to distinguish one high energy photon from two lower energy photons. Pile-up can also occur when photons striking adjacent pixels are confused with a single photon that deposits charge in more than one pixel. Depending on how many pixels are involved, this is called a single-, double-, triple-, or quadruple- pixel event. The SAS task EPATPLOT measures the pile-up in an observation and the results for our target with the highest count rate, MKN 421 (Table 1), are shown in Figure \[epatplot\]. When we compare the expected fractions of pixel events (solid lines) with those actually measured in the data (histograms) for the range 0.75 to 10 keV we see that a larger than expected fraction of double events (third histogram from top, dark blue in electronic edition) is measured as well as a larger fraction of triple and quadruple events (bottom two histograms), although to a lesser degree, while single events (second histogram from top) are lower than expected, indicating the presence of pile-up. Pile-up leads to a general reduction in the mean count rate as well as a reduction in the magnitude of variations. We will explore the influence of pile-up on our data in more detail when 