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 (39 events) and search for any divisions that may indicate that an NS-BH formation channel also contributes. Though no conclusive dichotomy is found, we find several lines of evidence that tentatively support the hypothesis that SGRBs with extended emission (EE; seven events) 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 anticorrelation between the offsets of SGRBs from their host galaxies and their prompt emission energies.