A Search for Neutron Star-Black Hole Binary Mergers in the Short Gamma-Ray Burst Population

Short gamma-ray bursts (SGRBs) are now known to be the product of the merger of two compact objects. However, two possible formation channels exist: neutron star - neutron star (NS - NS) or NS - black hole (BH). The landmark SGRB 170817A provided evidence for the NS - NS channel, thanks to analysis of its gravitational wave signal. We investigate the complete population of SGRBs with an associated redshift, and search for any divisions that may indicate that a NS - BH formation channel also contributes. We find several lines of evidence that support the hypothesis that SGRBs with extended emission (EE) constitute the missing merger population: they are unique in the large energy band-sensitivity of their durations, and have statistically distinct energies and host galaxy offsets when compared to regular (non-EE) SGRBs. If this is borne out via future gravitational wave detections it will conclusively disprove the magnetar model for SGRBs. Furthermore, we identify the first statistically significant anti-correlation between the offsets of SGRBs from their host galaxies and their prompt emission energies. We suggest that either the majority of high-offset SGRB hosts have been misidentified, or that a non-negligible fraction of bursts occur in globular clusters.