The potential role of binary neutron star merger afterglows in multimessenger cosmology

¹ Université de Paris, CNRS, Astroparticule et Cosmologie, 75006 Paris, France
e-mail: mastrosi@apc.in2p3.fr
² Sorbonne Université, CNRS, Institut d’Astrophysique de Paris, 75014 Paris, France
e-mail: duque@iap.fr

Received: 23 December 2020
Accepted: 13 May 2021

Abstract

Binary neutron star mergers offer a new and independent means of measuring the Hubble constant H₀ by combining the gravitational-wave inferred source luminosity distance with its redshift obtained from electromagnetic follow-up. This method is limited by the intrinsic degeneracy between the system distance and orbital inclination in the gravitational-wave signal. Observing the afterglow counterpart to a merger can further constrain the inclination angle, allowing this degeneracy to be partially lifted and improving the measurement of H₀. In the case of the binary neutron star merger GW170817, afterglow light-curve and imaging modeling thus allowed the H₀ measurement to be improved by a factor of three. However, systematic access to afterglow data is far from guaranteed. In fact, though each one allows a leap in H₀ precision, these afterglow counterparts should prove rare in forthcoming multimessenger campaigns. We combine models for emission and detection of gravitational-wave and electromagnetic radiation from binary neutron star mergers with realistic population models and estimates for afterglow inclination angle constraints. Using these models, we quantify how fast H₀ will be narrowed down by successive multimessenger events with and without the afterglow. We find that because of its rareness and though it greatly refines angle estimates, the afterglow counterpart should not significantly contribute to the measurement of H₀ in the long run.