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Accurate background estimation is essential for spectral and temporalanalysis in astrophysics. In this work, we construct the in-orbit backgroundmodel for the High-Energy Telescope (HE) of the Hard X-ray Modulation Telescope(dubbed as Insight-HXMT). Based on the two-year blank sky observations ofInsight-HXMT/HE, we first investigate the basic properties of the backgroundand find that both the background spectral shape and intensity have long-termevolution at different geographical sites. The entire earth globe is thendivided into small grids, each with a typical area of 5x5 square degrees ingeographical coordinate system. For each grid, an empirical function is used todescribe the long-term evolution of each channel of the background spectrum;the intensity of the background can be variable and a modification factor isintroduced to account for this variability by measuring the contemporary fluxof the blind detector. For a given pointing observation, the background modelis accomplished by integrating over the grids that are passed by the track ofthe satellite in each orbit. Such a background model is tested with both theblank sky observations and campaigns for observations of a series of celestialsources. The results show an average systematic error of 1.5% for thebackground energy spectrum (26-100 keV) under a typical exposure of 8 ks, and<3% for background light curve estimation (30-150 keV). Therefore, thebackground model introduced in this paper is included in the Insight-HXMTsoftware as a standard part specialized for both spectral and temporalanalyses.

Background Model for the High-Energy Telescope of Insight-HXMT