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

Abstract

The High Energy X-ray telescope (HE) 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 (μs) and large effective area (up to thousands cm2). 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/HE data, our targeted search recovered 40 GRBs, which were detected by either Swift/BAT or Fermi/GBM but too weak to be found in our blind search. With this coherent search pipeline, the GRB detection sensitivity of Insight-HXMT/HE is increased to about 1.5E-08 erg cm−2 (200 keV–3 MeV). We also used this targeted coherent method to search Insight-HXMT/HE data for electromagnetic (EM) counterparts of LIGO-Virgo GW events (including O2 and O3a runs). However, we did not find any significant burst associated with GW events.

Insight-HXMT dedicated 33-day observation of SGR J1935+2154 I. Burst Catalog

“…Indeed, Insight-HXMT/HE has been served as a wide-field gamma-ray monitor for GRBs, mainly using the CsI detectors of HE Cai et al 2021). We find that some short GRBs (nominally with duration less than 2 s) would look similar to the SGR bursts in NaI detectors when they incident the HE telescope from the front side.…”

“…The blind search is based on the signal-to-noise ratio (i.e. SNR) method which has been applied to gamma-ray burst search in the HE CsI detectors of Insight-HXMT (see Cai et al (2021) for more details). This method is used to search for magnetar bursts in the HE NaI detectors of Insigth-HXMT.…”

“…Bottom: The difference between the raw light curve and the light curve after removing spikes shown in the top panel. of Insight-HXMT to jointly search for bursts (e.g., (Guidorzi et al 2020a,b)), covering a series of timescales ranging from 5 ms to 40 ms, with two phase offsets ( (Cai et al 2021)). Given the limited counts for weak bursts, Poisson statistics is assumed (e.g., (Younes et al 2020)).…”

“…Therefore, in this paper, considering the quality of the burst sample and available data, we require that a burst should be observed by at least two telescopes during the burst search and verification, thus three telescopes of Insight-HXMT could be divided into four combinations: HE&ME, HE&LE, ME&LE and HE&ME&LE. Following the experience in previous search studies (Cai et al 2021;Guidorzi et al 2020a,b), all un-blinded detector units are summed together for ME and LE, respectively, while the 17 NaI units of HE are used separately.…”

“…It is composed of three telescopes with different energy bands: the High Energy X-ray telescope (HE, 20-250 keV) (Liu et al 2019), the Medium Energy X-ray telescope (ME, 5-30 keV) (Cao et al 2019) and the Low Energy X-ray telescope (LE, 1-15 keV) (Chen et al 2019). With wide energy band and high time resolution, Insight-HXMT plays an important role in searching for the X-ray and gamma-ray transient, with duration from milliseconds to seconds, as gamma-ray bursts (GRBs) (Cai et al 2021), high-energy emission of FRBs (Guidorzi et al 2020a,b) and electromagnetic counterpart of gravitational waves (Li et al 2018;Cai et al 2021).…”