The X-ray Imaging Spectrometer (XIS) on board the Suzaku satellite is an X-ray CCD camera system that has features of a low background, high quantum efficiency, and good energy resolution in the 0.2 - 12 keV band. Because of the radiation damage, however, the energy resolution of the XIS has been degraded since Suzaku was launched (July 2005).
One of the major advantages of the XIS over the other X-ray CCDs in orbit is the provision of a precision charge injection (CI) capability. In order to improve the energy resolution, the precise measurement of charge transfer inefficiency (CTI) is essential. For this purpose, we applied the checker-flag CI, and we were able to measure the CTI of each CCD column. Furthermore, we were able to obtain the pulse height dependency of the CTI.
Our precise CTI correction using these results improved the energy resolution from 193 eV to 173 eV in FWHM at 5.9 keV in July 2006 (one year after the launch).
The energy resolution can be improved also by reducing the CTI. For this purpose, we applied the spaced-row charge injection (SCI); periodically injected artificial charges
work as if they compensate radiation-induced traps and prevent electrons produced by X-rays from being captured by the charge traps. Using this method, the energy resolution improved from 210 eV to 150 eV at 5.9 keV in September 2006, which is close to the resolution just after the launch (145 eV).
We report the current in-orbit calibration status of the XIS data using these two techniques. We present the time history of the gain and energy resolution determined from onboard calibration sources (55Fe) and observed calibration objects like E0102-72.
The X-ray Imaging Spectrometers (XIS) on-board Suzaku is an X-ray CCD camera system that has features of low backgroud, good energy resolution, and high quantum efficiency (QE) at 0.2-12 keV band. However, an unexpected degradation of the QE at low energies (<1 keV) has emerged since November 2005. Some contaminants are considered to be adsorbed on the Optical Blocking Filter (OBF) for each sensor and cause the degradation. A suspected contamination source is rubber used in the shock absorber of the satellite gyro. For the recovery of the QE, we now design to remove the contaminants by increasing the OBF temperature. Before the on-board bakeout is performed, we need to confirm on the ground that it does not cause a serious damage to the OBF. In order to reproduce the on-board contamination, we adsorbed the contaminant of ~160 μg cm-2 from the rubber on a spare OBF and a Thermoelectric Quartz Crystal Microbalance simultaneously, which are cooled down to -40°C. Although enexpected wrinkles appeared on the OBF surface during the adsorption and they remained through the subsequent bakeout, we could not find any tears on it. In addition, we estimated the desorption rate at -15°C to be ~5 μg cm-2 per day. In our presentation, we also discuss the expected effect by the on-board bakeout based on these results.
The CCD detectors in the X-ray Imaging Spectrometers (XIS) aboard Suzaku have been equipped with a precision
charge injection capability. The purposes of this capability are to measure and reduce the detector degradation
caused by charged particle radiation encountered on-orbit. Here we report the first results from routine operation
of the XIS charge injection function. After 12 months' exposure of the XIS to the on-orbit charged particle
environment, charge injection already provided measurable improvements in detector performance: the observed
width of the 5.9 keV line from the onboard calibration source was reduced from 205 eV to less than 145 eV.
The rate of degradation is also significantly smaller with charge injection, so its benefit will increase as the
mission progresses. Measured at 5.9 keV, the radiation-induced rate of gain degradation is reduced by a factor
of 4.3 ± 0.1 in the front-illuminated sensors when injecting charge greater than 6 keV equivalent per pixel. The
corresponding rate of degradation in spectral resolution is reduced by a factor 6.5 ± 0.3. Injection of a smaller
quantity of injected charge in the back-illuminated XIS sensor produces commensurately smaller improvement
factors. Excellent uniformity of the injected charge pattern is essential to the effectiveness of charge injection in
The energy resolution of the X-ray CCDs onboard the Suzaku satellite (X-ray Imaging Spectrometer; XIS) has
been degraded since the launch due to radiation damage. To recover from this, we have applied a spaced-row
charge injection (SCI) technique to the Suzaku XIS in orbit. By injecting charge into CCD rows periodically,
the energy resolution 14 months after launch is improved from 210 eV to 150 eV at 5.9 keV, which is close to
the resolution just after the launch (140 eV). Additional information on these results is given in a companion
paper by the XIS team. In this paper, we report the details of CCD charge transfer inefficiency (CTI) in the
SCI mode, the correction method, and the implementation of it in ground analysis software for XIS data. In the
SCI mode, CTI depends on the distance of a charge packet from the nearest charge-injected row, and the gain
shows a periodic non-uniformity. Using flight data obtained with the onboard calibration sources, as well as a
cosmic source (the Perseus cluster of galaxies), we studied the non-uniformity in detail. We developed a method
to correct for the non-uniformity that will be valuable as the radiation damage progresses in future.
We developed the new readout system for the pixel-readout μ-PIC (micro pixel chamber), which is one of the micro-pattern gas detectors that have been developed as a X-ray polarimeter so far. By using this system, we succeeded in achieving the sensitivity predicted by the simulation, i.e, the modulation factors, which is one of the most important factors for X-ray polarimeter as defined later in this paper, 0.24±0.08 at 8 keV, 0.18±0.07 at 15 keV in the neon-based gas mixture, and 0.18±0.04 in the argon-based gas although there still remain problems such as the pitch size among pixels and the non-uniformity of the response.