (GRB) and promising sources of r-process nucleosynthesis. In this talk, I will explore the signatures of collapsars from progenitors with extremely massive helium cores >= 130Msun above the pair-instability mass gap. While rapid collapse to a black hole likely precludes a prompt explosion in these systems, we demonstrate that disk outflows can generate a large quantity (up to >= 50Msun) of ejecta, comprised of >= 5 10Msun in r-process elements and 0.1 1M of 56Ni, expanding at velocities 0.1 c. Radioactive heating of the disk-wind ejecta powers an optical/infrared transient, with a characteristic luminosity 1042 erg s1and spectral peak in the near-infrared (due to the high optical/UV opacities of lanthanide elements) similar to kilonovae from neutron star mergers, but with longer durations >= 1 month. These “super-kilonovae” (superKNe) herald the birth of massive black holes >= 60M, which— as a result of disk wind mass-loss—can populate the pair-instability mass gap “from above” and could potentially create the binary components of GW190521. SuperKNe could be discovered via wide-field surveys such as those planned with the Roman Space Telescope or via late-time infrared follow-up observations of extremely energetic GRBs. Gravitational waves of frequency 0.1 50 Hz from non-axisymmetric instabilities in self-gravitating massive collapsar disks are potentially detectable by proposed third-generation intermediate and high-frequency observatories at distances up to hundreds of Mpc; in contrast to the “chirp” from binary mergers, the collapsar gravitational-wave signal decreases in frequency as the disk radius grows (“sad trombone”).