Observational prospects of double neutrons star mergers and their multi-messenger afterglows: LIGO discovery power, event rates and diversity

Abstract

The double neutron star (DNS) merger event GW170817 signifies the first multimessenger (MM) event with electromagnetic-gravitational (EM-GW) observations. LIGO-Virgo-KAGRA observational runs O4-5 promise to detect similar events and as yet unknown GW signals, which require confirmation in two or more detectors with comparable performance. To this end, we quantify duty cycles of comparable science quality of data in coincident H1L1-observations, further to seek consistent event rates of astrophysical transients in upcoming EM-GW surveys. Quite generally, discovery power scales with exposure time, sensitivity, and critically depends on the percentage of time when detectors operate at high quality. We quantify coincident duty cycles over a time-frequency domain $W\times B$, defined by segments of duration $W=8$s, motivated by a long-duration descending GW-chirp during GRB170817A, and the minimum detector noise over about $B=100-250\,$Hz. This detector yield factor satisfies $1\%-25 \%$ in S5-6 and O1-O3ab, significantly different from duty cycles of H1 and L1 individually with commensurable impact on consistency in event rates in EM-GW surveys. Significant gain in discovery power for signals whose frequency varies slowly in time may be derived from improving detector yield factors by deploying time-symmetric data analysis methods. For O4-5, these can yield improvements by factors up to $\mathcal{O}(10^5)$ relative to existing data and methods. Furthermore, the diversity of MM afterglows to DNS mergers may be greatest for systems similar to GW170817 but possibly less so for systems of substantially different mass such as GW190425. We summarize our findings with an outlook on EM-GW surveys during O4-5 and perspectives for next-generation GRB missions like THESEUS.