The Hitomi (ASTRO-H) hard x-ray telescope (HXT): current status of calibration

Abstract

The Japanese X-ray Astronomy Satellite, Hitomi (ASTRO-H) carried two hard X-ray telescopes (HXT), covering the energy band from 5 keV to 80 keV. In the initial functional verification phase of the onboard instruments, G21.5-0.9 and Crab nebula were observed with HXT. The data of G21.5-0.9 observation indicated that the hard X-ray imaging system worked well. Detail calibration of the Hitomi HXTs was performed with the observation data of Crab nebula. We extracted X-ray images of the Crab pulsar excluding the nebular emission, and confirmed that the imaging performance in orbit was satisfied with the requirement of the HXT. The 8-80 keV Crab spectrum was well fitted with a power-law model with the absorbed column of NH=3×1021 cm-2 . The estimated photon index of 2.122±0.003 was consistent with previous results of Crab observation, while the observed 2-10 keV flux of 2.3×10-8 erg s-1 cm-2 was slightly larger than the previous observation. We note that there was discrepancy between the simulated and the measured effective area on ground. Thus, we introduced a fudge factor to reproduce the effective area obtained in the ground calibration. The fudge factor of HXTs are included in the latest calibration database of Hitomi.