Binary neutron stars (BNSs) detected in the Milky Way have total masses distributing narrowly around similar to 2.6-2.7M circle dot, while the BNS merger GW190425 detected via a gravitational wave has a significantly larger mass (similar to 3.4M circle dot). This difference is not well understood, yet. In this paper, we investigate the BNS spin evolution via an improved binary star evolution model and its effects on the BNS observability, with the implementation of various relevant astrophysical processes. We find that the first-born neutron star component in low-mass BNSs can be spun up to millisecond pulsars by the accretion of Roche-lobe overflow from its companion and its radio lifetime can be comparable to the Hubble time. However, most high-mass BNSs have substantially shorter radio lifetimes than low-mass BNSs, and thus a smaller probability of being detected via radio emission. Adopting the star formation and metal enrichment history of the Milky Way given by observations, we obtain the survived Galactic BNSs with pulsar components from our population synthesis model and find that their distributions on the diagrams of the spin period versus the spin period time derivative ( P-P ) and the orbital period versus the eccentricity (Porb-e) can well match those of the observed Galactic BNSs. The total mass distribution of the observed Galactic BNSs can also be matched by the model. A significant fraction (similar to 19%-22%) of merging BNSs at redshift z similar to 0 have masses greater than or similar to 3M circle dot, which seems compatible with the GW observations. Future radio observations may detect many more Galactic BNSs, which will put strong constraints on the spin evolution of BNSs during their formation processes.

The cosmic variance on the star formation history may lead to bias in the merger rate density estimation of binary neutron star (BNS) mergers by compact binary population synthesis. In this paper, we take advantage of the large box size of the Millennium Simulation combined with the semianalytic galaxy formation model GABE and the parameterized population binary star evolution model to examine how much effect the cosmic variance will introduce on the estimation of the merger rate density of BNS mergers. We find that for subbox sizes of 100 and 200 Mpc, the variance of merger rate density sigma(R)/R at different redshifts is about 23%-35% and 13%-20%, respectively. On the one hand, as for the variance of the detection rate on BNS mergers with the current LIGO-Virgo-KAGRA (LVK) detector network, this value is very small at less than or similar to 10%, which indicates ignoring the cosmic variance is reasonable for estimating the merger rate density from current LVK observations. On the other hand, with next-generation gravitational wave detectors, it is possible to localize BNS mergers within subboxes possessing a length of 40 Mpc for a source redshift z(s) < 0.2. In such a small box, the cosmic variance of the merger rate density is significant, i.e., the value of sigma(R)/R is about similar to 55%. This hints that estimating the merger rate density of BNS in different sky areas may provide useful information on the cosmic variance.

Future ground-based gravitational wave (GW) detectors, i.e. Einstein telescope (ET) and Cosmic Explorer (CE), are expected to detect a significant number of lensed binary neutron star (BNS) mergers, which may provide a unique tool to probe cosmology. In this paper, we investigate the detectability of the optical/infrared electromagnetic (EM) counterparts (kilonov ae/afterglo ws) from these lensed BNS mergers by future GW detectors and EM telescopes using simple kilonov a, afterglo w, and lens models. ET and CE are expected to detect similar to 5 . 32(+26 . 1) (-5 . 10) and 67 . 3 (+ 332)( -64 . 7) lensed BNS mergers per year. We find that the EM counterparts associated with all these mergers will be detectable by an all sk y-surv e y in the H band with the limiting magnitude m(lim) (sic) 27, while the detectable fraction is (sic) 0 . 4 per cent in the g/z band if with m(lim) (sic) 24. Generally, it is more efficient to search the lensed EM counterparts by adopting the infrared bands than the optical/UV bands with the same m(lim). Future telescopes like Vera C. Rubin Observatory, China Space Station Telescope, and Euclid can hardly detect the EM counterparts of even one lensed BNS merger. Roman Space Telescope (RST) and James Webb Space Telescope (JWST) have the capability to detect about a few or more such events per year. Moreover, the time delays and separations between the lensed image pairs are typically in the ranges from minutes to months and from 0.1 to 1 arcsec, suggesting that both the GW and EM images of most lensed BNS mergers can be well resolved by not only CE/ET in the time domain but also RST /JWST spatially.

ABSTRACT Kilonovae produced by mergers of binary neutron stars (BNSs) are important transient events to be detected by time domain surveys with the alerts from the ground-based gravitational wave detectors. The observational properties of these kilonovae depend on the physical processes involved in the merging processes and the equation of state (EOS) of neutron stars (NSs). In this paper, we investigate the dependence of kilonova luminosities on the parameters of BNS mergers, and estimate the distribution functions of kilonova peak luminosities (KLFs) at the u, g, r, i, y, and z bands as well as its dependence on the NS EOS, by adopting a comprehensive semi-analytical model for kilonovae (calibrated by the observations of GW170817), a population synthesis model for the cosmic BNSs, and the ejecta properties of BNS mergers predicted by numerical simulations. We find that the kilonova light curves depend on both the BNS properties and the NS EOS, and the KLFs at the considered bands are bimodal with the bright components mostly contributed by BNS mergers with total mass $\lesssim 3.2\, \mathrm{M}_\odot$/$2.8\, \mathrm{M}_\odot$ and fainter components mostly contributed by BNS mergers with total mass $\gtrsim 3.2\, \mathrm{M}_\odot$/$2.8\, \mathrm{M}_\odot$ by assuming a stiff/soft (DD2/SLy) EOS. The emission of the kilonovae in the KLF bright components is mostly due to the radiation from the wind ejecta by the remnant discs of BNS mergers, while the emission of the kilonovae in the KLF faint components is mostly due to the radiation from the dynamical ejecta by the BNS mergers.

ABSTRACT In this paper, we investigate the properties of binary neutron stars (BNSs) and their mergers by combining population synthesis models for binary stellar evolution (BSE) with cosmological galaxy formation and evolution models. We obtain constraints on BSE model parameters by using the observed Galactic BNSs and local BNS merger rate density (R0) inferred from gravitational wave (GW) observations, and consequently estimate the host galaxy distributions of BNS mergers. We find that the Galactic BNS observations imply efficient energy depletion in the common envelope (CE) phase, a bimodal kick velocity distribution, and low mass ejection during the secondary supernova explosion. However, the inferred R0 does not necessarily require an extremely high CE ejection efficiency and low kick velocities, different from the previous claims, mainly because the latest inferred R0 is narrowed to a lower value ($320_{-240}^{+490}\, \rm Gpc^{-3}\, yr^{-1}$). The BNS merger rate density resulting from the preferred model can be described by R($z$) ∼ R0(1 + $z$)ζ at low redshift ($z$ ≲ 0.5), with R0 ∼ 316–$784\, \rm Gpc^{-3}\, yr^{-1}$ and ζ ∼ 1.34–2.03, respectively. Our results also show that R0 and ζ depend on settings of BSE model parameters, and thus accurate estimates of these parameters by future GW detections will put strong constraints on BSE models. We further estimate that the fractions of BNS mergers hosted in spiral and elliptical galaxies at $z$ ∼ 0 are ∼81–84 and ∼16–19 per cent, respectively. The BNS merger rate per galaxy can be well determined by the host galaxy stellar mass, star formation rate, and metallicity, which provides a guidance in search for most probable candidates of BNS host galaxies.

The exact relationship between the long gamma-ray burst (LGRB) rate and the cosmic star formation rate (CSFR) is essential for using LGRBs as cosmological probes. In this work, we collect a large sample composed of 371 Swift LGRBs with known redshifts and prompt emission properties. We first compare the rest-frame prompt properties of these bursts in different redshift bins, finding negligible redshift evolution of the luminosity of LGRBs with between z ∼ 1 and z ∼ 4. Then, by utilizing the CSFR obtained from the large-scale cosmological hydrodynamical simulation, the Illustris simulation, we calculate the cumulative redshift distribution of LGRBs under different metallicity thresholds. After comparing with our sample, we find that the predictions with a moderate threshold between are consistent with the sample between redshift 0 < z < 3, while at higher redshifts, between 3 < z < 5, all metallicity thresholds fit the data well. When changing to an empirical model based on observations, the predictions show similar results as well. After comparing with the metallicity distribution of the observed LGRB host galaxies between 0 < z < 1, we confirm that the production of LGRBs in galaxies with super-solar metallicity is suppressed. Nevertheless, considering that a significant fraction of stars are born in sub-solar metallicity environments at z ≳ 3, we suggest that, as a first approximation, LGRBs can be used as direct tracers of the CSFR in this redshift range.