Probing binary neutron star mergers in dense environments using afterglow counterparts

TL;DR

This paper proposes a new method to determine whether binary neutron star mergers occur in dense environments by analyzing their afterglow signals, which are highly sensitive to ambient density, thus helping to understand their formation and evolution.

Contribution

It introduces a novel multi-messenger observational approach to directly probe the density of merger environments, especially for high-density scenarios, using afterglow data.

Findings

Method can constrain merger environment density with small samples

High sensitivity of afterglow signals to ambient density

Potential to distinguish between low- and high-density merger populations

Abstract

The only binary neutron star merger gravitational wave event with detected electromagnetic counterparts recorded to date is GRB170817A. This merger occurred in a rarefied medium with a density smaller than $10^{-3}-10^{-2}~{\rm cm}^{-3}$. Since kicks are imparted to neutron star binaries upon formation, and due to their long delay times before merger, such low-density circum-merger media are generally expected. However, there is some indirect evidence for fast-merging or low-kick binaries, which would coalesce in denser environments. Nonetheless, present astronomical data are largely inconclusive on the possibility of these high-density mergers. We describe a method to directly probe this hypothetical population of high-density mergers through multi-messenger observations of binary neutron star merger afterglows, exploiting the high sensitivity of these signals to the density of the merger environment. This method is based on a sample of merger afterglows that has yet to be collected. Its constraining power is large, even with a small sample of events. We discuss the method's limitations and applicability. In the upcoming era of third-generation gravitational wave detectors, this method's potential will be fully realized as it will allow us to probe mergers that occurred soon after the peak of cosmic star formation, provided the follow-up campaigns are able to locate the sources.