Acta Polytechnica


doi:10.14311/AP.2013.53.0803
Acta Polytechnica 53(Supplement):803–806, 2013 © Czech Technical University in Prague, 2013

available online at http://ojs.cvut.cz/ojs/index.php/ap

THE ASTRO-H MISSION

Yoshitomo Maedaa,∗, Tadayuki Takahashia, Kazuhisa Mitsudaa,
Richard Kelleyb, on behalf of the ASTRO-H Team

a Institute of Space and Astronautical Science (ISAS), JAXA, Kanagawa, 252-5210, Japan
b NASA/Goddard Space Flight Center, Greenbert, MD 20771, USA
∗ corresponding author: ymaeda@astro.isas.jaxa.jp

Abstract. A review of the Astro-H mission is presented here on behalf of the Astro-H collaboration.
The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions
initiated by the Institute of Space and Astronautical Science (ISAS). One of the main uniquenesses of
the ASTRO-H satellite is the high sensitivity and imaging capability of the wide energy band from
0.3 keV to 600 keV. The coverage is achieved by combining the four instruments of the SXS, SXI, HXI,
and SGD. The other main uniqueness is a spectroscopic capability not only for a point-like source but
also for an extended source with high spectral resolution of ΔE ∼ 4 ÷ 7 eV of SXS. Using the unique
powers of these instruments, ASTRO-H will address unresolved issues in high-energy astrophysics.

Keywords: X-ray, hard X-ray, gamma-ray, X-ray astronomy, gamma-ray astronomy, micro-calorimeter.

1. Introduction and current
status

ASTRO-H (formerly known as “NeXT”) is a facility-
class mission to be launched on JAXA H-IIA into
low Earth orbit in 2014 [1–3]. NASA has selected
US participation in ASTRO-H as a Mission of Op-
portunity. Up to now, more than 160 scientists from
Japan/US/Europe/Canada have contributed to the
ASTRO-H mission.

ASTRO-H will be launched into a circular orbit
with altitude 500 ÷ 600 km, and inclination 31 degrees
or less. The total mass at launch will be as heavy
as 2700 kg while the length is about 14 m in orbit.
ASTRO-H is a pointing observation-type satellite;
each target is pointed until the integrated observing
time is accumulated, and then slews to the next target.
All instruments introduced in the later section are co-
aligned and will operate simultaneously.
ASTRO-H has just completed the first half of the

critical design review (CDR-1) which is required before
making the system tests using the Thermal Test Model
(TTM) or The Mechanical Test Model (MTM) and
producing a part of the flight modules. In this paper,
we will summarize the mission concept and the current
baseline configuration of instruments of ASTRO-H. A
comprehensive introduction and most recent reviews
of the ASTRO-H mission will be presented at the
SPIE meeting in July, 2012 [3].

2. Spacecraft and instruments
ASTRO-H will carry three focusing optics-type tele-
scopes (SXS, SXI, HXI) and one Compton telescope
(SGD). The conceptual design of each instrument is
shown in Fig. 1, and the requirements and specifica-
tions of the instruments based on the base-line design
are summarized in Tab. 1.

The Soft X-ray Spectrometer (SXS) achieves high
resolution spectroscopy with imaging [4, 5]. SXS has
the X-ray Calorimeter Spectrometer (XCS) focused by
the Soft X-ray Telescope (SXT-S) [6, 7]. The energy
resolution of the XCS is as good as ≤ 7 eV in the
0.3 ÷ 12 keV band pass with an effective area at 6 keV
of ∼ 210 cm2. The detector array corresponds to a
field of view of 3.04 arcmin on a side.
A filter wheel (FW) assembly, which includes a

wheel with selectable filters and a set of modulated
X-ray sources will be installed between the SXT-S and
XCS. The FW is able to rotate a suitable filter into the
beam to optimize the count rate [9]. An high count
rate degrades the energy resolution power. In addition
to the filters, a set of on-off-switchable X-ray calibra-
tion sources, using light sensitive photo-cathode, will
be implemented. These calibration sources will allow
proper gain and linearity calibration of the detector
in flight.
The X-ray sensitive silicon charge-coupled devices

(CCDs) of the Soft X-ray imaging system (SXI) pro-
vide a low background and high energy resolution
tool. SXI will consist of a telescope (SXT-I) and the
focal plane detector “Soft X-ray Imager” [11, 12]. The
design of SXT-I is the same as that of SXT-S [6, 7].
SXI will use next generation Hamamatsu CCD chips
with a thick depletion layer of 200 µm, low noise, and
almost no cosmetic defects. SXI features a large FOV
and covers a 38 × 38 arcmin2 region on the sky, com-
plementing the smaller FOV of the SXS calorimeter.
The effective area is larger than one unit of the Suzaku
XIS.

The hard X-ray imaging system (HXI) consists of
the Hard X-ray Telescope (HXT) and its Hard X-
ray Imager focal plane detector and enables imaging
spectroscopy in the 5 ÷ 80 keV band. The HXT has

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Yoshitomo Maeda et al. Acta Polytechnica

Figure 1. Configuration of the ASTRO-H satellite. [3].

Parameter Hard X-ray Soft X-ray Soft X-ray Soft γ-ray
Imager Spectrometer Imager Detector
(HXI) (SXS) (SXI) (SGD)

Detector Si/CdTe micro X-ray Si/CdTe
technology cross-strips calorimeter CCD Compton Camera
Focal length 12 m 5.6 m 5.6 m –
Effective area 300 cm2@30 keV 210 cm2@6 keV 360 cm2@6 keV > 20 cm2@100 keV

160 cm2@1 keV Compton Mode
Energy range 5 ÷ 80 keV 0.3 ÷ 12 keV 0.5 ÷ 12 keV 40 ÷ 600 keV
Energy 2 keV < 7 eV < 200 eV < 4 keV
resolution (@60 keV) (@6 keV) (@6 keV) (@60 keV)
(FWHM)
Angular < 1.7 arcmin < 1.3 arcmin < 1.3 arcmin –
resolution
Effective ∼ 9 × 9 ∼ 3 × 3 ∼ 38 × 38 0.6 × 0.6 deg2
Field of View arcmin2 arcmin2 arcmin2 (< 150 keV)
Time resolution 25.6 µs 5 µs 4 s/0.1 s 25.6 µs
Operating −20 °C 50 mK −120 °C −20 °C
temperature

Table 1. Summary of the four instruments of ASTRO-H [3].

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vol. 53 supplement/2013 The ASTRO-H Mission

Figure 2. (top) The effective area of SXS. (below)
The energy resolution obtained from Mn Kα1 using
a detector from the XRS program but with a new
sample of absorber material (HgTe) that has lower
specific heat, leading to an energy resolution of 3.7 eV
(FWHM). SXS could have energy resolution approach-
ing this value [8].

conical-foil mirrors with depth-graded multilayer re-
flecting surfaces that provide a 5 ÷ 80 keV energy
range [13, 14]. The focal plane imager consists of
four-layers of 0.5 mm thick Double-sided Silicon Strip
Detectors (DSSD) and one layer of 0.75 mm thick
CdTe double sided cross-strip detector [15, 16].

The Soft Gamma-ray Detector (SGD) is a Comp-
ton telescope which is sensitive to a broad band of
40 ÷ 600 keV [17, 18]. The sensitivity at 300 keV is
10 times better than the Suzaku HXD. It outper-
forms previous soft γ-ray instruments in background
rejection capability by adopting a new concept of
narrow-FOV Compton telescope. The SGD can be
functioned in two ways. One is the “photo absorption”
mode in which the larger effective area is achieved but
the imaging is not functional. The other is the Comp-
ton mode in which the SGD is capable of measuring
polarization as well as image.

Figure 3. Detection limits of the SXT-I/SXI,
HXT/HXI and SGD for point sources as functions of X-
ray energy, where spectral binning with ΔE/E = 0.5
and 1000 ks exposure are assumed. [3].

3. Expected Scientific
Performance

In contrast to the high-resolution gratings of Chan-
dra/HEG and XMM-Newton/RGS, at E > 2 keV,
SXS is the more sensitive and has higher resolution
(Fig. 2). It is notable that in the Fe K band SXS has
10 times the collecting area and much better energy
resolution than Chandra/HEG. The SXS band covers
the critical inner-shell emission and absorption lines
of Fe I–XXVI between 6.4 and 9.1 keV often seen in
the spectra from AGN or Galactic BH candidates.

SXS uniquely performs high-resolution spectroscopy
of extended sources such as cluster of galaxies and
supernova remnants because it is non-dispersive. For
sources with angular extent larger than 10 arcsec, the
Chandra MEG energy resolution is degraded down
to the level of CCD. The energy resolution of the
XMM-Newton RGS is similarly degraded for sources
with angular extent ≥ 2 arcmin. SXS makes possible
high-resolution spectroscopy of sources inaccessible to
current grating instruments.

Figure 3 shows the broad-band sensitivity achieved
by combing the two instruments, HXI and SGD for
point sources. The sensitivity at 30 and 300 keV is two
orders and one order of magnitude better than the
Suzaku HXD. A cloud of cool gas has been spotted
approaching the supermassive black hole SgrA* which

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Yoshitomo Maeda et al. Acta Polytechnica

lies at the center of our Milky Way galaxy [19]. In
2013, one year before the ASTRO-H launch, the cloud
may reach the event horizon, starting a bright X-ray
flaring activity [20] that will shed new light on the
black hole’s feeding behavior. With its broad band
capability, ASTRO-H will catch the flare with a flux
brighter than 1035÷36 erg s−1 that will shed new light
on the black hole’s feeding behavior.

Acknowledgements
The authors are deeply grateful for on-going contributions
provided by other members in the ASTRO-H team in
Japan, the US, Europe and Canada.

References
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	Acta Polytechnica 53(Supplement):803–806, 2013
	1 Introduction and current status
	2 Spacecraft and instruments
	3 Expected Scientific Performance
	Acknowledgements
	References