On the Rate of Neutron Star Binary Mergers from Globular Clusters

Abstract

The first detection of gravitational waves from a neutron star -neutron star (NS-NS) merger, GW170817, and the increasing number of observations of short gamma-ray bursts (SGRBs) have greatly motivated studies of the origins of NS-NS and neutron star -black hole (NS-BH) binaries. We calculate the merger rates of NS-NS and NS-BH binaries from globular clusters (GCs) using realistic GC simulations with the CMC cluster catalog. We use a large sample of models with a range of initial numbers of stars, metallicities, virial radii and galactocentric distances, representative of the present-day Milky Way GCs, to quantify the inspiral times and volumetric merger rates as a function of redshift, both inside and ejected from clusters. We find that over the complete lifetime of most GCs, stellar BHs dominate the cluster cores and prevent the mass segregation of NSs, thereby reducing the dynamical interaction rates of NSs so that at most a few NS binary mergers are ever produced. We estimate the merger rate in the local universe to be ∼ 0.02 Gpc −3 yr −1 for both NS-NS and NS-BH binaries, or a total of ∼ 0.04 Gpc −3 yr −1 for both populations. These rates are about 5 orders of magnitude below the current empirical merger rate from LIGO/Virgo. We conclude that dynamical interactions in GCs do not play a significant role in enhancing the NS-NS and NS-BH merger rates.

Discovery of the Optical Afterglow and Host Galaxy of Short GRB 181123B at z = 1.754: Implications for Delay Time Distributions

“…If we are indeed missing a population of high-z SGRBs, this would indicate overall shorter delay times and provide an additional constraint on the DTD. Some studies have also suggested a possible bimodal DTD distribution (Salvaterra et al 2008), but this is in tension with more recent theoretical studies showing that dynamical assembly of NS-NS and NS-BH mergers can only contribute a small fraction to the overall merger rates (Belczynski et al 2018;Ye et al 2020).…”

Common envelope episodes that lead to double neutron star formation

“…where E 2 = 1.592 × 10 −9 (M/M ) 2.84 is a second-order tidal coefficient as fitted by Hurley et al (2002) from the values given by Zahn (1975), under the assumption (violated for some of the systems we consider) that close binaries are nearly circular. For the dynamical tide, we set the tidal separation (D) to be the semilatus rectum D = a(1 − e 2 ).…”

Measuring the eccentricity of GW170817 and GW190425

“…Unfortunately, based on current merger rate estimates the detection of an eccentric binary neutron star merger will be difficult with current observatories (Lee et al 2010;Ye et al 2019;Nitz et al 2019), but gravitational-wave capture binaries that have e ≥ 0.8 and could form in the LIGO-Virgo band (Rodriguez et al 2018;Takátsy et al 2019). However, since the eccentricity of the detections is expected to be low and negligible, e ≤ 0.02, a circular search is effective in detecting them (Brown & Zimmerman 2010;Huerta & Brown 2013).…”

“…Dynamical interations may form binary neutron stars with residual eccentricity, although the rate of such mergers is expected to be small in current detectors (Lee et al 2010;Ye et al 2019) and a search for eccentric binary neutron stars in the O1 and O2 observing runs did not yield any candidates (Nitz et al 2019). However, since eccentricity is an interesting probe of binary formation channels and eccentric binaries may produce different electromagnetic emission than circular binary neutron stars (Radice et al 2016;Chaurasia et al 2018), it is important to accurately constrain the eccentricity of binary neutron stars as the number of observed mergers increases in the coming years.…”