This chapter is a brief introduction to the satellite and its instruments, and is intended as a simplified guide for the proposer. Reading it thoroughly should provide the reader with the necessary information to understand the capabilities of the instruments at a level sufficient to prepare the feasibility section of an Astro-E2 proposal.
Astro-E2 is scheduled to launch from the Uchinoura Space Center (USC) on a
Japanese M-V rocket in February 2005. After launch, the satellite
with undergo an initial checkout phase lasting approximately 4 weeks,
including instrument turn-on and initial calibration. Science
observations will begin immediately after this initial phase and will
focus on using the XRS for as long as the cryogenic system lasts,
which is expected to be years.
Astro-E2 is in many ways similar to ASCA in terms of its orbit, the
pointing and tracking capabilities of the spacecraft, and uses the
same station (USC) for uplink and downlink. Therefore, the
operational constraints for Astro-E2 are also similar to those of ASCA.
Astro-E2 will be placed in a near-Earth orbit, with inclination of 31
degrees, orbit altitude of roughly 550 km, and orbital period of about
96 minutes. The maximum slew rate of the spacecraft is
degrees/min, and settling to the final attitude will take about
10 minutes. The normal mode of operations will have the spacecraft
pointing in a single direction for at least 1/4 day (10 ksec; but see
``raster-scanning'' below). With this constraint, most targets will
be occulted by the Earth for about one third of each orbit, but some
objects near the orbital poles can be observed nearly continuously.
The current projection is that the observing efficiency of the
satellite will be about 45%.
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The scientific payload of Astro-E2 (Fig. 2.2) consists of
three types of co-aligned scientific instruments. High-resolution
X-ray spectroscopy will be carried out with the X-ray Spectrometer
(XRS). The XRS is an X-ray calorimeter array capable of precisely
measuring the energy of an incoming X-ray photon (between 0.3-12 keV)
by detecting the temperature rise resulting from the absorption of the
photon. This instrument uses consumable cryogens, and it is expected
that its lifetime will be years. The observatory also
features four X-ray sensitive imaging CCD cameras (X-ray Imaging
Spectrometers, or XISs), two front-illuminated (FI; energy range
0.4-12 keV) and two back-illuminated (BI; energy range 0.2-12 keV),
capable of moderate energy resolution. The XRS and each XIS are
located in the focal planes of five dedicated X-ray telescopes. The
third type of instrument is the non-imaging, collimated Hard X-ray
Detector (HXD), which extends the bandpass of the observatory to much
higher energies with its 10-600 keV bandpass.
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All of the instruments on Astro-E2 operate simultaneously. Each of the
five co-aligned XRTs features an X-ray mirror with an angular
resolution (expressed as Half-Power Diameter, or HPD) of
(cf. Fig.2.6). Figure 2.3 shows the total
effective area of the XRS+XRT, which includes features due to the
elemental composition of the XRS and XRT. K-shell absorption edges
from the oxygen (0.54 keV), aluminum (1.56 keV), and nickel (8.33 keV)
in the blocking filters can be seen, as well as a number of weak
M-shell features between 2-3 keV arising from the gold in the XRT.
Of course, similar features exist in the total effective area of the
XIS+XRT (for both FI and BI CCDs), seen in Figure 2.4, but
the energy resolution of the XRS makes these more significant if they
coincide with an astronomical feature.
The XRS includes a array of individual detector
elements (pixels), of which 30 are active2.1,
arranged as illustrated on Fig.2.5. There is also
one calibration pixel used for gain tracking, set away from the main
array and illuminated by a collimated
Fe source (see
§ 6.2). As of this writing, the final assembly of
the XRT for the XRS (the XRT-S) is in progress and its HPD has not yet
been measured, but it is expected to be between 1.6 and 2.0
arcminutes. Gravity will also slightly distort the ground-based HPD
measurements (cf. Fig.2.6) for all the XRTs in as-yet
unknown ways. If the HPD is
(a conservative estimate), an
on-axis point source will put
% of its flux onto the XRS
array, which is
on the sky.
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The four XISs (cf. Fig.7.2) are true imagers, and have a
larger field of view (
), but significantly lower
spectral resolution than the XRS (
eV at 6 keV; see
Fig.2.9). The XISs do not require cryogenic cooling,
and should continue to operate well after the cryogenic system has
been exhausted.
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The HXD (cf. Fig.8.1) is a non-imaging instrument
with an effective area of cm
, featuring a
compound-eye configuration and an extremely low background. It
dramatically extends the bandpass of the mission with its nominal
sensitivity over the 10 - 600 keV band (cf. Fig.2.7).
The HXD consists of two types of sensors: 2 mm thick silicon PIN
diodes sensitive over 10 - 60 keV, and GSO crystal scintillators
placed behind the PIN diodes covering 30 - 600 keV. The HXD field of
view is actively collimated to
by the
well-shaped BGO scintillators, which, in combination with the GSO
scintillators, are arranged in the so-called phoswich configuration.
At energies below
keV, an additional passive collimation
further reduces the field of view to
. The energy
resolution is
3.0 keV (FWHM) for the PIN diodes, and
% (FWHM) for the scintillators (where
is energy in MeV).
The HXD time resolution for both sensors is 61
s. While the HXD
is intended mainly to explore the faintest hard X-ray sources, it can
also tolerate very bright sources up to
Crab.
Although this document emphasizes the XRS, each observation will contain data from all instruments since they are co-aligned and operate simultaneously.
Astro-E2 carries a 6 Gbit data recorder. Data will be downlinked to USC at a rate of 4 Mbps for a total of 2 Gbits per pass, up to 5 times a day. This allows a maximum of 10 Gbits of data to be obtained per day, but fewer passes may be available to Astro-E2 as it will share the use of USC ground station with other ISAS satellites2.2. Data can be recorded at 4 different rates: Super-High (524 kbps), High (262 kbps), Medium (131 kbps), and Low (33 kbps). The recording rate will be changed frequently throughout an observation, according to a sequence that will be determined by the operations team at ISAS. This is to optimize the selection of the data rates and the usage of the data recorder, taking into account the expected count rates supplied by the proposers. Thus an accurate estimation of the count rates is important for the optimization of the mission operation. We emphasize that proposers cannot arbitrarily choose the data recording rate.
We expect that on-source data will usually be recorded at High (during contact orbits, during which the satellite passes over USC) or Medium (during remote orbits, without USC passes) data rate. The Low rate will be used mostly for times of Earth occultations and SAA passages, as it is probable that the background rates in the XIS and HXD exceed their telemetry allocation limit at Low data rate. The XRS telemetry allocation is a constant 160 ct/s regardless of the total data recording rate, while the telemetry limits for the XIS are presented in Chapter 7. Unless otherwise specified, the XIS data mode will be chosen for each data recording rate used to prevent telemetry saturation, based on the count rate supplied by the proposer.
The truly new and unique aspect of the Astro-E2 mission will be the
availability of the XRS to obtain high-resolution X-ray spectra of a
wide variety of celestial sources. The XRS differs from a grating
spectrometer in two important ways. First, it is a non-dispersive
detector so it can measure high-resolution spectra of extended sources
as well as point source. Secondly, since the XRS energy resolution is
nearly constant the resolving power increases with energy rather than decreasing.
The XRS absorbs the energy of each photon in a pixel
(one of the 30 active elements of the XRS detector), which is then
converted into heat (see Fig.6.3). This results in a
change of the temperature of the pixel
, which is roughly
, where
is the heat capacity of the
pixel. As discussed in Chapter 6, maintaining the
sensitivity of the detector requires keeping its operating temperature
to a minimum; at the operating temperature, the typical
of all
constituents of an XRS pixel is
erg/K. With this, a 1
keV photon with
of
erg produces
of
mK. The energy resolution of the XRS is
thus related to the accuracy with which the above
can be
measured. The pre-launch energy resolution
of the flight
detector pixels is 6.5 eV (FWHM) at 6 keV. With adequate counts, the
centroid energy of monochromatic photons can be determined to
eV; this is likely to be limited by the systematics of
the instrument, and specifically, the temperature stability of the
cooling system, but should be aided by the on-board calibration
sources (cf. §6.2). While at low energies, X-ray
spectrometers such as the LETG on Chandra and RGS on XMM-Newton have better
energy resolution, in the crucial Fe K region (6 - 7 keV), the energy
resolution of the XRS is superior (by
) to the
Chandra HETG (see Figs.2.8 & 2.9). Note that there are essentially no user-specified parameters for the
XRS, except for the setting of the Filter Wheel (cf.
§6.2), which is external to the instrument itself.
While both XMM-Newton and Chandra have high resolution spectroscopy
capabilities, there are several aspects where the Astro-E2 XRS will
provide unique data. First, both XMM-Newton and Chandra rely on dispersive
spectroscopy using diffraction gratings, and unambiguous spectra can
be obtained only for point or point-like sources with angular extent
(or small, well-defined structure) which is smaller than or comparable
to the PSF of the telescope. There are many classes of diffuse
celestial X-ray emitters (such as supernova remnants and clusters of
galaxies) with a larger angular size, and these simply cannot be effectively studied with gratings. The XRS, on the other hand, is
a non-dispersive instrument, and can study these sources and so
it will open a new, unique window on the X-ray Universe. Secondly,
the XRS has superior resolution at high energies compared to existing
X-ray telescopes. The XMM-Newton gratings have essentially no effective
area above keV and above
keV, the energy resolution
of the Chandra gratings is inferior to that of the XRS. Therefore,
the high resolution spectroscopy with the XRS lends itself well
towards answering such questions as:
By comparison with some other instruments (such as proportional
counters) the time to detect and measure the energy of an X-ray
photon with the XRS is relatively long-on the order of 100
milliseconds (although the arrival time can be measured to a few tens
of microseconds). As a result, the throughput of events whose energy
can be determined with high resolution will drop as the counting rate
increases beyond photons s
per pixel. The on-board
signal processing (see §6.3) distinguishes ``high
resolution'' events (entirely uncontaminated by temporally neighboring
events), ``medium resolution'' events (slightly contaminated events,
with
), and
``low resolution'' events, with
. Note that this limitation affects each pixel
independently, since all the channels are processed separately:
even when the count rate on one pixel may be excessive, another pixel,
receiving a lower photon rate, may be unaffected. As a result,
considering the point spread function of the XRT, it is possible that
the XRS pixels close to the nominal source position may be strongly
affected, while the pixels farther away from the peak may yield mostly
``high resolution'' and ``medium resolution'' events. The relative
fractions and event rates are illustrated in Figures. 2.10 &
2.11, which assume a Poisson distribution of arrival times
between events from a point source.
The relative fraction of event types will also be affected by cosmic
rays impinging on the XRS. Cosmic rays can heat the entire array,
leading to false events on every pixel. The rate will have to be
measured on orbit, but a conservative estimate is count/s.
The effect of these cosmic rays will be to decrease the number of high
resolution events, as it puts a lower floor on the count rate in any
pixel even for faint sources. Since the Hi-res record length is 0.16
s, the fraction of time affected by cosmic rays is 0.5 counts/s
s = 8%. In earlier X-ray detectors such as CCDs or
proportional counters, this would be considered unusable ``deadtime.''
In the XRS, cosmic rays do not cause true deadtime, but instead
migrate events toward lower resolution.
For the studies of very bright sources, there is a filter wheel
located in the X-ray beam between the mirror and the detector, capable
of reducing the photon flux incident on the detector. This filter
wheel (see § 6.2) has three different filter
elements; OPEN, BE, and ND, with optional Fe calibration
sources that illuminate the detector. BE is a soft X-ray cutoff filter
containing 300
m of beryllium; its effect can be seen in
Figure 2.3. ND is a neutral density filter that is
partially opaque with energy-independent transmission of 10%. It is
important to stress that except for the Crab nebula, there are no
known extended sources such as clusters of galaxies or supernova
remnants that would cause enough pulse pileup to warrant the use of
any filters in the filter wheel. We thus expect that the filter wheel
should be necessary only for bright Galactic binary sources (with flux
in excess of
milliCrab).
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The XRS has a relatively modest field of view, covering only
. For sources with larger angular extent, multiple
pointings may be required, increasing the integration time needed to
map a large diffuse region. In addition, for point sources, the energy
resolution of the XMM-Newton RGS or Chandra LETG is superior below
keV.
The XRS excels for observations such as:
The XRS is less appropriate for:
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Observations for scientific instrument calibration are to start
3 weeks after the launch. Those to calibrate the functions
essential for achieving the mission goals have the highest priorities,
and hence, all of them will be carried out by the end of Phase-1A.
After that, long term variation of the functions will have been
monitored every half a year or so. The list of all the calibration
targets during Phase-1A can be found on the Astro-E2 websites listed in
Appendix C.
Table 2.2 summarizes the calibration items of all
scientific instruments and their expected accuracy.
S/S | Calibration Item | Requirement | Goal |
XRT | absolute effective area (2-10 keV in XIS) | 5% | 5% |
XRT-S to XRT-I effective area cross-calibration | 5% | 5% | |
vignetting for all XIS f.o.v. | 2% | 2% | |
on-axis PSF (for all integration radii from 1![]() ![]() |
1% | 1% | |
off-axis PSF (as on-axis but for all XIS f.o.v.) | 2% | 2% | |
optical axis position in XRS | ![]() |
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|
optical axis position in XIS | ![]() |
![]() |
|
XRS | energy scale | 2 eV | 1 eV |
line spread function (FWHM) | 1 eV | 1 eV | |
effective area | 10%![]() |
5%![]() |
|
absolute timing | 300 ![]() |
100 ![]() |
|
relative timing | 80 ![]() |
10 ![]() |
|
XIS | Q.E. at 10 keV | 5%![]() |
5%![]() |
energy scale | 0.1%![]() |
0.1%![]() |
|
resolution (FWHM) at 5.9 keV | 1%![]() |
1%![]() |
|
Q.E. vs position | 5%![]() |
5%![]() |
|
OBF integrity | broken/unbroken | broken/unbroken | |
HXD | absolute effective area | 20% | 5% |
relative effective area | 10% | 5% | |
vignetting | 10% | 5% | |
background modelling![]() |
![]() ![]() |
5%![]() |
|
absolute timing | 300 ![]() |
100 ![]() |
|
relative timing | 10![]() |
10![]() |
|
GBST absolute timing | 100 ms | 15 ms |
Note ![]() a: Including the errors associated with the XRT. b: All values of the XIS are based on the ground calibration data and very preliminary. See the text in more detail. c: Study continues. The HXD team will prepare some documents on HXD BGD estimation for updated results by the start of AO-1 observations. |