The onboard calibration for the spaced-row charge injection of the Suzaku XIS

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.