Super-kilonovae from Massive Collapsars as Signatures of Black Hole Birth in the Pair-instability Mass Gap

The core collapse of rapidly rotating massive similar to 10M (circle dot) stars (collapsars), and the resulting formation of hyperaccreting black holes, comprise a leading model for the central engines of long-duration gamma-ray bursts (GRBs) and promising sources of r-process nucleosynthesis. Here, we explore the signatures of collapsars from progenitors with helium cores greater than or similar to 130M (circle dot) above the pair-instability mass gap. While the rapid collapse to a black hole likely precludes prompt explosions in these systems, we demonstrate that disk outflows can generate a large quantity (up to greater than or similar to 50M (circle dot)) of ejecta, comprised of greater than or similar to 5-10M (circle dot) in r-process elements and similar to 0.1-1M (circle dot) of Ni-56, expanding at velocities similar to 0.1 c. Radioactive heating of the disk wind ejecta powers an optical/IR transient, with a characteristic luminosity similar to 10(42) erg s(-1) and a spectral peak in the near-IR (due to the high optical/UV opacities of lanthanide elements), similar to kilonovae from neutron star mergers, but with longer durations greater than or similar to 1 month. These super-kilonovae (superKNe) herald the birth of massive black holes greater than or similar to 60M (circle dot), 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 IR follow-up observations of extremely energetic GRBs. Multiband gravitational waves of similar to 0.1-50 Hz from nonaxisymmetric instabilities in self-gravitating massive collapsar disks are potentially detectable by proposed observatories out 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).

Automated summary

The study explores the characteristics of collapsars from progenitors with helium cores above or equal to 130 solar masses, including a large quantity of ejecta and optical/IR transients. These superkilonovae indicate the birth of massive black holes and could potentially be detected in gravitational waves.