[Paper Review] In-flight calibration of the Insight-Hard X-ray Modulation Telescope

Xiaobo Li, Xufang Li+20 more|arXiv (Cornell University)|Mar 16, 2020

Particle Detector Development and Performance18 references9 citations

TL;DR

This paper presents in-flight calibration of the Insight-HXMT satellite's three X-ray telescopes (HE, ME, LE) using radioactive sources, fluorescence lines, and celestial calibrators like the Crab nebula. It achieves systematic errors below 2% in spectral fitting for HE (28–120 keV), less than 1.5% for ME (10–35 keV), and under 1% for LE (1–7 keV), with minor exceptions at the Si K-edge (1.839 keV).

ABSTRACT

We present the calibration of the Insight-Hard X-ray Modulation Telescope (Insight-HXMT) X-ray satellite, which can be used to perform timing and spectral studies of bright X-ray sources. Insight-HXMT carries three main payloads onboard: the High Energy X-ray telescope (HE), the Medium Energy X-ray telescope (ME) and the Low Energy X-ray telescope (LE). In orbit, the radioactive sources, activated lines, the fluorescence lines and celestial sources are used to calibrate the energy scale and energy resolution of the payloads. The Crab nebular is adopted as the primary effective area calibrator and empirical functions are constructed to modify the simulated effective areas of the three payloads respectively. The systematic errors of HE, compared to the model of the Crab nebular, are less than 2% in 28--120 keV and 2%--10% above 120 keV. The systematic errors of ME are less than 1.5% in 10--35 keV. The systematic errors of LE are less than 1% in 1--7 keV except the Si K--edge (1.839 keV, up to 1.5%) and less than 2% in 7--10 keV.

Motivation & Objective

• To refine the on-orbit calibration of Insight-HXMT’s three X-ray telescopes (HE, ME, LE) after launch.
• To improve energy scale and resolution calibration using in-flight radioactive sources, fluorescence lines, and celestial sources.
• To determine the effective area response using the Crab nebula as a primary calibrator with an empirical correction model.
• To quantify systematic errors in spectral fitting for all three instruments across their energy bands.
• To ensure high-precision timing and spectral analysis for bright X-ray sources using calibrated response matrices.

Proposed method

• Utilized in-flight radioactive 241Am sources embedded in AGC detectors to monitor and correct energy gain drifts in HE detectors.
• Applied fluorescence lines from activated materials and celestial sources (e.g., Crab nebula) to calibrate energy resolution and scale.
• Constructed empirical functions to correct simulated effective areas using simultaneous observations with NuSTAR and Crab nebula data.
• Reprocessed all Crab observations to generate instrument-specific response files at different exposure times.
• Calculated systematic errors and biases in the ratio of data to model (Crab spectrum) across energy channels using weighted mean and error propagation.
• Applied a simple absorbed power-law model (Γ = 2.11, N = 8.76 keV⁻¹cm⁻²s⁻¹, NH = 0.36×10²² cm⁻²) to calibrate effective areas.

Experimental results

Research questions

• RQ1 What are the in-flight energy gain and resolution stability levels for HE, ME, and LE detectors?
• RQ2 How accurate are the effective area calibrations of the three instruments using the Crab nebula as a standard candle?
• RQ3 What are the systematic errors in spectral fitting for HE, ME, and LE across their respective energy bands?
• RQ4 How do the calibration results compare with pre-launch ground calibration and external observations (e.g., NuSTAR)?
• RQ5 What are the dominant error sources affecting calibration accuracy at high energies and low energies?

Key findings

• The energy gain of HE detectors is stable within 1% accuracy after three months in orbit, with systematic errors below 2% in spectral fitting for energies below 120 keV.
• Systematic errors in HE increase to 2%–10% above 120 keV, primarily due to rising background levels and reduced Crab signal count.
• ME energy gain shows a slow evolution of less than 1% over time, with systematic errors in spectral fitting below 1.5% in the 10–35 keV band.
• LE energy gain uncertainty is less than ~20 eV in the 1–9 keV range, and systematic errors in spectral fitting are below 1% in 1–7 keV, except at the Si K-edge (1.839 keV, up to 1.5%).
• Systematic errors in LE rise slightly to less than 2% in the 7–10 keV band, with the highest precision achieved in the 1–7 keV range.
• The calibration model for effective areas, based on a Crab nebula power-law model with Γ = 2.11 and NH = 0.36×10²² cm⁻², successfully reduces calibration biases to within 2.8% for HE, 1.7% for ME, and 0.33% for LE in long exposures.