Performance of the Charge Injection Capability of Suzaku XIS

A charge injection technique is applied to the X-ray CCD camera, XIS (X-ray Imaging Spectrometer) onboard Suzaku. The charge transfer inefficiency (CTI) in each CCD column (vertical transfer channel) is measured by the injection of charge packets into a transfer channel and subsequent readout. This paper reports the performances of the charge injection capability based on the ground experiments using a radiation damaged device, and in-orbit measurements of the XIS. The ground experiments show that charges are stably injected with the dispersion of 91eV in FWHM in a specific column for the charges equivalent to the X-ray energy of 5.1keV. This dispersion width is significantly smaller than that of the X-ray events of 113eV (FWHM) at approximately the same energy. The amount of charge loss during transfer in a specific column, which is measured with the charge injection capability, is consistent with that measured with the calibration source. These results indicate that the charge injection technique can accurately measure column-dependent charge losses rather than the calibration sources. The column-to-column CTI correction to the calibration source spectra significantly reduces the line widths compared to those with a column-averaged CTI correction (from 193eV to 173eV in FWHM on an average at the time of one year after the launch). In addition, this method significantly reduces the low energy tail in the line profile of the calibration source spectrum.