Preamble

Vol. 66 No. 4

July 2025

Acta Astronomica Sinica

doi: 10.15940/j.cnki.0001-5245.2025.04.001

A Special Issue on Insight-HXMT Scientific Research: Preface

ZHANG Shu†, ZHANG Shuang-nan
(State Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049)

Abstract

Since its launch in June 2017, the Insight Hard X-ray Modulation Telescope (Insight-HXMT) has achieved a substantial body of important research results in high-energy astrophysics. The satellite continues to operate smoothly in orbit and is expected to remain operational for several more years. To enable the Chinese astronomical community to more fully understand and utilize the observational advantages of Insight-HXMT—particularly in light of the recent launch and upcoming deployment of a series of domestic high-energy astrophysical missions—and to continue exploring its scientific potential, the Insight-HXMT research team presents this special issue comprising 11 review papers that systematically introduce the progress made by Insight-HXMT across various research directions.

Keywords: high-energy objects, observations

1 Introduction

International space-based high-energy astronomy has undergone half a century of substantial development since the first X-ray astronomical satellite UHURU in the 1970s. Technologically, this field has experienced a leap from collimated telescopes to focusing imaging telescopes, while the number of detected X-ray sources has grown from a few hundred to several hundred thousand. Numerous high-performance space telescopes have emerged since the turn of the century, exemplified by the United States' Chandra X-ray Observatory, whose ultimate spatial angular resolution and high-sensitivity detection capabilities in the soft X-ray band (0.5–10 keV) will remain unsurpassed for a long time to come. Consequently, international space high-energy astronomy has gradually evolved from "seeing farther and clearer" toward "seeing more precisely and accurately," with the latter achieved primarily through detectors featuring large effective area, broad energy coverage, and high performance to obtain light curves on shorter timescales and precise measurements of more physical quantities.

Launched on June 15, 2017, the Insight Hard X-ray Modulation Telescope (Insight-HXMT) represents China's first X-ray astronomy satellite. It features a broad X-ray energy band including the soft X-ray regime, with good energy resolution in the soft X-ray band, large detection area and high time resolution in the hard X-ray band, and a gamma-ray burst (GRB) mode that extends the energy coverage to the MeV range with an exceptionally large field of view. These characteristics provide Insight-HXMT with unique advantages for "seeing more precisely and accurately" when observing explosive compact objects, enabling important contributions to studies of the fundamental properties, evolutionary dynamics, and radiation mechanisms of compact objects, as well as preliminary investigations into the extreme physical laws in their vicinity.

The detailed studies of compact objects with Insight-HXMT primarily encompass gamma-ray bursts, accretion evolution of galactic compact objects, and investigations into their fundamental properties, with the overarching goal of understanding physical processes in strong gravitational and magnetic fields. Galactic plane scanning is employed to search for and monitor new explosive phenomena, providing potential observational supplements for studies of bursts and activities in both galactic and extragalactic compact objects. Through these observations, researchers can further investigate astrophysical phenomena and physical processes under conditions of extreme magnetic fields, strong gravitational fields, and extreme densities. Studies of strong magnetic field physics are realized through observations of cyclotron absorption lines, polar cap radiation, and fast radio bursts (FRBs) from magnetars; strong gravitational field research is characterized through observations of the reflection component of hard X-rays from accretion disks; and neutron star properties are investigated through observations of thermonuclear bursts, particularly photospheric radius expansion phenomena.

China's space astronomy has now entered an unprecedented period of prosperity. In addition to Insight-HXMT, subsequent missions including GECAM, Einstein Probe (EP), the Sino-French SVOM satellite, CATCH experimental satellite, and the flagship eXTP (enhanced X-ray Timing and Polarimetry mission) space observatory currently under development have been launched or are in progress. Against this backdrop, and to enable readers to comprehensively understand the observational characteristics and scientific research progress of Insight-HXMT—thereby facilitating the exploration of its research potential in combination with other Chinese space X-ray astronomy projects—this special issue has been compiled for the community. Insight-HXMT has achieved fruitful results in studies of extreme explosive events, including the publication of the first Chinese GRB catalog \cite{1}, the first confirmation of the magnetar origin of FRBs \cite{2}, and observations of the brightest GRB in human history \cite{3}, with related achievements and research progress reported separately in dedicated articles. Since its launch, Insight-HXMT has published approximately 300 papers. As a collimated telescope, Insight-HXMT's unique design incorporating blind detectors provides advantages in background estimation and model construction compared to classical ON/OFF collimated telescopes \cite{4}.

This special issue includes progress reports on the following aspects: X-ray binaries (accounting for 60% of the total papers), galactic plane scanning, background calibration, and instrument performance (15%), and isolated neutron stars (4%). The Insight-HXMT team has innovatively employed a "map method" combined with extensive observations of empty sky regions to estimate the instrument background, achieving background model precision that meets the requirements for detailed spectral analysis with Insight-HXMT \cite{5}. As China's first space X-ray telescope, no accumulated experience in detector calibration could be referenced or borrowed, yet calibration precision directly affects the details of physical interpretation derived from observational data. During its in-orbit phase, Insight-HXMT has primarily completed energy and temporal response calibration and correction through observations of sources such as Crab, achieving an in-orbit calibration precision of approximately 2% that meets scientific analysis requirements \cite{6}. Insight-HXMT has conducted over 3,000 scanning observations of the galactic plane, accounting for approximately 30% of total observation time, monitoring the variability characteristics of a batch of X-ray sources and producing a monitoring catalog \cite{7}. The telescope has observed seven isolated pulsars and four millisecond pulsars, obtaining high-precision spin pulse profiles and broad-energy phase-resolved spectra, revealing the phase evolution characteristics of spectral parameters \cite{8}.

X-ray binaries represent the most productive research direction for Insight-HXMT, encompassing both neutron star and black hole X-ray binary systems. For weakly magnetized neutron stars in low-mass X-ray binaries, studies have focused on the energy upper limits of quasi-periodic oscillations (QPOs) in Z and Sco X-1 and deciphering the possible origin of hard X-ray tails in their energy spectra \cite{9}. Insight-HXMT possesses unique advantages in probing the accretion environment of X-ray binaries through thermonuclear bursts; for instance, significant cooling signals of the corona by bursts have been observed in individual events, providing high-quality observational clues for studying coronal geometry and possible origins \cite{10}. For high-mass accretion neutron star X-ray binaries, Insight-HXMT has provided the first observational evidence for radiation pressure-dominated accretion disks, established a world record for the directly observed strongest magnetic field of a neutron star, and revealed detailed evolutionary relationships between cyclotron absorption lines in polar cap radiation and luminosity. These results suggest the possible existence of multipolar magnetic fields and intrinsic super-Eddington accretion in ultraluminous X-ray sources, challenging current popular radiation models for polar cap regions \cite{11}.

Insight-HXMT has achieved remarkable scientific output in black hole X-ray binary burst evolution and fundamental black hole properties. The telescope first extended the energy upper limit of QPO signals from the previous ~30 keV to above 200 keV, discovering a batch of high-energy QPO signals and establishing a physical picture of QPO generation through precession of twisted jets. By introducing new analysis methods, it has provided, for the first time, detailed phase evolution of spectral parameters associated with QPOs, offering a novel interpretive framework for revealing the physical origin of QPOs \cite{12}. Insight-HXMT's high-cadence, high-statistics observation mode enables the acquisition of detailed temporal and spectral information throughout all stages of outbursts, providing opportunities for comprehensive interpretation of disk-corona-jet characteristics and fundamental black hole properties \cite{13}. The telescope has produced a batch of black hole mass and spin measurements and, based on Insight-HXMT-led observational data, systematically studied and compared the differences between two classical methods for measuring black hole spin \cite{14}. Based on observations of MAXI J1820+070, Insight-HXMT systematically investigated magnetic field transport and jets in black hole X-ray binary systems, discovering for the first time the phenomenon of jets shrinking toward the black hole while simultaneously accelerating. Combined with multi-band temporal behavior during the decay phase of the outburst, it provided the first direct observational evidence for the formation process of a magnetically arrested disk \cite{15}.

The operation of Insight-HXMT has secured China a position in the international space X-ray astronomy observation field. Its rich scientific output, particularly breakthrough research results and observational discoveries in certain areas and directions, demonstrates Insight-HXMT's unique capabilities in compact object research. As this special issue illustrates, combining other satellites and ground-based facilities across broad energy bands, multi-wavelength coverage, multi-messenger approaches, and innovative analysis methods, Insight-HXMT possesses tremendous research potential and opportunities for further scientific breakthroughs in the future. Insight-HXMT observation data are now essentially fully open, and its stable operation will continue to provide sustained service for researchers both domestic and international.

References

Song X Y, Xiong S L, Zhang S N, et al. ApJS, 2022, 259: 46
Li C K, Lin L, Xiong S L, et al. NatAs, 2021, 5: 378
Zheng C, Zhang Y Q, Xiong S L, et al. ApJ, 2024, 962:
Rothschild R E, Blanco P R, Gruber D E, et al. ApJ, 1998, 496: 538
LIAO Jin-yuan. Acta Astronomica Sinica, 2025, 66: 37
LI Xiao-bo, SONG Li-ming, JIA Shu-mei, et al. Acta Astronomica Sinica, 2025, 66: 36
LIAO Jin-yuan, GUAN Ju, WANG Chen. Acta Astronomica Sinica, 2025, 66: 38
GE Ming-yu, LI Zhao-sheng. Acta Astronomica Sinica, 2025, 66: 46
BU Qing-cui. Acta Astronomica Sinica, 2025, 66: 45
CHEN Yu-peng. Acta Astronomica Sinica, 2025, 66: 44
JI Long. Acta Astronomica Sinica, 2025, 66: 43
ZHANG Liang, QU Jin-lu, ZHANG Shu. Acta Astronomica Sinica, 2025, 66: 41
MA Rui-can. Acta Astronomica Sinica, 2025, 66: 42
WANG Wei, ZHU Hai-fan. Acta Astronomica Sinica, 2025, 66: 39
YANG Shuai-kang, YOU Bei. Acta Astronomica Sinica, 2025, 66: 40