The origin of the first neutron star – neutron star merger

Abstract

The first neutron star-neutron star (NS-NS) merger was discovered on August 17, 2017 through gravitational waves (GW170817) and followed with electromagnetic observations (Abbott et al. 2017). This merger was detected in an old elliptical galaxy with no recent star formation (Blanchard et al. 2017;Troja et al. 2017). We perform a suite of numerical calculations to understand the formation mechanism of this merger. We probe three leading formation mechanisms of double compact objects: classical isolated binary star evolution, dynamical evolution in globular clusters and nuclear cluster formation to test whether they are likely to produce NS-NS mergers in old host galaxies. Our simulations with optimistic assumptions show current NS-NS merger rates at the level of 10 −2 yr −1 from binary stars, 5 × 10 −5 yr −1 from globular clusters and 10 −5 yr −1 from nuclear clusters for all local elliptical galaxies (within 100 Mpc 3 ). These models are thus in tension with the detection of GW170817 with an observed rate 1.5 +3.2 −1.2 yr −1 (per 100 Mpc 3 ; LIGO/Virgo 90% credible limits). Our results imply that either (i) the detection of GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is a statistical coincidence; or that (ii) physics in at least one of our three models is incomplete in the context of the evolution of stars that can form NS-NS mergers; or that (iii) another very efficient (unknown) formation channel with a long delay time between star formation and merger is at play.

On the Rate of Neutron Star Binary Mergers from Globular Clusters

“…Assuming a few km s −1 for the velocity dispersion instead will slightly increase the NS retention rate, but is not likely to have a large effect on the NS-NS merger rate. Indeed, Belczynski et al (2018) used zero natal kicks for ECSN NSs in their NS-NS merger rate calculations and estimated a merger rate of about 10 times higher than ours, but still not high enough to explain the empirical LIGO/Virgo NS-NS merger rate. Although we have limited this analysis to those globular clusters that survive to the present day, there may also exist a potentially significant number of additional massive clusters that disrupted at earlier times (e.g., Fragione & Kocsis 2018;Rodriguez & Loeb 2018;Krumholz et al 2019, and references therein).…”

“…For typical clusters with initial N = 8×10 5 , (1)- (7) show merger rate estimates from GCs. Note that the merger rate estimates in Belczynski et al (2018) are for GCs in local elliptical galaxies only. Previous studies may use different number densities of GCs in the local universe than ours (ρ GC = 0.77 Mpc −3 ).…”

Double Neutron Star Populations and Formation Channels

“…On the other hand, the detailed properties of DNS populations modelled with binary population synthesis do not always match the observed Galactic systems. Models predict too many systems with high eccentricity relative to observations (Ihm et al 2006;Chruslinska et al 2017), under-predict DNS masses (Vigna-Gómez et al 2018, and predict lower DNS merger rates than the point estimate based on the single gravitationalwave observation to date (Belczynski et al 2018, although a single fortuitous detection is by no means implausible given the predicted range of merger rates).…”

X-Ray Binary Luminosity Function Scaling Relations for Local Galaxies Based on Subgalactic Modeling

“…Depending on the binary parameters, subsequent evolution beyond the XRB phase is expected to result in a variety of as-trophysical systems, including, e.g., gravitational wave (GW) mergers, millisecond pulsars, and short gammaray bursts (GRBs). Recent discoveries of gravitational waves (GWs) from merging BHs and NSs from LIGO (e.g., Abbott et al 2016Abbott et al , 2017 have prompted a resurgence in efforts to self-consistently model close binary populations and their evolution (e.g., Belczynski et al 2016Belczynski et al , 2018Mandel & de Mink 2016;Marchant et al 2017;Kruckow et al 2018;. As such, statistically meaningful constraints on XRB populations are critical to such efforts.…”