Search for gamma-ray bursts and gravitational wave electromagnetic counterparts with High Energy X-ray Telescope of Insight-HXMT

The High Energy (HE) X-ray telescope on board the Hard X-ray Modulation Telescope (Insight-HXMT) can serve as a wide field of view (FOV) gamma-ray monitor with high time resolution (mu s) and large effective area (up to thousands cm(2)). We developed a pipeline to search for gamma-ray bursts (GRBs), using the traditional signal-to-noise ratio (SNR) method for blind search and the coherent search method for targeted search. By taking into account the location and spectrum of the burst and the detector response, the targeted coherent search is more powerful to unveil weak and sub-threshold bursts, especially those in temporal coincidence with gravitational wave (GW) events. Based on the original method in literature, we further improved the coherent search to filter out false triggers caused by spikes in light curves, which are commonly seen in gamma-ray instruments (e.g. Fermi/GBM, POLAR). We show that our improved targeted coherent search method could eliminate almost all false triggers caused by spikes. Based on the first two years of Insight-HXMT IHE data, our targeted search recovered 40 GRBs, which were detected by either Swift/BAT or Fermi1GBM but too weak to be found in our blind search. With this coherent search pipeline, the GRB detection sensitivity of Insight-FIXMTIHE is increased to about 1.5E-08 erg cm(-2) (200 keV-3 MeV). We also used this targeted coherent method to search Insight-HXMTIHE data for electromagnetic counterparts of LIGO-Virgo GW events (including O2 and O3a runs). However, we did not find any significant burst associated with GW events.

Automated summary

The High Energy X-ray telescope on board the Insight-HXMT can serve as a gamma-ray monitor with high time resolution and large effective area. By improving the coherent search method, we were able to detect 40 GRBs that were too weak to be found in blind search, but did not find any significant burst associated with GW events.