Optical Loading

Bright optical sources can also liberate significant charge in the XRT CCD, affecting the detection of X-rays at the location of the optical source. Above a certain level the X-ray event energy calibration and grade are affected, and for brighter objects it can cause X-ray events to be rejected or spurious events to be detected. Due to the wavelength-dependent response of the XRT CCD, the impact of optical loading is a function of stellar type.

Which level is relevant depends on the expected X-ray count-rate of the source in question, and the exposure time. e.g., if you are studying a source with an X-ray flux of 1 cts s$^{-1}$, then optical loading at a level of 10$^{-2}$ cts s$^{-1}$ may be acceptable; but if you are searching for faint sources in deep exposures, optical loading of 10$^{-3}$ cts s$^{-1}$ is likely to contaminate your results. The level of optical loading can be estimated using the web tool provided by the UKSSDC.