On the Use of CHIME to Detect Long-duration Radio Transients from Neutron Star Mergers

The short gamma-ray burst (SGRB) GRB 170817A was found to be related to a binary neutron star (BNS) merger. It is uncertain whether all SGRBs are caused by BNS mergers and also under what conditions a BNS merger can cause an SGRB. As BNS mergers can cause SGRBs, afterglow observations will also provide an alternative measurement of the BNS merger rate independent of gravitational-wave observations. In previous work by Feng et al., the feasibility of the detection of afterglows was considered using a variety of radio observatories and a simple flux threshold detection algorithm. Here, we consider a more sophisticated detection algorithm for SGRB afterglows and provide an estimate of the trials factors for a realistic search to obtain an updated estimate of the possibility of observing afterglows with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). We estimate 893 and 312 afterglows per year can be detected using a 3 sigma confidence level threshold with two jet models, one with half-opening angle uniformly distributed in 6 degrees-30 degrees and the other uniformly distributed in 3 degrees-8 degrees with the median of 6 degrees. We also find that 88% and 98%, respectively, of the detectable afterglows for each jet-opening distribution are off axis, which are candidates for orphan afterglows. Our result predicts fewer detectable sources per year than the earlier analysis but confirms the essential conclusion that using CHIME to search for afterglows will be effective in constraining the astrophysical merger rate.