Kilonovae

Abstract

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.

Gravitational-wave astrophysics from neutron star inspiral and coalescence

“…From ultraviolet through infrared, observations of the GW170817 remnant (labeled the astronomical transient AT2017gfo) strikingly confirmed the behavior of a radioactively-powered kilonova (see, e.g., 6 and references therein). A spectrum initially peaked in ultraviolet progressed through blue to red over about three days and then to infrared, 88-90 supporting models whose ejecta had a range of neutron fractions.…”

“…5,58,59 To reproduce the observed abundance of lighter r-process nuclides, the ejecta must also include a less neutron rich component, with the ratio proton/baryon 0.25, and that is seen in simulations of mergers with M < M thres , mergers in which a massive neutron star briefly supports itself against collapse. 6,[59][60][61][62] In merger simulations, the first ejecta are neutron rich. If a high-mass neutron star is formed, the e ± production and subsequent weak interactions that lead to loss of most of the merger energy in neutrinos also deplete the number of neutrons: The higher density of neutrons means a higher rate of n → p than p → n conversions, from positron and neutrino capture.…”

“…The brightness and timescale are key signatures of the formation of r-process material whose radioactive decay powers the kilonova, and we give brief estimates here. Li and Paczynski 80 first suggested an observable optical glow from merger ejecta in 1998; see Metzger 6 for a review and references.…”

Upper Limit on the Dissipation of Gravitational Waves in Gravitationally Bound Systems

“…The above results also limit a possible electromagnetic (EM) counterpart to the GW signal from its environment (Kocsis & Loeb 2008;Li et al 2012), unrelated to the possible EM emission by the source itself (Loeb 2016;D'Orazio & Loeb 2018;Metzger 2019). The dissipation of a fraction ǫ diss of the GW energy, E GW , in a baryonic system surrounding the GW source, would lead to an EM counterpart with a luminosity,…”

Gamma-Rays from Kilonovae and the Cosmic Gamma-Ray Background

“…Concerning the second point, the amount and composition of the ejecta depend on how and when dur-ing the merger the material has become unbound (for a review, see Metzger 2019). The most neutron-rich component known as "dynamical ejecta" gets unbound by dynamical tide and attains ideal conditions for robust r-process nucleosynthesis (Freiburghaus et al 1999;Korobkin et al 2012;Bauswein et al 2013).…”