Simulation of the in-flight calibration of the collimator alignment and PSF for HXMT

Abstract

The Hard X-ray Modulation Telescope (HXMT) is an X-ray astronomical satellite in 1-250 keV, consisting of three collimated instruments. We present the in-flight calibration approach of the collimator alignment and Point Spread Function (PSF) for HXMT, using both the direct fitting method and the imaging method. According to observational simulations of the Crab Nebula, we find that these two methods produce almost the same calibration accuracy of the alignment, and with a one-day scanning observation, the alignment can be calibrated to better than 0.45′ and 0.1′ along the wide and narrow directions of the Field of View (FOV) for a detector module, which corresponds to a localization accuracy of better than 0.1′ and meets the scientific requirement. HXMT, alignment, PSF, calibrationThe Hard X-ray Modulation Telescope (HXMT) [1, 2] is an X-ray satellite working in 1-250 keV, consisting of three collimated instruments: the High Energy X-ray telescope (HE, 20-250 keV), the Medium Energy X-ray telescope (ME, 5-30 keV) and the Low Energy X-ray telescope (LE, 1-15 keV). HXMT will perform an all-sky scan survey with the highest sensitivity and angular resolution in 20-250 keV by using the Direct Demodulation (DD) image reconstruction method [3,4]. It will also carry out pointing observations to investigate the temporal and spectral properties of compact objects such as black hole and neutron star binaries.For a collimator type X-ray telescope, the boresight and PSF of the collimator need to be accurately calibrated to eliminate the systematic errors and to obtain reliable scientific results. For HXMT, the boresights of all the collimators of HE, ME and LE are designed to be parallel to that of the star tracker (see Figure 1). In order to decompose the background from the observation data and to improve the sensitivity, the combined Field of View (FOV) method and the off-axis method [5] are employed, demanding that each of the HE, ME and LE is configured with several kinds of FOVs (i.e. PSFs).However, small deviations in both boresight and PSF are expected due to the fabrication uncertainty, launch vibrations as well as relaxation in zero gravity in orbit. The deviations of boresight and PSF will contribute errors to the source localization, background calculation, spacecraft attitude control, flux estimation of the observed sources and so on. Consequently, the accurate in-flight calibration of the collimator alignment and PSF is strongly required. Actually, alignment and PSF calibrations have been executed for almost all X-ray and gamma-ray telescopes, including collimator type telescopes, such as HEAO-1 [6], HEXTE (private communication), and non-collimator type telescopes, like HETE-2 [7], Suzaku [8] and Fermi [9].