Examining CCDs for Anomalous States

Figure 5: MOS1 spectra from FWC data with CCDs in their nominal state (red), CCDs #4 (green) and #5 (blue) in their anomalous states. Note the excess at energies less than 1 keV for the anomalous states. The data have been normalized so that the peaks of the Al K$\alpha$ lines ($E\sim1.5$ keV) are equal to 1.0. The variations in the strength of the Si K$\alpha$ is due to the relative variations of the line strengths over the area of the detector.

Some of the individual CCDs in the MOS detectors occasionally operate in anomalous states where the background at $E<1$ 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 $\S$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:

G

The hardness ratio indicates that the chip is not in an anomalous state. The QPB spectra constructed by ESAS will be an adequate description of the actual QPB spectrum for this chip.

I

The hardness ratio indicates that the chip is not in its normal state, but the chip is not too deep in an anomalous state. The QPB spectra constructed by ESAS may be adequate to describe the actual QPB spectrum for this chip.

B

The hardness ratio indicates that the chip is in an anomalous state. The QPB spectra constructed by ESAS will not be adequate to describe the actual QPB spectrum for this chip.

O

The chip is not active in this observation. (Thus far, this will only apply to MOS1-3 and MOS1-6 which have become inactive due to micrometeorite strikes.)

U

The count rate is sufficiently low that there are no counts in the soft band. It would be impossible to determine if the chip were in an anomalous state or not.

The anomalous state information is also printed to an ASCII text file. For the above example the output text files would be mos1S001-anom.log and mos2S002-anom.log. One has the option to save the corner data used for the anomalous state check, but this is not particularly useful.

Figure 6: MOS1 and MOS2 event images in the $0.2-0.9$ keV band showing the CCD IDs and two CCDs in anomalous states. The observation (ObsID 0402530201) was performed in 2006 June after the loss of MOS1 CCD#6 due to a meteorite hit (2005 March 9, $\sim$01:30 UT). MOS2 CCD#5 is clearly in an anomalous state, however MOS1 CCD#5 is in a fainter anomalous state as well.