First results of terrestrial gamma-ray flash observed by Insight-HXMT

Abstract

As China’s first X-ray astronomy satellite, the hard X-ray modulation telescope (Insight-HXMT) carries three sets of X-ray telescopes. The high energy X-ray telescope (Insight-HXMT/HE) could serve as an all-sky gamma-ray monitor with a detection area of up to 5000 cm² and energy range from about 200 keV to 3 MeV. These characteristics, together with the high orbital inclination angle (43°) of the satellite, make the HE very suitable for detecting terrestrial gamma-ray flashes (TGFs). In this work, we implemented a dedicated TGF search algorithm for Insight-HXMT/HE, and identified 282 bright TGFs in its first four years of operation. We made a systematic study on the properties of these TGFs, including trigger time, duration, intensity, as well as the lightning association. We found that TGFs detected in mid-latitude regions (30° to 43°) are rare and they do not exhibit significantly different properties compared with TGFs in low-latitude (within 30°). Interestingly, the hardness ratio of TGF measured by Insight-HXMT/HE seems to be independent of the TGF duration, which differs from previous studies. These results show that, despite the dedicated design for astronomical observation, Insight-HXMT/HE is a versatile instrument to study energetic radiation phenomena from the Earth.