J. Plasma Fusion Res. SERIES, Vol.7 (2006) 157-160

Calibration and Performance of the X-Ray Spectrometer
on-board Astro-E2
FURUSHO Tae, BOYCE Kevin R.1 , BROWN Greg V.1 , COTTAM Jean1 , FUJIMOTO Ryuichi, ISHISAKIYoshitaka2 , KELLEY Richard L.1 , KILBOURNE Caroline A.1 , MCCAMMON Dan3 , MITSUDA Kazuhisa,MORITA Umeyo2 , PORTER F. Scott1 , TAKEI Yoh and YAMAMOTO Mikio4
Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA),Sagamihara 229-8510, Japan
1 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA2 Tokyo Metropolitan University, Hachioji 192-0397, Japan
3 University of Wisconsin, Madison, WI 53706, USA
4 Miyazaki University, Miyazaki 889-2192, Japan（Received: 5 October 2004 / Accepted: 21 December 2005）

Abstract
The X-ray Spectrometer (XRS) is a non-dispersive X-ray spectrometer on-board the X-ray astronomy
satellite, Astro-E2, which is scheduled to be launched in 2005. The detector system consists of a 31-pixel
microcalorimeter array, and achieves an unprecedented energy resolution of ∼ 6 eV over the energy band
0.3–12 keV. Here we present the results of our ground calibration and performance tests that we have carriedout for the fully completed flight system of the XRS. Our latest results taken at ISAS with the XRS on thespacecraft show that the measured energy resolutions are about 5–6 eV FWHM at 6 keV for most of thepixels.
Keywords:
X-ray, calorimeter, astrophysic, spectroscopy, Astro-E2, XRS
1．Introduction
Astro-E2 is the fifth Japanese X-ray astronomy satellite developed under Japan-US international collaboration [1]. It is a rebuild of the Astro-E mission, which
was lost during launch in 2000, and is scheduled forlaunch in 2005. Currently, two large X-ray observatories are operating in orbit: Chandra from NASA, which
has a remarkable imaging capability (< 1 spatial resolution), and XMM-Newton from ESA (European SpaceAgency), which is superior in eﬀective area. Both carry
X-ray CCD cameras and dispersive grating spectrometers which have delivered various impressive results.
However, CCD cameras achieve an energy resolutionof about 100 eV at 6 keV at the best performance, andgrating spectrometers are chiefly usable for point-likesources in the soft energy band of less than 2 keV. AstroE2 is characterized by an extremely good energy resolution (∆E ∼ 6 eV at 6 keV) and wide energy coverage (0.3–700 keV) that will complement Chandra andXMM-Newton.
Astro-E2 has three co-aligned detectors: an Xray microcalorimeter array (X-Ray Spectrometer;XRS)[2, 3], four X-ray CCD cameras (X-ray Imaging
Spectrometer; XIS) [4], and well-type phoswitch scintillators in combination with silicon PIN diodes (HardX-ray Detector; HXD)[5]. The XRS and each XIS arelocated in the focal planes of five dedicated X-ray telescopes (XRT) [6], and cover the soft energy band of0.3–10 keV, while HXD extends the bandpass up to 700
keV. The spacecraft weighs approximately 1700 kg, andis about 5.0 m high as launched and 6.5 m high after deployment in orbit. It will be placed in a near-earth orbit
with a nominal altitude of 550 km, and an inclinationof 31◦ , using JAXA’s M-V rocket. As of October 2004,
final integration tests are in progress at ISAS. The left
panel of Fig. 1 is a photograph of the Astro-E2 spacecraft during a thermal-vacuum test phase. The detailed
description of Astro-E2 and its status are found at theweb site∗1 .
Since the XRS is a non-dispersive instrument, it isapplicable to extended sources such as galaxy clusters and supernova remnants. Thus, it will provide a
unique capability for studying physical state and chemC 2006 by The Japan Society of Plasma

Corresponding author’s e-mail: furusho@astro.isas.jaxa.jp∗1 http://heasarc.gsfc.nasa.gov/docs/astroe/prop tools/astroe2 td/

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Science and Nuclear Fusion ResearchCalibration and Performance of the X-Ray Spectrometer on-board Astro-E2

3.8m (without sunshade)

5 XRTFig. 2 View of the full 6 × 6, 3.8-mm-square XRS array.

four stages of cooling system inside a dewar: an adiabatic demagnetization refrigerator (ADR); superfluid
liquid helium; solid neon; and a Stirling cycle mechanical cooler. The total weight of the XRS dewar is about
400 kg, including the cryogens, and the expected lifetime is about 2.5–3.0 years. To reduce the incident Xray flux from bright sources, there is a filter wheel located about 400 mm above the sensor [8]. 55 Fe and 41 Ca
sources are attached to the filter wheel, and can be taken
in and out of the field of view for the drift monitoringin orbit. The Helium Insert, which includes the microcalorimeter array, the ADR, and a helium cryostat,and the signal processing electronics were developed byNASA/Goddard Space Flight Center (GSFC), while theneon dewar and the mechanical cooler were developedby ISAS and Sumitomo Heavy Industries, Ltd. (SHI).
Since the beginning of the Astro-E2 program, enormous eﬀorts were put into improving the performanceof the XRS based on our understanding of the original
XRS. The most significant improvement is the higher
energy resolving power of E/∆E = 1000 at 6 keV (FullWidth at Half Maximum; FWHM), which is a factor of
2 better than the original [9]. The high spectral resolution enables us to resolve Mn-Kα1 and Kα2 lines (11eV apart) much more clearly. With the newly added
mechanical cooler, a longer lifetime of the cryogens isexpected. A lower background should be achieved byusing the more collimated in-flight calibration source
which illuminates only one pixel. The XRS performance for both Astro-E and Astro-E2 is summarizedin Table 1.

XRS dewar
4 XIS and HXD (inside)
Fig. 1 A photograph of the Astro-E2 spacecraft during
the thermal-vacuum test at ISAS.

ical composition (from oxygen to nickel) of diﬀuse hotcosmic plasma. In addition, the XRS has superior energy resolution at high energies compared to any instruments so far in orbit, including grating spectrometers.
A detailed spectroscopy of Fe K lines (6–7 keV) will
enable us to perform plasma diagnostics as well as to reveal the dynamical motion on the order of ∼ 100 km s−1
from the Doppler shifts and line broadening.
To make the maximum use of its capability, it is very
important to understand the nature of the instrument andto calibrate its performance on ground. In this paper,
we briefly describe the design, performance, and calibration results obtained during ground testing.
2．Design of XRS
The sensor of the XRS is a 6 × 6 array of microcalorimeter pixels∗2 with HgTe absorbers attachedon silicon thermistors (Fig. 2) [3, 7]. The detector isoperated at 60 mK. The XRS precisely measures theenergy of an incoming X-ray photon by detecting thetemperature rise of a few mK resulting from the absorption of the photon. To maintain the sensor at a precisely
regulated cryogenic temperature, the XRS incorporates∗2 31 pixels are connected to readout electronics, including one calibration pixel with an 55 Fe source dedicated for gain drift monitoring.

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Calibration and Performance of the X-Ray Spectrometer on-board Astro-E2

Table 1 Performance of the original XRS of Astro-E and the new XRS of Astro-E2

Energy resolution (FWHM at 6 keV)
Field of view
Array format
Number of pixels
Pixel size
Lifetime

Astro-E12 eV
1.9 × 4.12 × 1632
1.23 × 0.318 mm< 2 years

3．Ground calibration
We have performed ground calibrations to measure
the energy resolution, energy scale, line spread function, quantum eﬃciency, filter transmittance, etc. The
characterizatiion and calibration of the sensor array
were performed at NASA/GSFC in early 2003 [10] andfrom December 2003 to February 2004. In March 2004,
the He Insert, which contains the detector and the ADR,
was shipped from US to Japan, and installed it into theflight dewar at SHI. Performance verification and calibration tests were carried out at SHI for four weeks in
July and August, with the neon tank filled with about
10 % solid neon. After the environment test of the XRS
dewar at ISAS, it was mounted onto the spacecraft in
September, and the neon tank was filled to 100 % withsolid neon. We then verified its performance through aone-week spacecraft running test in the end of September. The results presented here are mainly from the datataken at SHI and ISAS.

Astro-E2
6 eV2.9 × 2.96×6
31 +1(for calibration only)
0.624 × 0.624 mm
2.5–3 years

An electron impact X-ray source (Rotating TargetSource; RTS) with a target wheel including Ti, V, Fe,Co, Cu, Zn, GaAs and Au was used to produce fluorescence emission lines from 4.5 keV to 12 keV. We
generate the empirical enegy scale by fitting the measured pulse heights for the Kα lines in the RTS spectra
to a fourth order polynominal function. Figure 3 showsthe energy-corrected RTS spectrum and residuals to theline fits. Using only the simplest centroiding for thecomplex Kβ and Lα lines, the accuracy of the energy
scale has been achieved to be better than 1 eV in the
4.5–12 keV energy band.
3.2 Energy resolution and line spread function

The energy resolution and the line spread function(LSF; response to monocromatic X-ray photons) are
two other key parameters of the XRS. At NASA/GSFC,
we used a monochromator to measure the LSF, and at
SHI and at ISAS, we used a radioactive source of 55 Fe toverify it. Figure 4 shows a spectrum of Mn Kα1 and Kα2from the 55 Fe source attached on the filter wheel during the final spacecraft integration tests in the full flight
configuration at ISAS in the end of September, 2004.
The spectra of all the pixels except for two pixels whosecharacteristics are slightly worse than other pixels in the3.1 Energy scale

To determine X-ray energies of the incident photons
from the universe, it is necessary to know an energy
scale function to convert the measured pulse heights
into their X-ray energies. Hence, the energy scale isone of the most important properties of the XRS, andan accuracy of 1–2 eV is required.

Experiment: 04-09-26.06.45.27
Fit wave name: SumHist_fit

Counts

6000

4000

Fit Parameters
FWHM: 5.68 ± 0.027 eV
E_shift: -0.04 ± 0.012 eV
Amplitude: 6241.5 ± 2e+01 counts
y0: 0.0 ± 0.8 counts
χ 2: 6.12

2000

Residual

0200
0-200
5870

5880

5890
5900
Energy [eV]5910

5920eV

Fig. 4 Spectrum of MnKα1 and Kα2 from an 55 Fe sourcewith a ﬁtting to the instrinsic line shape and a
Gaussian instrumental function. The spectra of allpixels except for two were combined together. Theresolution was 5.68 ± 0.03 eV (FWHM).

Fig. 3 Composite energy spectra of Rotating Target
Source and residuals to the line ﬁts.

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Calibration and Performance of the X-Ray Spectrometer on-board Astro-E2

4．SummaryThe detailed ground calibration tests for the flightXRS instrument were carried out in US and Japan in
the last one year. On the spacecraft, an extremely low
noise environment has been realized, and the energy
resolution was better than 6 eV at 6 keV for most ofthe pixels. We also confirmed that the energy scale canbe reproduced with accuracy better than 1 eV across the
4.5–12 keV energy band, using an empirical fourth order polynominal function. The line spread function isexpressed well by a Gaussian function. Response matrices will be produced based on these results. One of
the remaining challenges is to understand the gain drift
due to the temperature change of the helium bath andthe neon tank interface. Futher study is needed using
the ground and the inflight data.
The XRS, the first X-ray microcalorimeter array inorbit, is almost ready for flight. The spectroscopic capability of the XRS will bring us completely new discoveries in cosmic plasmas and surely lead to great advances in high energy astrophysics.

10Number of channels

8642045678Resolution at 5.9 keV (eV, FWHM)

Fig. 5 Distribution of energy resolutions for each pixel
across the array. One pixel outlies in this plot,
which has an energy-dependant resolution of 14
eV at 6 keV.

higher energy band (see also Fig. 5), are combined together. The gray line shows the intrinsic line shape andthe black line shows the best fit curve with a Gaussian
instrumental function. The composite resolution was
5.68±0.03 eV (FWHM), which is even better than theresults of the ground calibrations at NASA/GSFC andSHI. The detector response has been measured to beentirely Gaussian to almost three orders of magnitude
down from the peak of the line or line complex.
Figure 5 shows a histogram of energy resolution foreach pixel across the array for a fit to Mn Kα lines.
The resolution distributes mainly between 5 and 6 eVwith a mean resolution of 5.4 eV. The resolution is only
slightly energy-dependent with an increase of about 1eV from 6 keV up to 10 keV. One pixel, which has
the strongest energy-dependant resolution of 14 eV at
6 keV, is not shown on this plot.

We would like to thank all NASA/GSFC I&T andSHI staﬀ working on the XRS. T. F. is supported by
the Japan Society for the Promotion of Science (JSPS)
Postdoctoral Fellowships.
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