HXMT Background

HXMT is a low-earth orbit satellite, with an altitude of ~five hundred and fifty kilometer and an inclination of forty-three degree. It is a collimated X-ray telescope and has a broad energy range, from 1 kev to 250keV.
Besides the three main scientific payloads, HXMT also has 18 plates of anti-coincidence detectors, covering HE collimators and scintillators, used to veto background events induced by charged particles. Three particle monitors (PM) are placed on the top of HXMT and are used to monitoring the space environment. Besides, the space environment monitor (SEM) can give the electron and proton spectra in SAA and their direction distribution.
Based on Geant4 simulation, the estimated in-orbit background level of HE is ~ 5.5e-4cnt/s/keV/cm2 after 100-days duration, which is consistent with the RXTE/HEXTE observation. HE is compared with HEXTE because of their similar sensitive detector and similar orbit parameters. The fluorescence lines of Ta, which is the main components of HE collimators, are clearly seen in the background spectrum. After PV phase, the radioactivity component dominates in the total background. The ME simulated background level is ~ 65cnt/s totally, and LE ~60cnt/s. In the LE background spectrum, cxb dominates before 7 keV, while protons become most important after 7keV.
Generally, the in-orbit background estimation methods for collimated instruments include:
1. on-off methods,
2. empirical model method,
3. background database method, and so on.
For HXMT, the empirical model method is suitable for HE, because its radioactivity background, while the datebase method may be available for ME and LE, considering the main background components are cxb and cosmic ray protons. If the modeled method is considered, many kinds of date can be used as the importance parameters that are correlated with the HE simultaneous background, such as the acd rate, the large events rate, CsI rate, the PM and SEM data, etc.
Beside these, due to its special design, i.e. the covered detectors and the different FOV detectors, another two methods could also be used, the combined FOV method and the off-axis method. Take HE as an example, among the eighteen modules, one is covered detector, another two large FOV detector, the rest 15 small FOV detectors. Beside these, although all of collimators are co-aligned with each other, they point to the same direction during pointing observation. While, the orientation of their major axes has 60 degrees difference between each other. For the 15 small FOV detectors, they can be classified into three groups according to their major axes orientation.
During pointing observation, there are two observation mode, one of which put the astronomical target in the center of the FOV, the other put the astronomical target slightly away from the FOV center.
During combined FOV observation mode, in principle, the observation of covered detector represents the leakage background contribution, and the difference between large and small FOV detectors represents the FOV contribution. This is the combined FOV background estimation method.
During off-axis observation mode, there are three kinds of observation data corresponding to the three FOV orientation groups. The number of unknown parameters are also three, the astronomical source, the leakage background, and the aperture background. Therefore, theoretically, the background could be derived from the observation data. This is called the off-axis background estimation method. The best off-axis angle, at which the instrument reaches best sensitivity theoretically, is found at the edge of FOV of two groups and about ~1.1deg away from FOV center along the major axis of the rest group’s FOV.
For more details, see “Simulation of the in-flight background for HXMT/HE” (Astrophys Space Sci, 360, 47. 2015).