Prospects of joint detections of neutron star mergers and short-GRBs with Gaussian structured jets

Abstract: GW170817 was the first ever joint detection of gravitational waves (GW) from a binary neutron star (BNS) merger with the detections of short $\gamma$-ray burst (SGRB) counterparts. Analysis of the multi-band afterglow observations of over more than a year revealed that the outflow from the merger end-product was consistent with structured relativistic jet models with the core of the jet narrowly collimated to half opening angles $\sim5$ deg. In this work, assuming all the BNS mergers produce Gaussian structured jets with properties as inferred for GW170817, we explore the prospects of joint detections of BNS mergers and prompt $\gamma-$ray emission, expected during the current and upcoming upgrades of LIGO-Virgo-KAGRA detectors. We discuss three specific observational aspects: 1) the distribution of detected binary inclination angles 2) the distance reach and 3) the detection rates. Unlike GW-only detections, the joint detections are greatly restricted at large inclination angles, due to the structure of the jets. We find that at lower inclination angles (say below 20 deg), the distance reach as well as the detection rates of the joint detections are limited by GW detectability while at larger inclinations (say above 20 deg), they are limited by the $\gamma$-ray detectability.