Living Reviews in Relativity: "Kilonovae" (Update)
Metzger, B., "Kilonovae", Living Rev Relativ (2020) 23: 1. https://doi.org/10.1007/s41114-019-0024-0
Open Access | Review Article
Published: 16 December 2019
Major revision, updated and expanded version of
"The author significantly revises the previous manuscript, which was written before the discovery of GW170817. The current manuscript extensively overviews what we have learned from GW170817 and includes a tremendous number of new papers published after the event. I appreciate the author's effort to update the review article and highly recommend this to be accepted in Living Reviews in Relativity. [Referee]"
The coalescence of double neutron star (NS–NS) and black hole (BH)–NS binaries are prime sources of gravitational waves (GW) for Advanced LIGO/Virgo and future ground-based detectors. Neutron-rich matter released from such events undergoes rapid neutron capture (r-process) nucleosynthesis as it decompresses into space, enriching our universe with rare heavy elements like gold and platinum. Radioactive decay of these unstable nuclei powers a rapidly evolving, approximately isotropic thermal transient known as a “kilonova”, which probes the physical conditions during the merger and its aftermath. Here I review the history and physics of kilonovae, leading to the current paradigm of day-timescale emission at optical wavelengths from lanthanide-free components of the ejecta, followed by week-long emission with a spectral peak in the near-infrared (NIR). These theoretical predictions, as compiled in the original version of this review, were largely confirmed by the transient optical/NIR counterpart discovered to the first NS–NS merger, GW170817, discovered by LIGO/Virgo. Using a simple light curve model to illustrate the essential physical processes and their application to GW170817, I then introduce important variations about the standard picture which may be observable in future mergers. These include ∼hour-long UV precursor emission, powered by the decay of free neutrons in the outermost ejecta layers or shock-heating of the ejecta by a delayed ultra-relativistic outflow; and enhancement of the luminosity from a long-lived central engine, such as an accreting BH or millisecond magnetar. Joint GW and kilonova observations of GW170817 and future events provide a new avenue to constrain the astrophysical origin of the r-process elements and the equation of state of dense nuclear matter.
Major revision, updated and expanded. Overall, the main changes were to 1) blend into the discussion GW170817 in many places; 2) substantially update the reference list in light of all the work post-170817; 3) re-worked the organization slightly, focusing in Sect. 4 on what aspects of the models largely confirmed in 170817 (blue/red kilonova) and making a new Sect. 6 on future speculations.
Brian D. Metzger is an Associate Professor in the Department of Physics at Columbia University, New York. His research is in theoretical high energy astrophysics, on topics including gamma-ray bursts, novae, supernovae, accretion processes, shocks, compact objects, nucleosynthesis (astrophysical origin of the elements), and the electromagnetic counterparts of gravitational-wave sources. His research utilizes both analytic calculations and numerical simulations, with the latter often pursued in collaboration with students and postdocs.