The Gravitational waves merger time distribution of binary neutron star systems

Abstract

Binary neutron stars (BNS) mergers are prime sites for r-process nucleosynthesis. Their rate determines the chemical evolution of heavy elements in the Milky Way. The merger rate of BNS is a convolution of their birth rate and the gravitational radiation spiral-in delay time.Using the observed population of Galactic BNS we show here that the lifetimes of pulsars in observed BNSs are sufficiently short that the ages of BNSs have little to no effect on the observed merger time distribution. We find that at late times (t ∼ > 1 Gyr) the gravitational wave delay time distribution (DTD) follows the expected t −1 . However, a significant excess of rapidly merging systems (between 40 − 60% of the entire population) is apparent at shorter times. Although the exact shape of the DTD cannot be determined with the existing data, in all models that adequately describe the data we find at least 40% of BNSs with merger times less than 1Gyr. This population of rapid mergers implies a declining deposition rate of r-process materials that is consistent with several independent observations of heavy element abundances in the Milky Way. At the same time this population that requires initial binary separations of roughly one solar radius clearly indicates that these binaries had common envelope progenitors. Our results suggest that a significant fraction of future LIGO/Virgo BNS mergers would reside in star forming galaxies.

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

“…Thus far, studies of the Galactic BNS population (Vigna-Gómez et al 2018), as well as SGRB host galaxy demographics, are in rough agreement with power-law DTDs with η1 (Zheng & Ramirez-Ruiz 2007;. We note that some studies have found an excess of more rapid mergers, finding steeper DTDs than η=1 (Beniamini & Piran 2019), but each provides fairly weak constraints. 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.…”

“…Many studies have constrained the SGRB DTD by fitting the SGRB redshift distribution, predominantly focused on the z<1 population (Nakar et al 2006;Berger et al 2007;Jeong & Lee 2010;Hao & Yuan 2013;Wanderman & Piran 2015;Anand et al 2018). Other constraints on the DTD have come from studies of the Galactic population of NS binaries (Champion et al 2004;Beniamini et al 2016a;Vigna-Gómez et al 2018;Beniamini & Piran 2019) and SGRB host galaxy demography, as longer delay times will result in an increase in host galaxies with old stellar populations (Zheng & Ramirez-Ruiz 2007;Behroozi et al 2014).…”

The Stars in M15 Were Born with the r-process*

“…Alternatively, Bekki & Tsujimoto (2017) proposed that the formation of M15 might have had a duration of ∼ 0.1 Gyr, long enough for a NSM to enrich the stars forming in the cluster. In support of this scenario, Beniamini & Piran (2019) found that about 40% observed binary neutron stars in the MW have short (< 1 Gyr) merging times. These include two systems discovered decades ago with merging times of 0.3-0.4 Gyr (e.g., Phinney 1991) and three others discovered more recently with merging times less than 0.1 Gyr (e.g., Stovall et al 2018).…”

Fast Radio Bursts from Activity of Neutron Stars Newborn in BNS Mergers: Offset, Birth Rate, and Observational Properties

“…In Figure 1, we show the delay time distribution and merger time distribution for NS-NS binaries. The merger times of 8 observed field Galactic NS-NS systems (Beniamini & Piran 2019) are shown as triangles. We also compare the cumulative distributions of merger time from population synthesis with these 8 observed field Galactic NS-NS systems.…”