Care needs to be taken when adding or subtracting XMM-Newton light curves to ensure that the start and stop times of the two time series are simultaneous. By default, evselect starts a time series at the time when the first event is found in the extraction region. This leads to source and background time series generally having different start / end times. In addition, time series extracted from the different cameras on board XMM-Newton will be non-synchronous by default.
This non-synchronicity of time series can cause artifacts to be introduced when they are added or subtracted by programs such as the ftool "lcmath" (see Barnard et al. 2006, A&A submitted, astro-ph/0610035).
To avoid this problem we recommend that when time series are extracted by evselect the limits of the series are explicitely set on the command line, e.g.
evselect table=M2EVL.ds:EVENTS timemin=151160560 timemax=151162600 withtimeranges=yes withrateset=yes rateset=m2_lc.ds timecolumn=TIME timebinsize=10In this case the same 'timemin' and 'timemax' values should be used for all of the time series which are later going to be combined.