Preamble

Vol. 66 No. 4

July 2025

Acta Astronomica Sinica Vol. 66 No. 4 Jul., 2025
doi: 10.15940/j.cnki.0001-5245.2025.04.002

The In-orbit Operation and Calibration of the Insight-HXMT Satellite

Li Xiaobo†, Song Liming, Jia Shumei, Zheng Shijie, Zhao Haisheng
(Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049)

Abstract

The "Insight" Hard X-ray Modulation Telescope (abbreviated as the Insight-HXMT or HXMT satellite), launched on June 15, 2017, marks the birth of China's first independently developed astronomical observatory-level X-ray telescope. Leveraging its comprehensive advantages of large detection area, broad energy band, high time resolution, and high energy resolution, the Insight-HXMT satellite has opened a new window for research on rapid hard X-ray variability and broadband spectral properties of black hole and neutron star systems. Having exceeded its designed lifespan, the satellite has been operating stably for over eight years, remains in good condition, and is expected to further extend its in-orbit service lifetime. As of October 2024, the Insight-HXMT satellite has issued seven public calls for observation proposals to the global scientific community, receiving 334 valid proposals and formulating 2,368 observation plans. Additionally, the satellite has released 13 batches of data to the public, with a cumulative data volume of 40 TB and a data release rate as high as 94%. The satellite also provides users with different versions of data analysis software and calibration databases, achieving an in-orbit calibration accuracy of approximately 2%, which meets the requirements for scientific analysis. Researchers from 17 international and 36 domestic research institutions have conducted scientific studies using Insight-HXMT data, publishing over 300 high-quality academic papers with approximately 7,300 citations in total.

Keywords: instrumentation: detectors, methods: data analysis, astronomical databases

1 Introduction

The "Insight" Hard X-ray Modulation Telescope (abbreviated as Insight-HXMT or HXMT) is China's first astronomical observatory-level X-ray satellite independently developed by China [1]. It was successfully launched on June 15, 2017, from the Jiuquan Satellite Launch Center into a 550 km altitude orbit with an inclination of 43°. The Insight-HXMT satellite can conduct wide-band, large-field X-ray surveys to discover new high-energy variable sources and new activities of known celestial objects. It is also capable of pointed observations of black holes, neutron stars, and other high-energy objects to study their short-timescale variability and energy spectra, thereby understanding the activity and evolution mechanisms of black holes and neutron stars. The satellite additionally possesses all-sky monitoring capability for gamma-ray bursts.

The Insight-HXMT satellite carries three main scientific payloads: the High Energy X-ray Telescope [2–3] (HE, operating energy range: 20–250 keV, geometric area: 5100 cm²), the Medium Energy X-ray Telescope [4–5] (ME, operating energy range: 8–35 keV, geometric area: 952 cm²), and the Low Energy X-ray Telescope [6–7] (LE, operating energy range: 1–10 keV, geometric area: 384 cm²), as shown in [FIGURE:1]. This detector configuration endows "Insight" with a broad X-ray detection energy band and large detection area in the 20–250 keV range. Meanwhile, the three payloads feature high time and energy resolution, small detection dead time, and the low-energy payload exhibits no photon pile-up effects in the soft X-ray band.

These characteristics enable "Insight" to uniquely study rapid multi-band X-ray variability of celestial objects, allowing exploration of regions closer to black hole event horizons or neutron star surfaces than previously possible, thus opening a new window for investigating the rapid variability and spectral properties of black holes and neutron stars. Since its launch, the satellite has provided valuable data services to the scientific community. Through compatibility testing with various analysis tools used by domestic and international users and cross-comparison with results from other satellites, the reliability and correctness of Insight-HXMT data products and user data analysis software have been verified. Based on these data and software, researchers have achieved numerous original and significant results: participating in the monitoring of the electromagnetic counterpart of the first binary neutron star merger gravitational wave event (GW170817) and placing the most stringent limits on the MeV-band radiation from this gravitational wave event [11–12]; repeatedly breaking records for direct measurements of the strongest magnetic fields in the universe [13–14]; discovering the highest-energy quasi-periodic oscillations in black hole binary systems [15]; for the first time confirming that fast radio bursts originate from magnetars [16]; and jointly with the GECAM telescope, precisely detecting the brightest gamma-ray burst to date [17].

2.1 Observation Proposal Collection and Evaluation

To fully utilize Insight-HXMT data and promote scientific output, the Insight-HXMT Science Center regularly issues public calls for observation proposals to the global astronomical community. Each proposal call is announced through an Announcement of Opportunity (AO) on the Insight-HXMT portal website. Our proposal calls not only cover annual observation proposals but also continuously accept Target of Opportunity (ToO) proposals through the proposal website (http://proposal.ihep.ac.cn/proposal/index.jspx) to enable rapid response to transient astronomical events or observation of temporarily appearing important scientific targets.

To date, the Insight-HXMT satellite has successfully conducted seven rounds of proposal calls. Detailed information for each round, including the call period, number of valid proposals received, observation requests, allocated observation time, and number of ToO targets, is recorded in [TABLE:1]. The seven proposal calls have accumulated 334 proposals, with their distribution across different research institutions shown in [FIGURE:2].

[FIGURE:1] shows the three main payloads on the Insight-HXMT satellite, covering a wide energy range of 1–250 keV. HE employs 18 thallium-doped sodium iodide (NaI(Tl)) crystal detectors, ME utilizes an array of 1728 SiPIN detectors, while LE is equipped with 96 Swept Charge Devices (SCD).

As an open observation platform for the international astronomical community, the daily operation and management of the "Insight" satellite are the responsibility of the "Insight" Science Center. The Science Center comprehensively supports the satellite's multi-target, multi-mode, multi-constraint observation requirements, providing a series of key services including efficient observation mission planning, standardized production and release of scientific data products, performance calibration of payloads, construction of space background models, and maintenance and updates of user data analysis software [8–10]. As of October 2024, the Science Center has publicly issued seven rounds of observation proposals to domestic and international scientific users, receiving 334 valid observation applications, formulating 2,368 observation plans, and publicly releasing 13 batches of data products totaling 40 TB, with a data release rate of 94%. This has provided observation and data services to over 500 researchers from 36 domestic universities and research institutes and 17 international research institutions.

After the proposal submission deadline, the Science Center conducts technical feasibility assessments on all submitted proposals and invites expert teams to perform scientific reviews to ensure that selected proposals both conform to the satellite's observation capabilities and satisfy observation constraints. Following a thorough review process, the selected proposals are ranked by grade.

To provide better user service and transparency, once the proposal grading is completed, all review results are published on the proposal website so that applicants can promptly check the status of their proposals.

2.2 Observation Plan Formulation

To optimize Insight-HXMT observation plans and meet user requirements, the Insight-HXMT Science Center must consider multiple factors to develop and execute observation plans while ensuring satellite safe operation.

Observation targets for Insight-HXMT primarily come from publicly solicited observation proposals from the international academic community or transient celestial activities. These observation requirements include various types such as small sky region scanning, short-term monitoring, joint observations, scheduled-time observations, and ToO observations.

Insight-HXMT observation proposals are divided into three grades: A, B, and C. Higher grades are prioritized in observation planning; for proposals of the same grade, joint observations and scheduled-time observations are given priority. When formulating observation plans, while satisfying satellite observation constraints (sun avoidance angle greater than 70°), the Science Center comprehensively considers proposal requirements, the South Atlantic Anomaly, Earth occultation, data reception, and other factors to maximize observation efficiency as much as possible. During proposal execution, Grade A proposals are guaranteed observation scheduling; those not executed due to reasons are automatically postponed to the next cycle. Grade B and C proposals are arranged as much as possible, with unexecuted ones not postponed. Based on these considerations, the Insight-HXMT Science Center first conducts overall planning for annual observation tasks. When executing observation tasks specifically, they are further refined into short-term observation plans every two days [10]. Additionally, to enable rapid response to transient celestial events, a rapid response mechanism has been established to handle such special observation applications (ToO observations).

As of September 30, 2024, Insight-HXMT has formulated 1,346 short-term observation plans, 65 ToO plans, 244 parameter update plans, and 713 gamma-ray burst plans, successfully completing all observation requirements. [TABLE:2] presents statistics of celestial sources observed by Insight-HXMT, including the number of different sources, observation times, and exposure time information. Most of the pointed observation time has been dedicated to observing black hole and neutron star X-ray binaries, which are also the fields where Insight-HXMT has produced the richest scientific output.

2.3 Data Product Production and Release

Given that Insight-HXMT observations are led by and deeply involve international astronomical users in data analysis, the Science Center converts raw data and auxiliary data from the satellite into standard data products after completing observations, provides instrument response files in the form of calibration databases to users, and designs standard data analysis procedures and algorithms, enabling users to focus on scientific analysis without needing to deeply understand instrument working principles and modes.

Insight-HXMT data products are designed to accommodate requirements for scientific research, engineering monitoring, and scientific quick-look, forming 13 levels of advanced data products totaling over 700 types. The project developed a "data template library software" to ensure consistency and traceability of data product files. The Science Center generates over 60 TB annually, approximately more than 2 million data product files at various levels, and has flexibly expanded over 100 product file types as needed for scientific research. According to Insight-HXMT's data management policy, data exceeding the proprietary period must be opened for download to all users, accessible through the Insight-HXMT website (http://archive.hxmt.cn/proposal). As of October 2024, 13 batches of Level 1 data products have been publicly released to scientific users, totaling 40 TB, with a data release rate of 94%. The remaining 6% of unreleased data are mainly those within the proprietary period. The data release status for each proposal round can be referenced in [FIGURE:3], which also shows the number of ToO observation IDs in each round.

2.4 User Data Analysis Software

The development of the Insight-HXMT Data Analysis Software package (HXMTDAS) was conducted after in-depth research into the satellite's data analysis procedures, algorithms, and data products, aiming to provide a data processing tool highly compatible with the internationally used HEAsoft package. HXMTDAS developed efficient intelligent data identification algorithms to address issues such as glitches caused by high-energy particle events, multi-pixel breakdown, instrument radiation damage, and dramatic changes in the space environment. These algorithms can automatically identify high-quality data, correct instrument response, and automatically match instrument background, significantly improving the automation level and accuracy of data processing.

HXMTDAS adopts a hierarchical architecture design, supporting multiple payloads to share the same common underlying layer, thereby possessing excellent scalability and inheritance. The software features low memory consumption, high computational speed, and good compatibility, making it convenient to install and run on servers and personal computers with different operating systems.

Since its release, the Insight-HXMT data analysis software has been operating stably for over six years. Based on user feedback and suggestions, the Science Center continuously optimizes and updates the software, having released nine versions to date, all downloadable from the official Insight-HXMT website (http://hxmten.ihep.ac.cn/software.jhtml). The software has gained wide recognition from domestic and international users, providing crucial technical support for scientific research on the Insight-HXMT project. Meanwhile, the framework and some common interfaces of HXMTDAS have been successfully applied to the data software development for the follow-up telescope of the Einstein Probe satellite, demonstrating its application potential for other astronomical satellite projects.

3 In-orbit Calibration

Monitoring payload performance and timely updating calibration databases constitute one of the main tasks of the Science Center during Insight-HXMT satellite operation. The Science Center monitors payload performance at multiple levels, including payload working parameters, quick-look analysis of detection data, and monitoring analysis of celestial sources, achieving high-precision monitoring of detector performance and effectively guaranteeing normal satellite observation and operation.

The Insight-HXMT satellite's detectors include compound crystals (18 units), semiconductors (1728 units), and swept charge devices (96 units), with large quantities and different responses. Their performance changes with time and space environment during in-orbit operation, posing challenges for high-precision calibration. To address these challenges, a high-precision detector performance response model was established for dramatic space environment changes, combining space and ground approaches. On the ground, through constructing accurate satellite mass models, interaction processes, and space environment models, extensive simulations and comparisons were conducted on in-orbit operation and ground calibration experimental results of payloads to ensure the effectiveness of payload simulations. After satellite launch, time- and temperature-dependent detector response models were established based on satellite working parameters and measured detector performance, with calibration parameters verified through joint observations [18].

To date, all 18 NaI(Tl) and CsI(Na) compound crystal detectors of the HE payload are operating normally. However, some detectors of the ME and LE payloads have been closed due to radiation damage in the space environment, including some hot detectors, excessively noisy detectors, and damaged detectors. Specifically, among the 1728 SiPIN pixels of the ME payload, 298 have been closed; among the 96 CCD detectors of the LE payload, 8 have been closed. The number of remaining detectors still satisfies scientific observation requirements.

3.1 Payload Gain and Energy Resolution

The operation of the Insight-HXMT satellite has provided us with valuable data to deeply understand the evolution of detector performance. We previously published the evolution of the three payloads' performance over more than five years in orbit in the journal Radiation Detection Technology and Methods (RDTM) [3, 5, 7]. In this work, we update the in-orbit performance of the three payloads.

When the HE payload observes empty sky regions or enters Earth occultation zones, four background spectral lines appear in the measured energy spectra due to activation of detector materials [3, 18]. These background lines can be used to calibrate HE gain. Since NaI(Tl) crystals suffer from deliquescence during ground storage, the resolution of all units is worse than during ground calibration. The width of the 31 keV line in orbit can be used to calibrate and evaluate changes in energy resolution of the 18 units. Meanwhile, we also use the 59.5 keV peak position and width from the onboard 241Am radioactive source and the 191 keV background line to monitor changes in gain and energy resolution of each unit over time, as shown in [FIGURE:4] and [FIGURE:5]. After three months in orbit, changes in gain and energy resolution of all units are relatively small, with most units showing a gradual improvement in energy resolution over time.

Each ME box carries two 241Am radioactive sources in orbit, which can illuminate 8 SiPIN pixels. By accumulating spectra from the radioactive sources during empty sky observations and using ground-calibrated gain coefficients, the central values and widths of the source spectral lines can be obtained. For pixels not illuminated by radioactive sources, calibration and monitoring can be performed using the silver line peak position (22.5 keV) during empty sky observations [5]. The gain and energy resolution variations of ME are shown in [FIGURE:6] and [FIGURE:7], demonstrating that ME gain variation does not exceed 1.5% and energy resolution variation does not exceed 3%. Such variations have minimal impact on data analysis, so ground-calibrated gain and energy resolution are still used in orbit.

LE uses spectral lines from supernova remnant Cas A observations and background lines from empty sky observations to verify the gain calibrated on the ground. Using observation data of Cas A from the MOS (Metal Oxide Semiconductor) detector onboard the European XMM-Newton (X-ray Multi-Mirror Mission) satellite, the theoretical energies of various lines from Cas A are obtained. If the ground-calibrated gain coefficients are applied to all line peak energies from Cas A, the gain continues to decrease over time, as shown in [FIGURE:8]. This indicates that LE gain has been continuously decreasing due to radiation damage after launch. We use a quadratic function to describe the evolution of different peak positions over time, so peak positions at any observation time can be obtained according to the evolution function. When Cas A cannot be observed due to sun avoidance angle constraints, gain changes can also be predicted through values from observations before and after. The evolution of LE energy resolution can also be characterized by the width evolution of Si, S, and Fe lines over time, as shown in [FIGURE:9]. Affected by radiation damage, LE energy resolution has also been continuously degrading.

3.2 Energy Response Matrix and Effective Area

Due to its stable flux, high brightness, and simple power-law spectrum in the 1–100 keV range, the Crab Nebula is often used as an in-orbit calibration source for hard X-ray telescopes. As a collimated telescope, Insight-HXMT can also use the Crab for calibration because of its relatively high background level and lack of pile-up effects in detectors. After subtracting the in-orbit background [19–21], the energy resolution obtained is shown in [FIGURE:10], which is then used to simulate the in-orbit effective area. The net counts from Crab observations for the three payloads are shown in [FIGURE:11], with the in-orbit background level indicated by the orange points. Finally, the simulated effective area is corrected using empirical functions, with the ratio distribution between corrected data and model shown in the bottom panel of [FIGURE:11]. Residuals for the three payloads are within 2% across most energy bands. This ratio structure indicates inaccuracies in effective area simulation and background estimation, which have been verified through joint observations of the Crab by the US NuSTAR (Nuclear Spectroscopic Telescope Array) satellite.

3.3 Timing Accuracy

Based on the design characteristics of the Insight-HXMT satellite time system, which utilizes the long-term stability of the standard second signal and short-term stability of the payload timing signals, an iterative calibration algorithm has effectively improved the timing precision of detected events. To address the new problem of long-term missing standard second signals during satellite operation, the timing algorithm was updated. The algorithm's effectiveness and systematic errors were verified through pulsar observations, forming an integrated workflow for the onboard time system, ground algorithms, and celestial source calibration. Currently, the solution precision of the Insight-HXMT time system is better than 16 μs, comparable to the intrinsic timing stability of calibration pulsars [22]. [TABLE:3] lists the timing offsets and systematic errors of various international X-ray astronomical satellites.

4 Summary and Outlook

As China's first astronomical observatory-level space X-ray telescope, the Insight-HXMT satellite has opened a new window for research on rapid hard X-ray variability and spectral properties of black holes and neutron stars, leveraging its comprehensive advantages of large area, broad band, high time resolution, and high energy resolution. It has achieved multiple major scientific results and provided an important astrophysical research platform for scientists both domestically and internationally, bringing profound impact to the development of China's space science and astronomy. Since its successful launch in 2017, the Insight-HXMT satellite has been operating stably for over eight years (designed lifespan of four years), remains in good condition, and will continue extended operation.

Since its launch and operation, the Insight-HXMT satellite has conducted over 300 joint observations with domestic and international space, ground-based optical, and radio telescopes, promoting the development of multi-messenger astronomy and obtaining large amounts of high-quality observational data on gamma-ray bursts, X-ray binary systems, supernovae, and other phenomena. Joint observations have demonstrated that Insight-HXMT's data format is correct, calibration procedures are accurate, and error estimation is reasonable, with its spectral and timing analysis results receiving cross-validation.

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