Subsections

• E..5.1 Actions
• E..5.2 Usage and Inputs
• E..5.3 Data Products

E..5 EXTRACT_BACKGROUNDS Stage

This stage is run for each observation to perform the final background spectral extraction in preparation for spectral fitting, as shown in Figure 8.

E..5.1 Actions

• For each source, a search is performed to find a local circular background region using the following criteria:
  1. The background scaling (BACKSCAL) associated with the region is constrained to fall within the target range defined for the source (§5.6 and 7.7).

  2. The algorithm tries to achieve a background scaling close to the goal defined for the source (§5.6 and 7.7).

  3. The algorithm tries to achieve a background region that contains at least the specified minimum number of total-band counts (default is 5 counts, modified via MIN_NUM_CTS option).

  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).

• Background event lists are extracted.

  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.

• Background spectra are extracted, and scaled using the integrated exposure in the region.

• Plots summarizing the results of EXTRACT_BACKGROUNDS are presented. These plots are diagnostic tools that help you spot problems in your data, in your catalog, in your CIAO configuration, in the CIAO software, or in the AE software. Some plots may also convey scientifically interesting information about your sources.

E..5.2 Usage and Inputs

acis_extract, srclist_filename, obsname, background_obsdata_filename, /EXTRACT_BACKGROUNDS, EMAP_FILENAME=filepath, MIN_NUM_CTS=value, ENERGY_RANGE=[0.5,8]

acis_extract, srclist_filename, obsname, /EXTRACT_BACKGROUNDS, /PLOT

EXAMPLE:
idl $\vert\&$ 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'

• The obsdata_filename and emap_filename should be paths to an event list and exposure map in which all point sources have been removed; see §7.6.2,7.6.3.

• The optional keyword MIN_NUM_CTS (default 5) should be the minimum number of in-band counts you prefer to have in the background spectrum for this single observation. The number of counts you wish to have in the final multi-ObsID background spectrum must be controlled via the MIN_NUM_CTS parameter to the ae_adjust_backscal_range tool (§5.6 and 7.7).

• The optional ENERGY_RANGE parameter defines the band over which statistics are computed; default is [0.5,8.0]. An energy filter is NOT applied to the spectra.

E..5.3 Data Products

For each source the following files are produced:

{sourcename}/{obsname}/background.emap:

the background exposure map (which defines the background region)

{sourcename}/{obsname}/background.evt:

the events falling in the background region

{sourcename}/{obsname}/background.pi:

the spectrum corresponding to background.evt

{sourcename}/{obsname}/obs.stats:

a FITS file containing keyword information about this observation of the source

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.