Comparison of simulated backgrounds with in-orbit observations for HE, ME and LE onboard Insight-HXMT

TL;DR

This study compares simulated and actual in-orbit backgrounds of Insight-HXMT's three telescopes, validating the simulation model and identifying discrepancies, especially in low-energy line features, to improve understanding of instrument performance.

Contribution

It provides a detailed comparison between simulated and observed backgrounds for Insight-HXMT, confirming the accuracy of the mass model and highlighting areas for improvement.

Findings

Simulated backgrounds agree within 5-15% with observations.

Long-term background increases observed in HE and simulations.

Discrepancies in low-energy line features suggest uncertainties in detector modeling.

Abstract

Insight-HXMT, the first X-ray astronomical satellite in China, aims to reveal new sources in the Galaxy and to study fundamental physics of X-ray binaries from 1\,keV to 250\,keV. It has three collimated telescopes, the High Energy X-ray telescope (HE), the Medium Energy X-ray telescope (ME) and the Low Energy X-ray telescope (LE). Before the launch, in-orbit backgrounds of these three telescopes had been estimated through Geant4 simulation, in order to investigate the instrument performance and the achievement of scientific goals. In this work, these simulated backgrounds are compared with in-orbit observations. Good agreement is shown for all three telescopes. For HE, 1) the deviation of the simulated background rate after two years of operation in space is $\sim5\%$ from the observation; 2) the total background spectrum and the relative abundance of the $\sim$67\,keV line show long-term increases both in simulations and observations. For ME, 1) the deviation of simulated background rate is within $\sim15\%$ from the observation, and 2) there are no obvious long-term increase features in the background spectra of simulations and observations. For LE, the background level given by simulations is also consistent with observations. The consistencies of these comparisons validate that the Insight-HXMT mass model, i.e. space environment components and models adopted, physics processes selected and detector constructions built, is reasonable. However, the line features at $\sim$7.5\,keV and 8.0\,keV, which are obvious in the observed spectra of LE, are not evident in simulations. This might result from uncertainties in the LE constructions.