Joint analysis of gravitational-wave and electromagnetic data of mergers: breaking an afterglow model degeneracy in GW170817 and in future events

TL;DR

This paper presents a combined analysis of gravitational-wave and electromagnetic data from neutron star mergers, demonstrating how joint fitting can break degeneracies in afterglow models and improve parameter constraints for future events.

Contribution

The study introduces a methodology for joint GW and EM data analysis that reduces parameter degeneracies, enhancing the accuracy of viewing angle and jet structure estimates.

Findings

Joint analysis reduces uncertainties in viewing angle estimates.

Including GW data breaks degeneracies present in EM-only fits.

Method improves parameter constraints for future GW events at various distances.

Abstract

On August 17, 2017, Advanced LIGO and Virgo observed GW170817, the first gravitational-wave (GW) signal from a binary neutron star merger. It was followed by a short-duration gamma-ray burst, GRB 170817A, and by a non-thermal afterglow emission. In this work, a combined simultaneous fit of the electromagnetic (EM, specifically, afterglow) and GW domains is implemented, both using the posterior distribution of a GW standalone analysis as prior distribution to separately process the EM data, and fitting the EM and GW domains simultaneously. These approaches coincide mathematically, as long as the actual posterior of the GW analysis, and not an approximation, is used as prior for the EM analysis. We treat the viewing angle, $\theta_v$, as shared parameter across the two domains. In the afterglow modelling with a Gaussian structured jet this parameter and the jet core angle, $\theta_c$, are correlated, leading to high uncertainties on their values. The joint EM+GW analysis relaxes this degeneracy, reducing the uncertainty compared to an EM-only fit. We also apply our methodology to hypothetical GW170817-like events occurring in the next GW observing run at $\sim$140 and 70 Mpc. At 70 Mpc the existing EM degeneracy is broken, thanks to the inclusion of the GW domain in the analysis. At 140 Mpc, the EM-only fit cannot constrain $\theta_v$ nor $\theta_c$ because of the lack of detections in the afterglow rising phase. Folding the GW data into the analysis leads to tighter constraints on $\theta_v$, still leaving $\theta_c$ unconstrained, requiring instruments with higher sensitivities, such as Athena.