The ASTRO-H SXT performance to the large off-set angles

摘要

The X-ray astronomy satellite ASTRO-H, which is the 6th Japanese X-ray astronomy satellite and is renamed Hitomi after launch, is designed to observe celestial X-ray objects in a wide energy band from a few hundred eV to 600 keV. The Soft X-ray Telescopes (SXTs) onboard ASTRO-H play a role of collecting and imaging X-rays up to ～12 keV. Although the field of view of the SXT is ～15＇ (FWHM), due to the thin-foil-nested Wolter-I type optics adopted in the SXTs, X-rays out of the field of view can reach the focal plane without experiencing a normal double reflection. This component is referred to as "stray light". Owing to investigation of the stray light so far, "secondary reflection" is now identified as the main component of the stray light, which is composed of X-rays reflected only by secondary reflectors. In order to cut the secondary reflections, a "pre-collimator" is equipped on top of the SXTs. However, we cannot cut all the stray lights with the pre-collimator in some off-axis angle domain. In this study, we measure the brightness of the stray light of the SXTs at some representative off-axis angles by using the ISAS X-ray beam line. ASTRO-H is equipped with two modules of the SXT; one is for the Soft X-ray Spectrometer (SXS), an X-ray calorimeter, and the other is for the Soft X-ray Imager (SXI), an X-ray CCD camera. These SXT modules are called SXT-S and SXT-I, respectively. Of the two detector systems, the SXI has a large field of view, a square with 38＇ on a side. To cope with this, we have made a mosaic mapping of the stray light at a representative off-axis angle of 30＇ in the X-ray beam line at the Institute of Space and Astronautical Science. The effective area of the brightest secondary reflection is found of order ～0.1% of the on-axis effective area at the energy of 1.49 keV. The other components are not so bright (<5×10~(-4) times smaller than the on-axis effective area). On the other hand, we have found that the effective area of the stray light in the SXS field of view (～3＇×3＇) at large off-axis angles (>15＇) are ～10~(-4) times smaller than the on-axis effective area (～590 cm~2 at 1.49 keV).