3. Mission Description

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 a Suzaku proposal.

To date Suzaku has accumulated extensive data from calibration, SWG and GO observations. The list of all observations performed is available in the Browse master catalog at the High Energy Astrophysics Science Archive Research Center (HEASARC) at http://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3browse.pl and at
http://heasarc.gsfc.nasa.gov/docs/suzaku/docs/suzaku/aehp_time_miss.html.

Figure 3.1: The 96 minute Suzaku orbit.

Suzaku is in many ways similar to ASCA in terms of orbit, pointing, and tracking capabilities. Suzaku uses the same station (USC) as ASCA did for up-link and down-link, although down-link at NASA DSN is not possible with Suzaku (see footnote in subsection 3.2.1). As a result, the operational constraints for Suzaku are also similar to those of ASCA. Suzaku is placed in a near-circular orbit with an apogee of 568km, an inclination of 31.9degrees, and an orbital period of about 96minutes. The maximum slew rate of the spacecraft is $6$degrees/min, and settling to the final attitude takes $\sim 10$minutes, using the star trackers. The normal mode of operations will have the spacecraft pointing in a single direction for at least 1/4day (10ks net exposure time). 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 observing efficiency of the satellite as measured after four years of operation is about 45%.

Figure 3.2: [Left] Schematic picture of the bottom of the Suzaku satellite. [Right] A side view of the instrument and telescopes on Suzaku.

3.1 Brief Introduction to Suzaku

The scientific payload of Suzaku (Fig. 3.2) initially consisted of three distinct co-aligned scientific instruments. There are four X-ray sensitive imaging CCD cameras (X-ray Imaging Spectrometers, or XISs), three front-illuminated (FI; energy range 0.4-12keV) and one back-illuminated (BI; energy range 0.2-12keV), capable of moderate energy resolution. Each XIS is located in the focal plane of a dedicated X-ray telescope. The second instrument is the non-imaging, collimated Hard X-ray Detector (HXD), which extends the bandpass of the observatory to much higher energies with its 10-600keV pointed bandpass. The X-Ray Spectrometer (XRS) is no longer operational, and will not be discussed further. Interested readers are invited to access the XRS instrument paper at http://www.astro.isas.jaxa.jp/suzaku/doc/suzakumemo/suzakumemo-2006-38.pdf.

Table 3.1: Overview of Suzaku capabilities.

S/C	Orbit apogee	568km
	Orbital period	96minutes
	Observing efficiency	$\sim 45$%
XRT	Focal length	4.75m
	Field of view	$17'$ at 1.5keV
		$13'$ at 8keV
	Plate scale	0.724arcmin/mm
	Effective area	440cm$^2$ at 1.5keV
		250cm$^2$ at 8keV
	Angular resolution	$2'$ (HPD)
XIS	Field of view	$17.8'\times17.8'$
	Bandpass	0.2-12keV
	Pixel grid	1024$\times$1024
	Pixel size	24$\mu$m$\times$24$\mu$m
	Energy resolution	$\sim130$eV at 6keV
	Effective area	340cm$^2$ (FI), 390cm$^2$ (BI) at 1.5keV
	(incl XRT-I)	150cm$^2$ (FI), 100cm$^2$ (BI) at 8keV
	Time resolution	8s (Normal mode), 7.8ms (P-Sum mode)
HXD	Field of view	$4.5^{\circ}\times4.5^{\circ}$ ($\gtrsim 100$keV)
	Field of view	$34'\times34'$ ($\lesssim 100$keV)
	Bandpass	10-600keV
	- PIN	10-70keV
	- GSO	40-600keV
	Energy resolution (PIN)	$\sim 4.0$keV (FWHM)
	Energy resolution (GSO)	$7.6 / \sqrt{E_{\rm MeV}}$ % (FWHM)
	Effective area	$\sim 160$cm$^2$ at 20keV, $\sim 260$cm$^2$ at 100keV
	Time resolution	61$\mu$s
HXD-WAM	Field of view	2$\pi$ (non-pointing)
	Bandpass	50keV-5MeV
	Effective area	800cm$^2$ at 100keV / 400cm$^2$ at 1MeV
	Time resolution	31.25ms for GRB, 1s for All-Sky-Monitor

.

Figure 3.3: XIS effective area of one XRT + XIS system, for both the FI and BI chips.(No contamination included.)

All of the instruments on Suzaku operate simultaneously. Each of the co-aligned XRTs features an X-ray mirror with an angular resolution (expressed as Half-Power Diameter, or HPD) of $\sim 2'$ (Fig. 3.4). Figure 3.3 shows the total effective area of the XIS+XRT, which includes features due to the elemental composition of the XIS and XRT. K-shell absorption edges from oxygen (0.54keV) and aluminum (1.56keV) in the blocking filters are present, as well as a number of weak M-shell features between 2-3keV arising from the gold in the XRT.

The four XISs (Fig. 7.3) are true imagers, with a large field of view ( $\sim 18' \times 18'$), and moderate spectral resolution.

Figure 3.4: The Encircled Energy Function (EEF) showing the fractional energy within a given radius for one quadrant of the XRT-I telescopes on Suzaku at 4.5 and 8.0keV.

The HXD (Fig. 8.1) is a non-imaging instrument with an effective area of $\sim$260cm$^{2}$, featuring a compound-eye configuration and an extremely low background. It dramatically extends the bandpass of the mission with its nominal sensitivity over the 10-600keV band (Fig. 3.5). The HXD consists of two types of sensors: 2mm thick silicon PIN diodes sensitive over 10-70keV, and GSO crystal scintillators placed behind the PIN diodes covering 40-600keV. The HXD field of view is actively collimated to 4.5$^\circ\times$4.5$^\circ$ by the well-shaped BGO scintillators, which, in combination with the GSO scintillators, are arranged in the so-called phoswich configuration. At energies below $\sim$100keV, an additional passive collimation further reduces the field of view to 34$'\times$34$'$. The energy resolution is $\sim$4.0keV (FWHM) for the PIN diodes, and 7.6 / $\sqrt{E}$ % (FWHM) for the scintillators (where $E$ is energy in MeV). The HXD time resolution for both sensors is 61$\mu$s. While the HXD is intended mainly to explore the faintest hard X-ray sources, it can also tolerate very bright sources up to $\sim$10Crab. The HXD also performs as an all-sky monitor (the Wide-band All-sky Monitor (WAM), which can detect GRBs and other sources. Although observers will receive data from the WAM, it cannot be proposed for directly and has special rules regarding data rights; see Chapter 4.

Figure 3.5: Total effective area of the HXD detectors, PIN and GSO, as a function of energy.

Because the HXD bore-sight axis, with the highest effective area, is about 3.5arcmin shifted from that of the XISs, the Suzaku operations team supported two aim points, XIS and HXD oriented, in the past. The XIS aim point provides a $\sim$10$\%$ larger XIS effective area than the HXD aim point. Conversely for the HXD, the HXD aim point provides a $\sim$ 10$\%$ larger HXD effective area than the XIS aim point. A 10% increase in effective area corresponds to a 10% and 20% increase in observing time for source and background dominated observations, respectively. In order to mitigate effects due to the increased attitude jitter of Suzaku since the end of 2009 the HXD aim point is not supported anymore in AO-7.

3.2 Operational Constraints

3.2.1 Telemetry Rates

Suzaku carries a 6Gbit data recorder. Data will be down-linked to USC at a rate of 4Mbps for a total of 2Gbits per pass, up to 5 times a day. This allows a maximum of 10Gbits of data to be obtained per day, but fewer passes may be available to Suzaku as it will share the use of USC ground station with other ISAS satellites^3.1. Data can be recorded at 4 different rates: Super-High (524kbps), High (262kbps), Medium (131kbps), and Low (33kbps). 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.

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 primarily be used for times of Earth occultations and SAA passages, as the background rates in the XIS and HXD exceed their telemetry allocation limit at Low data rate. The telemetry limits for the XIS are presented in Chapter 7. 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.

3.2.2 Summary

Suzaku excels for observations such as:

• Studies of diffuse soft X-ray sources with low surface brightness: the low background, soft X-ray sensitivity, and near-Gaussian response of the XIS BI CCD makes such targets an excellent use of Suzaku.
• Observations requiring sensitivity both above and below 10keV, especially measuring the Fe K complex simultaneously with the hard ($>$10keV) continuum.
• Rapid variability studies on 10ms time scales. The best time resolution available on the XIS is $\sim 7.8$ms, while the HXD time resolution is $\sim 61$$\mu$s (see Table 3.1)

Suzaku is less appropriate for:

• Studies requiring primarily high spatial resolution. Chandra's PSF is $\sim 100\times$ smaller than Suzaku's, while the XMM-Newton PSF is $\sim 10\times$ smaller.
• Studies requiring primarily high spectral resolution. The gratings on Chandra and XMM-Newton have significantly higher resolution than the XIS.

3.3 Calibration

Table 3.2 summarizes the calibration items of all scientific instruments, the current status, and their expected and measured accuracy.

These values are the 90% limits, equivalent to 1.6$\sigma$. Note that the values listed are those required from the scientific purpose and ultimate goals which are possible to be realized on the basis of the instrument design.

Table 3.2: Error Budgets of Scientific Instrument Calibrations.

	Calibration Item	Oct 2008	Requirement	Goal
XRT$-$I/XIS	On-axis effective area $^{\rm a}$	$\sim$2%	5%	5%
	Vignetting$^{\rm a}$	$\sim$10%	5%	2%
	On-axis EEF $^{\rm b}$	$\sim$3%	5%	1%
	Off-axis EEF $^{\rm c}$	$\sim$3%	20%	2%
	Optical axis position in XIS	$\sim$0.5$^\prime$	$<$0.2$^\prime$	$<$0.2$^\prime$
	Energy scale$^{\rm d}$	max(0.2%, 5eV)	0.1%	0.1%
	Energy resolution (FWHM) at 5.9keV	5%$^{\rm e}$	1%	1%
	Contamination thickness$^{\rm f}$	10$^{18}$cm$^{-2}$	N/A	N/A
	OBF integrity	unbroken	broken/unbroken	broken/unbroken
HXD	Absolute effective area	20%	20%	5%
	Relative effective area	10%	10%	5%
	Vignetting	5%	10%	5%
	Background modeling (PIN)$^{\rm g}$	$3\sim5$%	10%	1%
	Background modeling (GSO)$^{\rm g}$	$1.5\sim2$%	10%	3%
	Absolute timing$^{\rm h}$	300$\mu$s	300$\mu$s	100$\mu$s
	Relative timing$^{\rm h}$	1.9$\times$10$^{-9}$	10$^{-8}$	10$^{-10}$
	GRB absolute timing	$\sim$2ms	10ms	1ms

a: Valid in the 2-10 keVband. Calibration uncertainty may become larger outside this energy range, especially below 0.3keV (BI chip) and above 10keV. We calibrated the effective area using spectral parameters of the Crab emission as those given by Toor & Seward (1974, AJ, 79, 995).
b: For all integration radii from 1$^\prime$-6$^\prime$. No error on attitude control is included.
c: As on-axis but for all XIS f.o.v. No calibration is currently scheduled.
d: For the normal mode data. Uncertainties of the energy scale increase when the Burst and/or Window options are applied.
e: When xisrmfgen is used. Note that an error of 5% in the energy resolution could produce an artificial line width of as large as $\sim$25eV in sigma at the iron band. Energy resolution with the spaced-row charge injection is under investigation.
f: Uncertainty represented as the carbon-equivalent column density. Valid only at the center of the field of view.
g: Modeling accuracy depends on energy-band and exposure. See Chapter 8.5 for typical examples.
h: The Crab and PSR B1509-58 pulses are clearly detected in the quick look analysis of calibration data.