XMM-Newton SAS Home Page
XMM-Newton Science Analysis System

arfgen (arfgen-1.104) [xmmsas_20230412_1735-21.0.0]

CCD gap and bad pixel corrections

By default the task will reduce the effective area produced in the ARF if the source area contains bad pixels/columns or contains area which lies outside the CCD boundaries. For an extended source this reduction is simply the ratio of the dead area within the source region to the good area if badpixmaptype=flat is used. For point sources a fine grid is used to determine the contribution to the enclosed energy of each dead point, i.e. the dead regions are normalised by the fraction of the energy-dependent point spread function lying within that region before being subtracted from the total effective area. The task uses the parameter extendedsource to choose between these options. A better method for correcting for lost flux in extended sources which have a large variation in surface brightness, is to use an image from another camera or even another mission as a detector map (see section 5.3.2). In this way the code can calculate the fraction of counts which are contained within the dead regions. This is turned on by setting the parameter badpixmaptype=dataset.

A more direct method of avoiding potential errors due to chip gaps is to apply a detector mask in the accumulation of extended source spectra. The detector mask is a binary (1/0) detector image, where all CCD "dead" areas are set to 0. The masked out pixels are not used in the calculation of the effective area (see section 5.2.1). Hence, the integrated effective area over the extended source surface brightness takes into account only the fraction of the source which has been effectively imaged.
A detector mask is generated with emask starting from an exposure map as generated by eexpmap, which in turn requires an image generated by evselect or xmmselect. In command-line terms:

eexpmap imageset=field_of_view.img attitudeset=0060_0122700101_AttHk.ds
expimageset=exposure_map.fits pimin=1000 pimax=10000

emask expimageset=exposure_mask.fits detmaskset=detector_mask.fits

If the fraction of counts, lost to bad pixels and chip gaps is large, then it is recommended to use a finer resolution when determining this contribution. A warning is issued by the program if it sees that the resolution used is too coarse.

In a similar way the task corrects for area of the source region which is outside the FOV (selectable with the parameter ignoreoutoffov), outside the observing window or on a CCD explicitly excluded by a CCD selection within the data subspace.

This behaviour can be turned off by setting withbadpixcorr=false. CCD gaps and area outside the FOV are automatically corrected for if withbadpixcorr=true. To correct for bad pixels, a file containing bad pixel extensions must be specified on the command line by badpixlocation=file. The bad pixel information is usually stored in the event file from which the spectrum was constructed. Bad columns and bad rows (MOS only) are also handled if they are specified in the OFFSETS column of the badpixlocation file. Pixels adjacent to chip gaps, bad pixels and bad columns are also corrected for if the FLAG selection in the datasubspace has been set accordingly. The code checks each bit mask in the event flag to see if pixels next to a particular type of bad pixel or bad column should be excluded. In this way spectra created with, for example, the #XMMEA_EM and #XMMEA_SM flagging can produce different effective areas for certain selection regions.

The flags used to extract spectra are used to calculate the area of the extraction region. For extended sources, dead area (chip gaps, bad pixels, bad columns, columns next to bad columns, pixels next to bad pixels) is subtracted from the total area. For point sources, the fraction of the PSF which has been lost in the dead area is removed from the total effective area.

We recommend users to utilize #XMMEA_EM for accumulating EPIC-MOS spectra. Spectra extracted using this selection criteria have been demonstrated to be in agreement with the EPIC calibration status document (http://xmm2.esac.esa.int/docs/documents/CAL-TN-0018.pdf).

The task creates temporary files called BADPIXnn.ds, where nn is the CCD number, in the current directory while calculating the bad pixel correction. To make these files permanent specify withfilteredset $=$ yes on the command line.

XMM-Newton SOC -- 2023-04-16