Merger and Mass Ejection of Neutron Star Binaries

Abstract

Mergers of binary neutron stars and black hole-neutron star binaries are one of the most promising sources for the ground-based gravitationalwave (GW) detectors and also a high-energy astrophysical phenomenon as illustrated by the observations of gravitational waves and electromagnetic (EM) waves in the event of GW170817. Mergers of these neutron-star binaries are also the most promising site for r-process nucleosynthesis. Numerical simulation in full general relativity (numerical relativity) is a unique approach to the theoretical prediction of the merger process, GWs emitted, mass ejection process, and resulting EM emission. We summarize our current understanding for the processes of neutron star mergers and subsequent mass ejection based on the results of the latest numerical-relativity simulations. We emphasize that the predictions of the numerical-relativity simulations agrees broadly with the optical and infrared observations of GW170817.

General-relativistic rotation laws in fluid tori around spinning black holes

“…Approximately stationary disks can exist in tight accretion systems with central black holes. These highly relativistic systems can be created in the merger of compact binaries consisting of pairs of black holes and neutron stars [30][31][32]. In the case of light disks -with disks's masses much smaller than the mass of a black hole -their motion is quite likely ruled by the generalrelativistic Keplerian rotation law [15,16].…”

“…in powers of c −2 , as in (9) and (10). Then, keeping terms up to c −2 , replacing a by J/(cM ) and using (32), one gets (after suitable reordering and simplification) the left hand side of Eq. (31),…”

Gravitational-wave astrophysics from neutron star inspiral and coalescence

“…12 Finally, even if M < M max,rot , collapse remains a short-term threat, because the newly generated magnetic field will spin the star down, and radiation will cool its outer part. The spin-down time from a magnetic field of order 10 15 G is a few minutes, and unless the remnant's mass is below the maximum mass M max of a cold, nonrotating star, the merging stars are fated to end as a black hole surrounded by a disk (see, e.g., 5 and references therein).…”

“…Some ejected matter must be highly neutron rich to reach the second and third peaks: To form elements from lanthanides (rare earth elements) through uranium, a ratio of protons to the total number of baryons d less than 0.25 is needed. 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.…”

“…Because tides change the rate at which the orbit loses energy at order (R/d) 5 , with d the distance between stars, they are important in late inspiral; 99, 101 phase accumulated from the earlier inspiral, however, induces a time shift that corresponds to a larger phase difference in the later high-frequency cycles. 102 Since tidal effects influence the internal structure of neutron stars, the calculation of the relevant coupling constants needs to be carried out in full general relativity.…”