In-orbit performance of ME onboard Insight-HXMT in the first 5 years

Introduction: The medium-energy X-ray telescope (ME) is a collimated X-ray telescope onboard the Insighthard X-ray modulation telescope (Insight-HXMT) satellite. It has 1728 Si-PIN pixels readout using 54 low noise application-specific integrated circuits (ASICs). ME covers the energy range of 5–30 keV and has a total detection area of 952 cm<inline-formula id="IEq1"><alternatives><math><msup><mrow></mrow><mn>2</mn></msup></math><tex-math id="IEq1_TeX">\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2$$\end{document}</tex-math><inline-graphic href="41605_2023_395_Article_IEq1.gif"></inline-graphic></alternatives></inline-formula>. The typical energy resolution of ME at the beginning of the mission is 3 keV at 17.8 keV (full width at half maximum, FWHM), and the time resolution is 255 <inline-formula id="IEq2"><alternatives><math><mi>μ</mi></math><tex-math id="IEq2_TeX">\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu $$\end{document}</tex-math><inline-graphic href="41605_2023_395_Article_IEq2.gif"></inline-graphic></alternatives></inline-formula>s. In this study, we present the in-orbit performance of ME in its first 5 years of operation. Methods: The performance of ME was monitored using onboard radioactive sources and astronomical X-ray objects. ME carries six <inline-formula id="IEq3"><alternatives><math><msup><mrow></mrow><mn>241</mn></msup></math><tex-math id="IEq3_TeX">\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{241}$$\end{document}</tex-math><inline-graphic href="41605_2023_395_Article_IEq3.gif"></inline-graphic></alternatives></inline-formula>Am radioactive sources for onboard calibration, which can continuously illuminate the calibration pixels. The long-term performance evolution of ME can be quantified using the properties of the accumulated spectra of the calibration pixels. In addition, observations of the Crab Nebula and the pulsar were used to check the long-term evolution of the detection efficiency as a function of energy. Conclusion: After 5 years of operation, 742 cm<inline-formula id="IEq4"><alternatives><math><msup><mrow></mrow><mn>2</mn></msup></math><tex-math id="IEq4_TeX">\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2$$\end{document}</tex-math><inline-graphic href="41605_2023_395_Article_IEq4.gif"></inline-graphic></alternatives></inline-formula>of the Si-PIN pixels were still working normally. The peak positions of <inline-formula id="IEq5"><alternatives><math><msup><mrow></mrow><mn>241</mn></msup></math><tex-math id="IEq5_TeX">\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{241}$$\end{document}</tex-math><inline-graphic href="41605_2023_395_Article_IEq5.gif"></inline-graphic></alternatives></inline-formula>Am emission lines gradually shifted to the high-energy region, implying a slow increase in ME gain of <inline-formula id="IEq6"><alternatives><math><mrow><mn>1.43</mn><mo>%</mo></mrow></math><tex-math id="IEq6_TeX">\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.43\%$$\end{document}</tex-math><inline-graphic href="41605_2023_395_Article_IEq6.gif"></inline-graphic></alternatives></inline-formula>. A comparison of the ME spectra of the Crab Nebula and the pulsar shows that the E–C relations and the redistribution matrix file are still acceptable for most data analysis works, and there is no detectable variation in the detection efficiency.