The Hard X-ray Modulation Telescope, dubbed Insight-HXMT after it was launched into orbit on June 15th, 2017, is the first X-ray astronomical satellite in China. It contains 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). The main scientific objective of Insight-HXMT is to study neutron star and black hole X-ray binaries in the Galaxy by observing their temporal and spectral properties in 1–250 keV. Collimated telescopes usually have relatively high backgrounds that are comparable to the source fluxes. There is no active temperature control for the ME and LE detectors, whose working temperatures and response functions change significantly in space. Therefore, a good calibration of the background and energy response matrices is essential for obtaining reliable observational results.

Telescope performance can gradually degrade in space. The primary detectors for HE are NaI(Tl)/CsI(Na) scintillators. High energy charged particles can activate iodine element in the detectors, and the decay of the iodine isotope increases the background of the telescope at specific energies. The Si-PIN detectors of ME and the SCD detectors of LE are sensitive to radiation damage, which increases the noise level and worsens the spectral resolution of these detectors. The interaction between high energy particles and satellite materials can contribute additional backgrounds for these telescopes. Studying the evolution of telescope performance will reveal how often the instruments need to be calibrated to meet the accuracy requirements of scientific data analyses.

Six papers on the Insight-HXMT mission are included in this special issue,Footnote 1 three of which focus on the performances of the three telescopes. In the paper on HE by X. F. Li et al., the gain and energy resolution of the NaI(Tl) and CsI(Na) detectors, the performance of the pulse-shape discriminator, and system dead-time were reviewed over a span of five years. The overall performance of NaI(Tl) remained stable, whereas the gain of CsI(Na) continuously increased. The paper on ME by Y. Tan et al. presents the monitoring of the Si-PIN detectors using both onboard radioactive sources and observations of the Crab Nebula. After five years of operation, there were still 742 cm2 of normally operating Si-PIN detector pixels out of a total of 952 cm2 of pixels, and the gain of ME detectors slowly increased by approximately 1.43%. The paper by X. B. Li et al. presents the performance evolution of LE by utilizing observations of the Cas A supernova remnant, blank sky, and the Crab Nebula. Changes in LE were significantly greater than those in HE and ME. For example, the energy resolution at 6.4 keV worsened by 100 eV (FWHM) over the 5 five years. However, the response files were well calibrated. The uncertainty of LE gain was less than 20 eV, and that of the energy resolution was less than 15 eV. The systematic errors of the effective area curve of LE, shown by spectral fitting of the Crab Nebula, were less than 1.5% in 1–10 keV.

The paper entitled “Five-year in-orbit background of Insight-HXMT” presents the background evolution of the three telescopes, including their flux and spectral changes and geographical distributions. The background models are still valid even for the latest observations, implying that the knowledge of the satellite structure, composition as well as space environment is reliable and complete. For the fifth paper, the researchers used the HE and LE data to study the long-term evolution of the Crab pulsar. They found that the observed flux of this pulsar is closely correlated with its spin-down power, that is, it decreases slowly with time, which is consistent with results obtained by other space X-ray telescopes.

I would also like to note that the results presented in these papers will benefit not only scientific data analyses but also detector technologies. For example, the HE study has shown that the overall performance of NaI(Tl) remains stable, whereas the gain of CsI(Na) continuously increases. The reason for this is not yet understood and is worth exploring in the future because the scintillation lights of NaI(Tl) and CsI(Na) are collected and read out by the same photomultiplier tube and electronics.