[Paper Review] Background Model for the Low-Energy Telescope of Insight-HXMT

J. Y. Liao, Shu Zhang+17 more|arXiv (Cornell University)|Apr 3, 2020

Particle Detector Development and Performance4 citations

TL;DR

This paper develops a comprehensive background model for the Low Energy Telescope (LE) on Insight-HXMT using over 150 blank sky observations. It separates the background into particle-dominated (above 7 keV) and diffuse X-ray background (below 7 keV), using blind detector data for particle background estimation and sky-subtracted spectra for diffuse background, achieving <3.7% systematic error in 1–10 keV spectra for exposures >1 ks and <8% in light curves across time bins.

ABSTRACT

With more than 150 blank sky observations at high Galactic latitude, we make a systematic study to the background of the Low Energy Telescope (LE) of the Hard X-ray Modulation Telescope (dubbed as Insight-HXMT). Both the on-ground simulation and the in-orbit observation indicate that the background spectrum mainly has two components. One is the particle background that dominates above 7 keV and its spectral shape is consistent in every geographical locations. Another is the diffuse X-ray background that dominates below 7 keV and has a stable spectrum less dependent of the sky region. The particle background spectral shape can be obtained from the blind detector data of all the blank sky observations, and the particle background intensity can be measured by the blind detector at 10-12.5 keV. The diffuse X-ray background in the high Galactic latitude can also be obtained from the blank sky spectra after subtracting the particle background. Based on these characteristics, we develop the background model for both the spectrum and the light curve. The systematic error for the background spectrum is investigated with different exposures (T_exp). For the spectrum with T_exp=1 ks, the average systematic errors in 1-7 keV and 1-10 keV are 4.2% and 3.7%, respectively. We also perform the systematic error analyses of the background light curves with different energy bands and time bins. The results show that the systematic errors for the light curves with different time bins are &lt;8% in 1-10 keV.

Motivation & Objective

• To model the instrumental and cosmic background in the Low Energy Telescope (LE) of Insight-HXMT for accurate X-ray source detection.
• To separate the particle background (dominant above 7 keV) from the diffuse cosmic X-ray background (dominant below 7 keV) using in-orbit blank sky observations.
• To quantify systematic errors in background spectrum and light curve estimation as a function of exposure time and time binning.
• To develop a robust, exposure-dependent background model applicable to both spectral and temporal analysis of LE data.
• To enable accurate background subtraction for pointed observations, especially in regions with elevated diffuse background like the Galactic center.

Proposed method

• Utilized over 150 blank sky observations at high Galactic latitudes to characterize the background spectrum and light curve.
• Separated the background into two components: particle background (above 7 keV) and diffuse X-ray background (below 7 keV) based on spectral and spatial consistency.
• Used blind detector data (10–12.5 keV) to estimate particle background intensity, leveraging its consistent spectral shape across all sky locations.
• Subtracted particle background from blank sky spectra to isolate the diffuse X-ray background, which was found to be stable in high Galactic latitude regions.
• Applied a GTI selection optimization to remove flares and improve data quality in preliminary reduction.
• Quantified systematic errors in background spectra and light curves using Monte Carlo residual analysis across varying exposures (1–8 ks) and time bins (1–16 s).

Experimental results

Research questions

• RQ1 What are the dominant components of the background in the Insight-HXMT LE detector, and how do they vary with energy?
• RQ2 Can the particle background spectrum be reliably modeled using data from blind detectors, and is its shape invariant across different sky positions?
• RQ3 How does the systematic error in background spectrum estimation depend on observation exposure time?
• RQ4 What is the systematic error budget for background light curves across different time binning and energy bands?
• RQ5 How does the diffuse X-ray background vary across different sky regions, particularly near the Galactic center?

Key findings

• The particle background dominates above 7 keV and has a consistent spectral shape across all geographic locations observed.
• The diffuse X-ray background dominates below 7 keV and exhibits a stable spectrum in high Galactic latitude regions.
• The particle background intensity can be accurately measured using the contemporaneous flux from blind detectors in the 10–12.5 keV band.
• For a 1-ks exposure, the systematic error in the 1–10 keV background spectrum is 3.7%, decreasing to 2.0% for an 8-ks exposure.
• Systematic errors in background light curves are below 8% across all tested time bins (1–16 s) in the 1–10 keV band.
• The diffuse X-ray background in the Galactic center region is approximately twice as high as in high Galactic latitude regions, necessitating region-specific modeling for pointed observations.