Investigating the detection rates and inference of gravitational-wave and radio emission from black hole neutron star mergers

Abstract

Context. Black hole neutron star (BHNS) mergers have recently been detected through their gravitational-wave (GW) emission. While no electromagnetic emission (EM) has yet been confidently associated with these systems, observing any such emission could provide information on, for example, the neutron star (NS) equation of state (EOS). BHNS mergers could produce EM emission as a short gamma-ray burst (sGRB), and/or an sGRB afterglow upon interaction with the circummerger medium. Aims. Here, we make predictions for the expected detection rates with the Square Kilometre Array Phase 1 (SKA1) of sGRB radio afterglows associated with BHNS mergers. We also investigate the benefits of a multimessenger analysis in inferring the properties of the merging binary. Methods. We simulate a population of BHNS mergers, making use of recent stellar population synthesis results, and estimate their sGRB afterglow flux to obtain the detection rates with SKA1. We investigate how this rate depends on the GW detector sensitivity, the primary black hole spin, and the NS EOS. We then perform a multimessenger Bayesian inference study on a fiducial BHNS merger. We simulate its sGRB afterglow and GW emission, as input to this study, using recent models for both and take systematic errors into account.Results. The expected rates of a combined GW and radio detection with the current generation GW detectors are likely low. Due to the much increased sensitivity of future GW detectors like the Einstein Telescope, the chances of an sGRB localisation and radio detection increase substantially. The unknown distribution of the BH spin has a big influence on the detection rates, however, and it is a large source of uncertainty. Furthermore, for our fiducial BHNS merger we are able to infer both the binary source parameters as well as the parameters of the sGRB afterglow simultaneously, when combining the GW and radio data. The radio data provides useful extra information on the binary parameters such as the mass ratio but this is limited by the systematic errors involved. Conclusions. The probability of finding an sGRB afterglow of a BHNS merger is low in the near future but rises significantly when the next generation GW detectors come online. Combining information from GW data with radio data is crucial to characterise the jet properties. A better understanding of the systematics will further increase the amount of information on the binary parameters that can be extracted from this radio data.

Pulsar revival in neutron star mergers: multi-messenger prospects for the discovery of pre-merger coherent radio emission

“…This is because the more massive star in a binary has a shorter lifetime, will undergo supernovae first and is more likely to result in a BH, such that the younger compact object is the (unrecycled) NS. Therefore such systems may still contribute significantly to the volumetric rate of coherent bursts from compact object mergers, and will also emit multi-wavelength and multi-messenger signals discussed in Section 4 that can provide paths for detection (McWilliams & Levin 2011;Boersma & van Leeuwen 2022). Lastly, the gravitational wave detection horizon for NS-BH mergers is larger than for NS-NS mergers by a factor of 2 (for a recent detection see Abbott et al 2021), which may allow automatic triggering of buffer boards or observations on a much larger number of gravitational wave events than discussed in Section 4.3 10 .…”