Unraveling merging neutron stars with population synthesis

On September 14 2015, the two advanced LIGO detectors obtained the first direct detection of gravitational waves (GWs). Predicted by Einstein about 100 years ago, GWs are ripples in the fabric of space-time which propagate as waves, traveling at the speed of light. They can be produced by the motion of very massive bodies, such as binary systems composed of two black holes (BHs). To date, three GW events and one candidate event have been observed, opening the era of GW astrophysics. While only GW signals from BH binary mergers have been reported so far, double neutron star (NS) binaries and BH-NS binaries are also expected to be powerful sources of GWs in the frequency range of advanced LIGO and Virgo, possibly associated with electromagnetic counterparts. Thus, astrophysicists must be ready to meet the challenge of interpreting GW emission from merging NS binaries. The population of merging NS binaries and the properties of their environment are still largely elusive, because only few recent models of double NS and BH-NS binary demography are available in the literature and none of them characterizes the host galaxies of such systems across cosmic time. This project (hereafter, the PopNeS project) aims at studying the demography of double NS binaries and BH-NS binaries by means of novel population-synthesis simulations. The most important tool of PopNeS will be our new code `SEVN`, which couples advanced stellar evolution models with several prescriptions for supernovae and binary evolution processes. SEVN has already been successfully used to predict and constrain the properties of the first GW events. In the frame of PopNeS, we will update SEVN including new prescriptions, and we will run a large grid of isolated binary systems to study the formation of double NS and BH-NS binaries. The outputs of SEVN will then be interfaced with several publicly available state-of-the-art cosmological simulations, to reconstruct the cosmic merger history of double NS binaries and BH-NS binaries. This procedure will also enable us to characterize the properties of the host galaxies of merging NS binaries, providing precious insights for the search of electromagnetic counterparts. Our models will be crucial to interpret future detections of NS binaries by ground-based GW interferometers.

In 2015, LIGO, the Laser Interferometer Gravitational-Wave Observatory detected for the first time fluctuations in spacetime caused by gravitational waves (GW) generated by two colliding black holes located 1.3 billion light-years away. This ushered in a new era for GW astrophysics and led to the awarding of the Nobel Prize in Physics in 2017. By now the LIGO-Virgo-KAGRA detectors collaboration has detected about 90 GW event candidates in its three observing runs. These 'inspiral' events were caused by orbiting pairs of massive and dense ("compact") objects such as binary black holes (BBH), binary neutron stars (BNS), or black hole-neutron star binaries (BHNS). The goal of the PopNeS project is to use state-of-the-art astrophysical simulations to understand the demography of BNS and BHNS merging systems and the connection to the galaxies where they reside, their host galaxies, especially in the context of star formation and galaxy evolution models. The method proposed for the PopNeS project combines stellar evolution models from population-synthesis simulations, with galaxy catalogs from cosmological simulations. This approach allowed us to study: i) the characteristics of the host galaxies of binary compact object mergers and ii) the possible different formation channels of these merging binary compact objects. Among the most interesting results, we find that the host galaxies of binary compact objects tend to be massive and have a preference for elliptical (early-type) galaxies in the nearby universe. Our results also indicate that the GW events detected by LIGO-Virgo-KAGRA might be described by a combination of binary systems that formed in star clusters and others that formed in isolation. The results of the PopNeS enable us to understand the properties of the host galaxies of the compact binaries, and provide precious insights for the search of electromagnetic counterparts, thus representing a kickoff on a recent and vivid area such as GW astrophysics.