Hard X-ray Telescope for the FFAST project

1. The FFAST project

Hironori Matsumoto (Nagoya University) Miyata, Y., Furuzawa, A., Kunieda, H. (Nagoya Univ.),

Ishida, N., Furuta, H., Yamamoto, Y. (Tamagawa Engineering), Tsunemi, H. (Osaka Univ.) & FFAST team

2. The Hard X-ray Telescope (HXT) The FFAST telescope satellite carries a Hard X-ray Telescope (HXT). An Scintillator-Deposited CCD (SD-CCD; see poster P-7 Nagino et al.) is placed at the foci of the HXT, and this combination makes it possible to take pictures of the high-energy universe with X-rays up to 80keV. Also it is possible to obtain X-ray spectra with good energy resolution.

The HXT mirror employs tightly-nested, conically-approximated thin-foil Wolter-I optics (a schematic view is shown in the right figure).

X-ray source

http://www.x-ray-optics.com/

Image point

12m

The HXT utilizes two principles for reflecting X-rays: the total reflection for low-energy X-rays (E<10keV) and the Bragg reflection for high-energy X-rays (E>10keV). The mirror surfaces are coated with Pt/C depth-graded multilayers for the Bragg reflection.

A cross section of the ASTRO-H HXT mirror surface is shown in the left figure below. The layer thickness (d) gradually changes to reflect X-rays of various energies. In the case of the ASTRO-H HXT, the thickness ranges from 24 to 136Å. The right panel shows reflectivity curves for a single layer

(black; d=100Å), a multilayer (red; d=40Å, N=30, Γ=0.4), and a depth-

graded multilayer (blue; d=26-50Å, N=78, Γ=0.4), where N is the number of

layers, and Γ is a fraction of Pt.

4. Mirror Production Below is a summary of the mirror production of the HXT. The mirror production for the HXT of FFAST is now going on in Nagoya university.

Telescope satellite

Detector satellite

FFAST (Formation Flight Astronomical Survey Telescope) will cover a large sky area in the hard X-ray band up to 80 keV by a formation flight; two small satellites fly in tandem, and one of them carries a hard X-ray telescope and the other carries an SD CCD detecor. The hard X-ray telescope is designed and manufactured by Nagoya University.

The HXT of FFAST is designed based on the HXT of ASTRO-H (see the figure below); ASTRO-H is the 6th Japanese astronomical X-ray satellite and will be launched in 2015. The diameter and height of the mirror housing of FFAST are 450 mm and 400 mm which are the same as those for the ASTRO-H HXT. The number of nested mirrors is not yet fixed, though the number for the ASTRO-H HXT is 213.

The Hard X-ray Telescope for ASTRO-H. The design of the HXT for FFAST is almost the same as the HXT for ASTRO-H, but the number of the nested mirrors is different.

3. Mirror design As mentioned in section 2, the design of the HXT for FFAST is based on that for ASTRO-H. However, ASTRO-H will conduct mainly pointing observations, while FFAST will conduct mainly survey observations. Thus the grasp (𝑆Ω), which is defined as an effective area (𝑆) times a field of view (Ω), is also important for the HXT of FFAST. It would be possible to change the design of the ASTRO-H HXT to obtain larger 𝑆Ω.

First we studied the possibility of changing the space between mirror foils. If the foil spacing is increased, the number of nested foils is reduced and the effective area is decreased. However, the filed of view is increased. Thus it would be possible to obtain large 𝑆Ω with a reduced total weight.

Changing the foil spacing

𝑑 + Δ𝑑

Effective Area(Ω) Field of View (𝑆) Grasp(𝑆Ω) Weight

The effective area, field of view, and grasp are calculated for various energy of X-rays assuming the focal length of 12m as shown below. Also the total weight is shown. The grasp depends on the foil spacing very weakly as expected. The grasp will be maximum at around Δ𝑑 𝑑 ∼ 0.3. In this case, the total weight will be reduced by 11kg compared to the ASTRO-H HXT. However, increasing the foil spacing will increase the stray light. To determine the foil spacing, a detailed simulation study is required.

The critical angle of total reflection of Pt for X-rays with an energy of 30keV is 0.18deg. Thus it would be possible to use a single layer instead of the multi layer for foils of an incident angle less than 0.18 deg in order to enhance an effective area for low energy X-rays and to make a foil production process simple. However, the integrated effective area from 20 keV to 30keV is not increased.

The right figure shows the relation between the grasp and the angle; foils whose incident angle is less than that angle are covered by a single layer. If we use a single layer, the grasp for 30keV X-rays is decreased.

Thus using a single layer for inner foils is not a good idea for the HXT of FFAST. We use the same multilayer design of the ASTRO-H HXT.

Multilayer coated