i) A radio isotope (55Fe) continuously illuminates two corners of each CCD for the purpose of calibration. This calibration data is not removed from the event file for the convenience of the users. Users need to remove the data manually when calculating the image, spectrum, etc.
ii) XIS data may be contaminated by the scattered and Fluorescent X-rays from the sun-lit earth's atmosphere. We screened out such data, but some contamination may be remained. The contamination may be noticed in the soft energy band especially as oxygen and nitrogen lines.
iii) Because of the presence of bad columns, which are removed during the ground processing, the XIS image may contain dark lines.
i) Energy scale may have a systematic error of 1-2% level. This includes both the relative uncertainties among the sensors and the absolute uncertainty. The uncertainties are time and energy dependent; they are larger at lower energy (in terms of fractional uncertainties) and for more recent observations.
ii) Effective area (or detection efficiency) of XIS+XRT have systematic errors both in soft (below ~1 keV) and hard (above ~7-8 keV) ends of the energy band. The error is especially large in the soft band (below ~0.7 keV) of XIS-1 (BI chip). Effective area of XIS-1 in this energy range could be wrong by more than a factor of two. Note that current XIS-1 rmf does not support an energy range below 0.3 keV. Relative effective areas among the sensors also do not coincide below ~1 keV.
iii) Only a single rmf (for XIS-2) is supplied for XIS-0,2,3 (FI CCDs). Difference of the performance among the FI CCDs is not very large compared to the systematic errors mentioned above.
iv) Only a single arf file for an on-axis of the telescope is currently available.
XIS response matrices