Since pile-up most often results in the combination of two first-order events to become a single apparent second order event, a useful technique to assess its importance is the comparison of first and second order fluxed spectra. In the absence of pile-up, the effective area calibration ensures that first and second order fluxes at a given wavelength agree within a few percent. Hence, examining the ratio between first and second order can help determine whether pile-up has played a role.
Details about pile-up in RGS and the relevant methods for the
assessment and management of this effect are given in the XMM-SOC
Technical Note CAL-TN-0075, available from