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.
In its first four years of operation, Suzaku has accumulated 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://suzaku.gsfc.nasa.gov/docs/suzaku/aehp_time_miss.html.
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 degrees/min, and settling to the final attitude takes 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%.
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.
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 (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 ( ), and moderate spectral resolution.
The HXD (Fig. 8.1) is a non-imaging instrument with an effective area of 260cm, 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 4.54.5 by the well-shaped BGO scintillators, which, in combination with the GSO scintillators, are arranged in the so-called phoswich configuration. At energies below 100keV, an additional passive collimation further reduces the field of view to 3434. The energy resolution is 4.0keV (FWHM) for the PIN diodes, and 7.6 / % (FWHM) for the scintillators (where is energy in MeV). The HXD time resolution for both sensors is 61s. While the HXD is intended mainly to explore the faintest hard X-ray sources, it can also tolerate very bright sources up to 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.
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 10 larger XIS effective area than the HXD aim point. Conversely for the HXD, the HXD aim point provides a 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.
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 satellites3.1. 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.
Suzaku excels for observations such as:
Suzaku is less appropriate for:
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. 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.
|Calibration Item||Oct 2008||Requirement||Goal|
|XRTI/XIS||On-axis effective area||2%||5%||5%|
|Optical axis position in XIS||0.5||0.2||0.2|
|Energy scale||max(0.2%, 5eV)||0.1%||0.1%|
|Energy resolution (FWHM) at 5.9keV||5%||1%||1%|
|HXD||Absolute effective area||20%||20%||5%|
|Relative effective area||10%||10%||5%|
|Background modeling (PIN)||%||10%||1%|
|Background modeling (GSO)||%||10%||3%|
|GRB absolute timing||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 1-6. 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 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.