RGS observations of very bright sources may show the effects of pile-up, the arrival of more than one X-ray photon in one camera pixel before it is read out, in their spectra. The potential for pile-up in the RGS spectra is discussed in this section. It is shown that pile-up is not likely to be a problem for the greatest majority of the X-ray cosmic sources. As a rule-of-thumb, pile-up effects in bright continuum sources should only be considered if integrated fluxes within one RGS CCD exceed 1.5-310 erg cm s. This level of flux integrated over the 17-21 Å (0.6-0.7 keV) range, where the RGS effective area is nearly highest, can give rise to a 2% pile-up. Only some two dozens of objects with fluxes higher than that are identified in the ROSAT All Sky Survey.
The effects of pile-up on spectra are two-fold: migration of photons from first to second order, in particular, and to higher orders generally, and rejection of events with complicated patterns by the on-board processing.
The characteristics of pile-up are likely to differ between RGS1 and RGS2 and even between CCD locations due to event-pattern variations over the detectors. Since August 2007, RGS2 is operated in single-node readout, and hence from that date on the effects of pile-up are more acute in RGS2, with double the readout time of RGS1, which is read out via two nodes.