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Overview to the Hard X-ray Modulation Telescope (Insight-HXMT) Satellite

ShuangNan Zhang, TiPei Li, FangJun Lu, LiMing Song, YuPeng Xu, CongZhan Liu, Yong Chen, XueLei Cao, QingCui Bu, Ce Cai, Zhi Chang, Gang Chen, Li Chen, TianXiang Chen, Wei Chen, YiBao Chen, YuPeng Chen, Wei Cui, WeiWei Cui, JingKang Deng, YongWei Dong, YuanYuan Du, MinXue Fu, GuanHua Gao, He Gao, Min Gao, MingYu Ge, YuDong Gu, Ju Guan, Can Gungor, ChengCheng Guo, DaWei Han, Wei Hu, Yan Huang, Yue Huang, Jia Huo, ShuMei Jia, LuHua Jiang, WeiChun Jiang, Jing Jin, YongJie Jin, Lingda Kong, Bing Li, ChengKui Li, Gang Li, MaoShun Li, Wei Li, Xian Li, XiaoBo Li, XuFang Li, YanGuo Li, ZiJian Li, ZhengWei Li, XiaoHua Liang, JinYuan Liao, Baisheng Liu, GuoQing Liu, HongWei Liu, ShaoZhen Liu, XiaoJing Liu, Yuan Liu, YiNong Liu, Bo Lu, XueFeng Lu, Qi Luo, Tao Luo, Xiang Ma, Bin Meng, Yi Nang, JianYin Nie, Ge Ou, JinLu Qu, Na Sai, RenCheng Shang, GuoHong Shen, XinYing Song, Liang Sun, Ying Tan, Lian Tao, WenHui Tao, YouLi Tuo, ChunQin Wang, GuoFeng Wang, HuanYu Wang, Juan Wang, WenShuai Wang, YuSa Wang, XiangYang Wen, BoBing Wu, Mei Wu, GuangCheng Xiao, Shuo Xiao, ShaoLin Xiong, He Xu, LinLi Yan, JiaWei Yang, Sheng Yang, YanJi Yang, Qibin Yi, JiaXi Yu, Bin Yuan, AiMei Zhang, ChunLei Zhang, ChengMo Zhang, Fan Zhang, HongMei Zhang, Juan Zhang, Liang Zhang, Qiang Zhang, ShenYi Zhang, Shu Zhang, Tong Zhang, Wei Zhang, WanChang Zhang, WenZhao Zhang, Yi Zhang, Yue Zhang, YiFei Zhang, YongJie Zhang, Zhao Zhang, Zhi Zhang, ZiLiang Zhang, HaiSheng Zhao, JianLing Zhao, XiaoFan Zhao, ShiJie Zheng, Yue Zhu, YuXuan Zhu, Renlin Zhuang, ChangLin Zou

arXiv:1910.09613v1astro-ph.IMastro-ph.HE

TL;DR

Hard-X-ray astronomy requires instruments that address difficult photon collection and high background, motivating Insight-HXMT’s development from China’s X-ray program and direct demodulation work. The paper overviews the satellite’s multi-band payload, observing modes, calibration, ground systems, and early operations. By June 2018, Insight-HXMT had completed verification and calibration, conducted extensive surveys and observations, detected more than 60 GRBs, and constrained soft gamma-ray emission from GW170817.

  • Problem

    Hard-X-ray observations face difficult photon collection, high background, and low sensitivity, motivating a dedicated broad-band X-ray astronomy mission.

  • Method

    The paper presents an overview of Insight-HXMT’s multi-telescope, slat-collimator mission, including direct-demodulation imaging, observing modes, calibration, ground systems, and operations.

  • Results

    By June 2018, Insight-HXMT had completed performance verification and calibration, conducted more than 20 Galactic-plane surveys, observed 44 bright sources, detected more than 60 GRBs, and constrained GW170817 soft gamma-ray emission.

  • Takeaways & Limitations

    Early science results support Insight-HXMT’s performance for broad-band source studies, Galactic surveys, GRB detection, and gravitational-wave electromagnetic follow-up.

Abstract

from arXiv · show

As China's first X-ray astronomical satellite, the Hard X-ray Modulation Telescope (HXMT), which was dubbed as Insight-HXMT after the launch on June 15, 2017, is a wide-band (1-250 keV) slat-collimator-based X-ray astronomy satellite with the capability of all-sky monitoring in 0.2-3 MeV. It was designed to perform pointing, scanning and gamma-ray burst (GRB) observations and, based on the Direct Demodulation Method (DDM), the image of the scanned sky region can be reconstructed. Here we give an overview of the mission and its progresses, including payload, core sciences, ground calibration/facility, ground segment, data archive, software, in-orbit performance, calibration, background model, observations and some preliminary results.

1 Introduction

Insight-HXMT emerged from China’s development of X-ray astronomy and the direct demodulation method, addressing the challenges of hard-X-ray observation. Its mission concept evolved from an initial AGN survey toward broad-band observations of bright X-ray binaries.

  • Motivation: Hard-X-ray astronomy historically lagged soft-X-ray astronomy because high-energy photons were difficult to collect, producing high background and low sensitivity.The paper traces this disparity to the limited maturity of hard-X-ray detector and collection technologies.
  • Background: China’s X-ray astronomy program began with balloon-borne instruments for bright pulsars and binaries, leading to the direct demodulation method and the HXMT proposal.The direct demodulation method enabled image reconstruction using non-imaging instruments.
  • Mission evolution: First proposed in 1993, Insight-HXMT initially targeted a 20–250 keV all-sky survey for obscured supermassive black holes and an AGN catalog.That original objective was later achieved by INTEGRAL and Swift before Insight-HXMT was funded.
  • Mission status: Insight-HXMT was launched on June 15, 2017 as China’s first X-ray astronomy satellite, and the paper reviews its mission components and progress.The overview includes payloads, science goals, calibration, ground systems, software, performance, observations, and preliminary results.

2 The Insight-HXMT Mission

Insight-HXMT combines multiple telescopes, observing modes, slat-collimator fields of view, and particle monitoring to support pointed, scanning, and GRB observations. Its payload spans soft X-rays through MeV gamma rays, with dedicated systems for background control and environmental monitoring.

  • Mission modes: Insight-HXMT supports all-sky survey, pointing, and small-region scan modes, with scan observations designed to locate new sources and monitor known-source variability.The Galactic plane is divided into 22 patches, each 20 deg×20 deg, for small-area scans.
  • Payload: The satellite carries HE, ME, and LE telescopes plus the Space Environment Monitor for charged-particle monitoring.The telescopes are aligned to observe the same source simultaneously.
  • Scientific payload: The payload covers 1–15, 5–30, and 20–250 keV for LE, ME, and HE pointed or scan observations, while HE monitors 0.2–3 MeV across the sky.HE uses 18 NaI/CsI detectors, ME uses 1728 Si-PIN detectors, and LE uses SCD detectors.
  • Scientific payload: Slat collimators define telescope fields of view, and blocked detectors measure local background.The FWHM fields differ across instruments, including 1.1°×5.7° and 5.7°×5.7° for HE.
  • High Energy telescope: HE combines NaI(Tl)/CsI(Na) phoswich detectors, tantalum shielding, and plastic-scintillator veto detectors to observe X-rays and reject charged-particle events.Its CsI detector also supports all-sky monitoring beyond 200 keV, reaching about 3 MeV in low-gain or GRB modes.
  • Supporting instruments: ME uses a modular Si-PIN architecture, LE uses cooled SCD modules, and SEM measures particle spectra and directional fluxes for background estimation.SEM provides one data point every second and includes separate electron and proton spectrometers.

2.3 The Direct Demodulation Method

The Direct Demodulation Method reconstructs images from observations by solving statistically distorted observational equations under physical constraints.

  • Direct Demodulation Method: DDM suppresses statistical distortions while solving observational equations to recover celestial-source fluxes and positions.A non-negativity constraint on flux is an example of the physical constraints introduced during the solution process.

2.4 The Insight-HXMT ground calibration

Insight-HXMT calibration accounts for detector responses that vary with operating conditions, especially temperature. Dedicated monochromatic-beam facilities and fluorescence-line sources measure response matrices across the relevant energy ranges and conditions.

  • Calibration requirements: Detector energy resolution and signal amplitude depend on operating temperature, so calibration measures or estimates response matrices under different working conditions.The temperature dependence is illustrated for LE and also occurs in HE and ME.
  • Calibration facilities: Two dedicated facilities provide mono-energy X-ray beams for HE at 15–100 keV and ME/LE at 0.8–30 keV.Double crystal monochromators produce beams with roughly 0.1%-1% intrinsic energy dispersion.
  • Calibration facilities: Additional vacuum chambers use fluorescent iron, copper, molybdenum, and tin lines to supplement monochromatic-beam calibration across detector temperatures.These sources calibrate variations in the detectors’ energy response matrices.

3. The core sciences

Insight-HXMT’s core science program targets variable and transient Galactic sources, broad-band X-ray binaries, and GRBs in a relatively unexplored high-energy band.

  • High-cadence Galactic-plane surveys are expected to build a source catalogue capturing short-timescale behavior at hard X-rays.
  • The mission will scan the Galactic plane for new transients and monitor known variable sources.
  • Broad-band observations of X-ray binaries will probe dynamics and emission mechanisms in strong gravitational or magnetic fields.
  • GRB observations will cover a relatively rarely explored range from a few hundred keV to a few MeV.

4 The Insight-HXMT ground segment

The Insight-HXMT ground segment combines mission operations with science support, data services, planning, rapid ToO response, and analysis software for producing calibrated scientific products.

  • 4 The Insight-HXMT ground segment: The ground segment has mission and science components: MOGS handles commands, reception, preprocessing, and archiving, while SGS provides user, support, operation, and data centers.
  • 4.1 Data products: HXMT data are distributed in FITS format through level 0 primary data, level 1 user releases, and level 2 scientific results.
  • 4.2 Analysis software: HXMTDAS converts level 1 data into cleaned, calibrated event files and higher-level products through calibration, screening, and extraction steps.
  • 4.3 Science programs: The first AO round received 90 proposals covering over 300 targets and requested 331 days of normal observations plus 1140 days of ToO observations.
  • 4.4 Observation planning: Observation planning uses long-, medium-, and short-period plans, while short-term schedules can change frequently to support ToO observations.
  • 4.5 ToO response: After a ToO trigger, the SGS can arrange an observation within as little as 5 hours.

5. IN-ORBIT PERFORMANCE

Following its June 15, 2017 launch, Insight-HXMT completed platform, payload, and ground-segment testing successfully and entered scientific service after performance verification.

  • 5.1 Launch and commissioning: Insight-HXMT was delivered to its preset orbit about 600 seconds after launch from JiuQuan on June 15, 2017.
  • 5.3 Verification outcome: The performance verification phase found the satellite operating smoothly and healthily, with scientific observations beginning at the end of 2017.
  • 5.2 In-orbit measurements: HE in-orbit measurements showed readout-electronics dead time of about 6 μs.
  • 5.2 In-orbit measurements: In-orbit and ground spectra were compared for ME and LE pixels illuminated by calibration radioactive sources.
  • 5.2 In-orbit measurements: Particle monitors mapped the SAA region during June 15–July 11, 2017, verifying their functionality.

6 IN-ORBIT CALIBRATION

Insight-HXMT’s in-orbit calibration covered timing, PSF, energy response, effective area, and GRB-mode response, using pulsar observations, radioactive sources, scans, simulations, and Crab spectra.

  • 6. Overview: The calibration program also included timing, PSF, energy response, effective area, and GRB-mode calibration.
  • 6.1 Timing calibration: Timing calibration with Crab pulse profiles achieved accuracies of 22 μs for HE, 51 μs for ME, and 21 μs for LE.LE’s systematic pulse-profile shift was understood and can be corrected.
  • 6.2 PSF calibration: Crab scan observations were used to modify the ground PSF so it fitted the source signal better and reduced residuals.
  • 6.3 Energy response: Energy-response calibration addressed the energy-channel relation and resolution using radioactive-source lines and observed emission lines.
  • 6.4 Effective area calibration: ARF calibration used the Crab’s pulse-off emission as background and pulse-on emission as the known source spectrum while background modeling continued.
  • 6.5 GRB-mode calibration: GEANT4 simulations and Crab observations showed that platform structures used as a coded mask can localize a typical GRB to a few degrees.

7. IN-ORBIT BACKGROUND

Insight-HXMT required a dedicated instrumental-background strategy because conventional on/off measurements were unavailable. Its model uses blocked detectors and is checked against blank-sky observations through spectral residuals and residual-to-data ratios.

  • Conventional on/off background measurements were unavailable because Insight-HXMT was initially designed for scanning surveys rather than pointed observations.
  • Some detectors had their collimators completely blocked to estimate the instrumental background.
  • Figure 32 compares HE module No.15 blank-sky data with contemporary background measured by blind module No.16 and model-estimated background.
  • The blank-sky spectral assessment includes fit residuals and residual-to-real-data ratios for HE module No.15.

8. OBSERVATIONS

During its first year, Insight-HXMT conducted extensive pointed and scanning observations, with sky coverage including pointed targets, small-area scans, all-sky-survey tests, and slew tracks.

  • Over 680 pointings and roughly 460 scanning surveys were completed by June 2018, during Insight-HXMT’s first year in service.
  • Table 5 summarizes the Insight-HXMT observations conducted during its first year.
  • Figure 33 maps red-star pointed observations, green small-area scans, yellow all-sky-survey tests, and blue slew tracks through April 2018.

9. PRELIMINARY RESULTS

Insight-HXMT’s early science demonstrations span direct-demodulation imaging, compact-object timing and spectroscopy, gravitational-wave counterpart monitoring, and gamma-ray-burst detection.

  • More than 60 GRBs were detected by HXMT/HE by April 2018, including the short GRB 170921C detected at 12σ significance.
  • The DDM imaging method reconstructed a Galactic-center sky map from LE scan light curves.
  • Table 6 lists identified sources in the reconstructed Galactic-center map.
  • HE, ME, and LE detected low-frequency QPOs from the black-hole candidate MAXI J1535-571 during its low/hard outburst state.The exposure was 3 ks.
  • Pulse profiles from Swift J0243.6+614 evolved with the outburst and showed complicated patterns across HE, ME, and LE measurements.
  • Insight-HXMT monitored GW170817 around the gravitational-wave merger and GRB trigger and reported stringent soft-gamma-ray emission constraints.

10. SUMMARY

By June 2018, Insight-HXMT had completed calibration and entered normal operations, delivering broad-band observations and early science results that met mission requirements.

  • Insight-HXMT covers 1-15, 5-30, and 20-250 keV for pointed and scanning observations, plus 200-3000 keV for all-sky monitoring.
  • By June 2018, the mission had performed more than 20 Galactic-plane scans and monitoring campaigns, 183 normal observations, and 13 ToO observations on 44 bright sources.
  • More than 60 GRBs had been detected, while GW170817 follow-up constrained soft-gamma-ray emission from its counterpart.
  • Early science results indicated that the satellite, instruments, operations, and user-support software met mission requirements.
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