The observer may supply an arbitrary16 background region for an individual source by placing the region specification in a file named background.reg in the source's extraction directory for this observation (e.g. srcname/obs/background.reg). When a background region file is supplied then the two criteria above are ignored. Note that for a source observed multiple times, there are theoretical reasons to design the background regions so that the scaling values they require with respect to their source apertures are similar (§5.6.3).
Note that the background region (whether an auto-generated circle or an observer-supplied file) is NOT directly used to filter the event list. In order to accurately measure the exposure-area of the background spectrum, the background region specification is applied to the masked exposure map supplied, and the emap pixels that are found to be ``inside'' the DS9/CIAO region define the actual set of events that form the background spectrum. Thus, the effective background region is a pixelized (as defined by the exposure map) version of the DS9/CIAO region.
acis_extract, srclist_filename, obsname, /EXTRACT_BACKGROUNDS, /PLOT
idl tee extract_backgrounds_1875.log
acis_extract, 'all.srclist', '1875', '../obs1875/background.evt', /EXTRACT_BACKGROUNDS, EMAP_FILENAME='../obs1875/background.emap', MIN_NUM_CTS=20
acis_extract, 'all.srclist', '1875', /EXTRACT_BACKGROUNDS, /PLOT, REGION_FILE='background.reg'
Several interactive plots show characteristics of the source extraction; see our AE recipes (§7.1) for descriptions of each.
The optional REGION_FILE parameter to the /PLOT stage specifies the name to use for a region file AE will construct containing the background regions for all the sources in the catalog.