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ebkgreg (ebkgreg-0.8) [xmmsas_20211130_0941-20.0.0]


The task ebkgreg searchs for the most suitable position of a region to get the background estimation of any source in an EPIC image (PN or MOS).

This task has two processing modes: multiple sources from an input source list file or single-source from command-line arguments.

In the multiple sources case the input source list has to contain the following basic parameters for each source: celestial coordinates (RA, DEC), EPIC-pn counts (PN_CTS) and EPIC-pn offaxis information (PN_OFFAX). The extraction region radius for each source is obtained from a tabulated table and it depends on the counts and the off-axis values of the source. All of that mandatory information is included in the 'EPIC summary source list' from the standard distribution of the pipe-line processing. Additionally that source list can be created from srcmatch (follow the link for further information).

Alternatively the extraction region radius for each source can be present in the input source list as SRC_RAD. In this case this is the mandatory column in the input, but not PN_CTS and PN_OFFAX.

Only in the single-source mode the source basic information can be given via the command-line arguments, x, y and r, where x,y are the celestial coordinates of the source and r is the source extraction radius for source products in arcsec. Depending on the coordtype the source position could be RA,Dec or X,Y Sky coord.

The algorithm will search for a circular background region in the same CCD where the source is located, except for the source in the central CCD of a MOS observation in SmallWindow mode (PrimePartialW2/3). In that case the background is estimated from an annulus (inner radius = 5.5 arcmin, outer radius = 11 arcmin) centered in the center of the image. Thus the background is estimated from the peripheral CCDs and the central CCD is completely excluded. For EPIC-pn sources the algorithm avoids the same RAWY column of the source in order to exclude out-of-time events from the background estimate.

It is very important to notice that the algorithm search for the best background region (position and size) only based on geometrical criteria, but not looking for a source-free region in the image. The algorithm assumes that some contaminating sources could be contained in the background region. It is the user's responsability to exclude those contaminating sources from the background estimate calculation. That is very critical for some crowded sky regions.

Contaminating sources could be found by the source detection task edetect_chain and then the FITS Region file created by the task region.

Any SAS task which accepts an input FITS Region file allows also to specify exclusion regions (e.g.: evselect, especget). Those exclusion regions are identified by an exclamation mark "!" before the region shape (e.g.: !CIRCLE) and they must appear in the next rows after the including region in the Region file (see Table 1).

Example: evselect expression=region(FITSRegionFile.fits,X,Y) ...

Table 1: FITS Region file structure
SHAPE X Y R Comment
CIRCLE include_X include_Y include_R Included region
!CIRCLE exclude_X1 exclude_Y1 exclude_R1 Excluded region
!CIRCLE exclude_X2 exclude_Y2 exclude_R2 Excluded region
... ... ... ... ...

The XMM Pipeline is able to exclude any contaminating source from the selected background region in an automatic process. That excluding process is based on the knowledge of the brightness and position of all of the sources in the image.

The background extraction region always have a radius larger than 3 pixels, otherwise no background is calculated.

Table 2: Input and Output quantities
Value Description Units
x Source RA or X Sky coord deg. or 0.05 arcsec pixel
y Source Dec or Y Sky coord deg. or 0.05 arcsec pixel
r Source extraction radius arcsec
bkg_ra_out Background region center RA deg.
bkg_dec_out Background region center Dec deg.
bkg_rad1_arcsec_out Background region: circle or inner annulus radius arcsec
bkg_rad2_arcsec_out Background region: outer annulus radius arcsec
bkg_x_out Background region center X sky coord 0.05 arcsec pixel
bkg_y_out Background region center Y sky coord 0.05 arcsec pixel
bkg_rad1_phys_out Background region: circle or inner annulus radius 0.05 arcsec pixel
bkg_rad2_phys_out Background region: outer annulus radius 0.05 arcsec pixel

XMM-Newton SOC -- 2021-11-30