Pulsars, transients and relativistic astrophysics

Abstract:

The high rate of black hole (BH) mergers detected by LIGO/Virgo opened questions on their astrophysical origin. One possibility is the dynamical channel, in which binary formation and hardening is catalyzed by dynamical encounters in globular clusters (GCs). Previous studies have shown that the BH merger rate from the present day GC density in the Universe is lower than the observed rate. In this \textit{Letter}, we study the BH merger rate by accounting for the first time for the evolution of GCs within their host galaxies. The mass in GCs was initially $\sim 8\times$ higher, which decreased to its present value due to evaporation and tidal disruption. Many BH binaries that were ejected long before their merger, originated in GCs that no longer exist. We find that the comoving merger rate in the dynamical channel from GCs varies between $18$ to $35\,{\rm Gpc}^{-3}\,{\rm yr}^{-1}$ between redshift $z=0.5$ to $2$, and the total rate is $1$, $5$, $24$ events per day within $z=0.5$, $1$, and $2$, respectively. The cosmic evolution and disruption of GCs systematically increases the present-day merger rate by a factor $\sim 2$ relative to isolated clusters. Gravitational wave detector networks offer an unique observational probe of the initial number of GC populations and their subsequent evolution across cosmic time.

Abstract:

© 2018. The American Astronomical Society. All rights reserved.. Black hole (BH) mergers detectable with the Laser Interferometer Gravitational-wave Observatory (LIGO) can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum, and the spin spectrum of BHs in AGN disks. The predicted merger rate spans ∼10-3-104 Gpc-1 yr-1, so upper limits from LIGO (<212 Gpc-1 yr-1) already constrain it. The predicted mass spectrum has the form of a broken power law, consisting of a pre-existing BH power-law mass spectrum and a harder power-law mass spectrum resulting from mergers. The predicted spin spectrum is multipeaked with the evolution of retrograde spin BHs in the gas disk playing a key role. We outline the large uncertainties in each of these LIGO observables for this channel and we discuss ways in which they can be constrained in the future.

Abstract:

Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.2 solar masses) and low kinetic energy (≈2 × 1050 ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.