Suzaku has five light-weight thin-foil X-Ray Telescopes (XRTs). The XRTs have been developed jointly by NASA/GSFC, Nagoya University, Tokyo Metropolitan University, and ISAS/JAXA. These are grazing-incidence reflective optics consisting of compactly nested, thin conical elements. Because of the reflectors' small thickness, they permit high density nesting and thus provide large collecting efficiency with a moderate imaging capability in the energy range of 0.2-12 keV, all accomplished in telescope units under 20 kg each.
Four XRTs onboard Suzaku (XRT-I) are used on the XIS, and the other XRT (XRT-S) is for the XRS. XRT-S is no longer functional. The XRTs are arranged on the Extensible Optical Bench (EOB) on the spacecraft in the manner shown in Figure 5.1. The external dimensions of the 4 XRT-Is are the same (See Table 5.1, which also includes a comparison with the ASCA telescopes).
The angular resolutions of the XRTs range from to , expressed in terms of half-power diameter, which is the diameter within which half of the focused X-ray is enclosed. The angular resolution does not significantly depend on the energy of the incident X-ray in the energy range of Suzaku, 0.2-12 keV. The effective areas are typically 440 cm at 1.5 keV and 250 cm at 8 keV. The focal lengths are 4.75 m for the XRT-I. Actual focal lengths of the individual XRT quadrants deviate from the design values by a few cm. The optical axes of the quadrants of each XRT are aligned within 2 from the mechanical axis. The field of view for XRT-Is is about 17 at 1.5 keV and 13 at 8 keV. (see also Table 2.1)
The Suzaku X-Ray Telescopes (XRTs) consist of closely nested thin-foil reflectors, reflecting X-ray at small grazing angles. An XRT is a cylindrical structure, having the following layered components: 1. a thermal shield at the entrance aperture to help maintain a uniform temperature; 2. a pre-collimator mounted on metal rings for stray light elimination; 3. a primary stage for the first X-ray reflection; 4. a secondary stage for the second X-ray reflection; 5. a base ring for structural integrity and interface with the EOB of the spacecraft. All these components, except the base rings, are constructed in 90 segments. Four of these quadrants are coupled together by interconnect-couplers and also by the top and base rings (Figure 5.2). The telescope housings are made of aluminum for an optimal strength to mass ratio. Each reflector consists of a substrate also made of aluminum and an epoxy layer that couples the reflecting gold surface to the substrate.
Including the alignment bars, collimating pieces, screws and washers, couplers, retaining plates, housing panels and rings, each XRT-I consists of over 4112 mechanically separated parts. In total, nearly 7000 qualified reflectors were used and over 1 million cm of gold surface was coated.
In shape, each reflector is a 90 segment of a section of a cone. The cone angle is designed to be the angle of on-axis incidence for the primary stage and 3 times that for the secondary stage. They are 101.6 mm in slant length and with radii extending approximately from 60 mm at the inner part to 200 mm at the outer part. The reflectors are nominally 178 m in thickness. All reflectors are positioned with grooved alignment bars, which hold the foils at their circular edges. There are 13 alignment bars at each face of each quadrant, separated at approximately 6.4 apart.
To properly reflect and focus X-ray at grazing incidence, the precision of the reflector figure and the smoothness of the reflector surface are important aspects. Since polishing of thin reflectors is both impractical and expensive, reflectors in Suzaku XRTs acquire their surface smoothness by a replication technique and their shape by thermo-forming of aluminum. In the replication method, metallic gold is deposited on extrusion glass mandrel (``replication mandrel''), of which the surface has sub-nanometer smoothness over a wide spatial frequency, and the substrate is subsequently bonded with the metallic film with a layer of epoxy. After the epoxy is hardened, the substrate-epoxy-gold film composite can be removed from the glass mandrel and the replica acquires the smoothness of the glass. The replica typically has 0.5 nm rms roughness in the mm or smaller spatial scale, which is sufficient for excellent reflectivity at incident angle less than the critical angle. The Suzaku XRTs are designed with on-axis reflection at less than critical angle, which is approximately inversely proportional to X-ray energy.
In the thermo-forming of the substrate, pre-cut, mechanically rolled aluminum foils are pressed onto a precisely shaped ``forming mandrel'', which is not the same as the replication mandrel. The combination is then heated until the aluminum softened. The aluminum foils acquire the figure of the properly shaped mandrel after cooling and release of pressure. In the Suzaku XRTs, the conical approximation of the Wolter-I type geometry is used. This approximation fundamentally limits the angular resolution achievable. More significantly, the combination of the figure error in the replication mandrels and the imperfection in the thermo-forming process (to about 4 micrometers in the low frequency components of the figure error in the axial direction) limits the angular resolution to about 1 minute of arc.
The pre-collimator, which blocks off stray light that otherwise would enter the detector at a larger angle than intended, consists of concentrically nested aluminum foils similar to that of the reflector substrates. They are shorter, 22 mm in length, and thinner, 120 micrometers in thickness. They are positioned in a fashion similar to that of the reflectors, by 13 grooved aluminum plates at each circular edge of the pieces. They are installed on top of their respective primary reflectors along the axial direction. Due to their smaller thickness, they do not significantly reduce the entrance aperture in that direction more than the reflectors already do. Pre-collimator foils do not have reflective surfaces (neither front nor back). The relevant dimensions are listed in Table 5.2.
The Suzaku XRTs are designed to function in a thermal environment of 20 7.5 C. The reflectors, due to its composite nature and thus its mismatch in coefficients of thermal expansion, suffer from thermal distortion that degrades the angular resolution of the telescopes in temperature outside this range. Thermal gradient also distorts the telescope in a larger scale. Even though sun shields and other heating elements on the spacecraft help in maintaining a reasonable thermal environment, thermal shields are integrated on top of the pre-collimator stage to provide the needed thermal control.
In this section we describe in-flight performance and calibration of the Suzaku XRTs. There are no data to verify the in-flight performance of the XRT-S, therefore we hereafter concentrate on the four XRT-I modules (XRT-I0 through I3) which focus incident X-rays on the XIS detectors.
A point-like source MCG630-15 were observed at the XIS aimpoint during August 17-18. Figure 5.4 shows the focal position of the XRT-Is, that the source is focused when the satellite points at the XIS aimpoint. The focal positions are located roughly within from the detector center with an deviation of . This implies that the fields of view of the XIS coincides each other within .
The maximum transmission of each telescope module is achieved when a target star is observed along the optical axis. The optical axes of the four XRT-I modules are, however, expected to scatter in an angular range of 1. Accordingly, we have to define the axis to be used for real observations that gives reasonable compromise among the four optical axes. We hereafter refer to this axis as the observation axis.
In order to determine the observation axis, we have first searched for the optical axis of each XRT-I module by observing the Crab nebula at various off-axis angles. The observations of the Crab nebula were carried out in the following three groups of time. Hereafter all the off-axis angles are expressed in the detector coordinate system DETX/DETY (see ).
By fitting a model comprising of a Gaussian plus a constant to the data of counting rate as a function of the off-axis angle, we have determined the optical axis of each XRT-I module. The result is shown in Fig. 5.5.
In-flight calibration of the effective area has been carried out with the version 0.7 processed data (see ) of Crab nebula both at the XIS/HXD-default positions. The observations were carried out in 2005 September 15-16 (§3.1). The data were taken in the normal mode with the 0.1 s burst option in which the CCD is exposed during 0.1 s out of the full-frame read-out time of 8 s, in order to avoid the event pile-up and the telemetry saturation. The exposure time of 0.1 s is, however, comparable to the frame transfer time of 0.025 s. As a matter of fact, the Crab image is elongated in the frame-transfer direction due to so-called the out-of-time events, as shown in Fig. 5.6.
After subtracting the background,
taking into account the sizes of the regions,
we have fitted the spectra taken with the four XIS modules with a model
composed of a power law undergoing photoelectric absorption
using XSPEC Version 11.2.
For the photoelectric absorption, we have adopted the model phabs
with the cosmic metal composition (Anders & Grevesse 1989, Geochim. Cosmochim. Acta, 53, 197)
First of all, we set all the parameters free to vary independently
for all the XIS modules.
The results are summarized for the XIS/HXD nominal positions
separately in table 5.3, and are shown
in Fig. 5.7.
|Sensor ID||Photon Index||Normalization||Flux||(d.o.f.)|
Toor & Seward (1974, AJ, 79, 995) compiled the results from a number of rocket and balloon measurements available at that time, and derived the photon index and the normalization of the power law of the Crab nebula to be and 9.7 photons cm s keV at 1 keV, respectively. Overlaying photoelectric absorption with cm, we obtain the flux to be erg cm s in the 2-10 keV band. The best-fit parameters of all the XIS modules at the XIS-default position are close to these standard values. Although those at the HXD-default position show similar values, the fluxes of XIS0 and XIS1 are smaller than the standard value by 6-7 %. Since the optical axes of these two detectors are farther away from the HXD-default position than those of the other two (Fig. 5.5), this may be due to insufficient calibration of the optical axes and/or the vignetting.
Since the best-fit parameters of the four XIS modules are close to the
standard values, we have attempted to constrain the hydrogen column
density and the photon index to be common among all the detectors.
The best-fit parameters are summarized in table 5.4.
|Sensor ID||Photon Index||Normalization||Flux||(d.o.f.)|
The vignetting curves calculated by the ray-tracing simulator is compared with the observed intensities of the Crab nebula at various off-axis angles in Fig. 5.8.
These figures roughly show that the effective area is calibrated within 5% over the XIS field of view, except for the 8-10 keV band of XIS1. The excess of the data point at the XIS-default position is already seen in Fig. 5.7. Although the reason for these excess is unclear, it is probably associated with insufficient calibration of the backside-illuminated CCD.
Verification of the imaging capability of the XRTs has been made with the data of SS Cyg in quiescence taken during 2005 November 2 01:02UT-23:39UT. The total exposure time was 41.3 ks. SS Cyg is selected for this purpose because it is a point source and moderately bright (3.6, 5.9, 3.7, and 3.5 c s for XIS0 through XIS3), and hence, it is needless to care about pile-up even at the image core.
In evaluating the imaging capability, it is found that variation of relative alignment between the XRT system and the Attitude and Orbit Controlling System (AOCS) becomes a significant problem. The variation is synchronized with the orbital motion of the spacecraft. This phenomenon is now understood to be due to the thermal distortion by the bright-earth illumination of the side panel #7 on which the instruments to measure the attitude of the spacecraft (the star trackers and the gyros) are mounted. The amplitude of the variation is as large as 50 at most, which cannot be neglected in evaluating the imaging capability. A series of software to correct this alignment variation has been developed. In the meanwhile, we simply accumulate the data taken while the pointing of the XRT is stable. In Fig. 5.9, we show the image, the Point-Spread Function (PSF), and the EEF of all the XRT-I modules thus obtained.
Observation of the stray light are carried out with the Crab nebula during 2005 August 22 - September 16 at the off-axis angles of , , , and in (DETX, DETY). An example of the stray-light image is shown in the right panel of Fig. 5.10.
In-flight stray-light observations were carried out with Crab at off-axis angles of (4 pointings), (4 pointing) and (4 pointing) in August and September. We show an example of -off image of XRT-I3 together with simulation results of the same off-axis angle for the cases with and without the pre-collimator in Fig. 5.10. It is seen that the pre-collimator works for reducing the stray light in orbit.
Figure 5.11 shows angular responses of the XRT-I at 1.5 and 4.5 keV up to 2 degrees. The effective area is normalized at on-axis. The integration area is corresponding to the detector size of XIS ( ). The plots are necessary to plan observations of diffuse sources or faint emissions near bright sources, such as outskirts of cluster of galaxies, diffuse objects in the Galactic plane, SN 1987A, etc.
The three solid lines in the plots correspond to different parameters of ray-tracing program while the crosses are the normalized effective area using the Crab pointings. For example, the effective area of the stray lights at 1.5 keV is 10 at angles smaller than 70 arcmin off axis and at angles larger than 70 arcmin off. The measured flux of stray lights are in good agreement with that of raytracing within an order. The solution of the black line is incorporated in the xissim for the AO2 simulation.