# Viewing angle constraints on S190425z and S190426c and the joint gravitational-wave/gamma-ray detection fractions for binary neutron star mergers

## TL;DR

This paper investigates viewing angle constraints on two binary neutron star merger candidates and estimates the joint gravitational-wave and gamma-ray detection fractions, highlighting the importance of jet structure and detector sensitivity.

## Contribution

It introduces a method to constrain viewing angles based on non-detections and models joint detection probabilities considering different gamma-ray detectors and future GW observatories.

## Key findings

- Viewing angles for S190425z and S190426c are constrained to > 0.09-0.41 radians.
- Estimated gamma-ray detection fractions are around 1.7-6.6% for current and future GW detectors.
- Joint detection probabilities vary significantly with detector sensitivity and field of view.

## Abstract

The LIGO and Virgo scientific collaboration (LVC) alerted two binary neutron star (BNS) merger candidates, S190425z and S190426c. Fermi-GBM observed 55.6\% (for S190425z) and 100\% (for S190426c) of the probability regions of both events at the respective merger times, but no gamma-ray burst (GRB) was detected in either case. The derived luminosity upper limits suggest that a short GRB similar to GRB 170817A would not be detectable for both cases due to their larger distances than GW170817. Assuming that the jet profile obtained from the GW170817/GRB 170817A is quasi-universal for all BNS-GRB associations, we derive that the viewing angles of S190425z and S190426c should be $> (0.11-0.41)$ and $> (0.09-0.39)$, respectively. Through Monte Carlo simulations, we show that with the GRB 170817A-like jet structure, all sky gamma-ray detectors, such as GBM and GECAM, are expected to detect $\sim 4.6\%$, $3.9\%$, $1.7\%$ and $6.6\%$, $5.7\%$, $2.8\%$ BNS mergers triggered by aLIGO, aLIGO A+ and ET, respectively. The joint detection fraction would be largely reduced for Swift-BAT, SVOM-ECLAIRS and Einstein Probe, whose sensitivities are better but fields of view are smaller.