Some of the individual CCDs in the MOS detectors occasionally operate in anomalous states where the background at keV is strongly enhanced (see Kuntz & Snowden 2008 and Figure 1). XMM-ESAS at this time does not adequately handle this situation and so the data must be screened and any affected CCDs excluded from further processing when soft data are of interest. Data above 2 keV are unaffected.
The effects of the anomalous state can usually be seen in the diagnostic plots created in 5.8. As the mission has progressed, those CCDs affected by anomalous states have in general been more frequently affected. Anomalous state CCDs can be excluded in further processing by an explicit CCD selection input in several of the tasks.
However, if the observation is short, visual screening may not be sufficient. In that case, examination of the diagnostic output from emanom will reveal the existence of anomalous states. The emanom task calculates the (2.5-5.0 keV)/(0.4-0.8 keV) hardness ratio from the corner data to determine whether a chip is in an anomalous state. However, it should be noted that the “anonymous” anomalous state of MOS1 CCD#4 is not always detectable from the unexposed corner data. Thus, comparing spectra from different CCDs is important for detecting (and removing) this anomalous state.
The emanom routine is invoked simply:
emanom eventfile=mos1S001.fits keepcorner=no
emanom eventfile=mos2S002.fits keepcorner=no
where the inputs are MOS event files. These need not have been filtered for the soft proton flares as the corner data required to determine whether a chip is in an anomalous state are shielded from the soft proton flares. In the default mode, the results are written to the header of the event file. For each chip n with corner data (chips 2 through 7), the header keywords ANOMHRn, ANOMHEn, and ANOMFLn contain the hardness ratio, uncertainty in the hardness ratio, and the anomalous state flag. The anomalous state flags are as follows: