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XMM-Newton Science Analysis System Page: 1
backcorr
November 22, 2004
Abstract
backcorr makes a background spectrum appropriate to the accumulation region
used for a given source spectrum. The task calls arfgen twice; once for the source
region and once for the background region. The regions are read directly from the
input datasets by arfgen . This gives the integrated eective area as a function of
energy over both regions. From these two eective area curves, a direct normalization
of the input background spectrum is performed.
1 Instruments/Modes
Instrument Mode
EPIC MOS IMAGING
EPIC PN IMAGING
2 Use
pipeline processing yes
interactive analysis yes
3 Description
backcorr makes a background spectrum appropriate to the accumulation region and position used for
the source spectrum. The output of the task is an \EPIC FITS source background spectrum" as dened
in the SSC data products ICD [1].
backcorr is a metatask that calls arfgen twice in its so-called extended source mode to compute the
integrated eective area curves over the source and background regions. The extraction regions, which
should be in detector co-ordinates, are read directly from the input spectra (both source and background),
as the information is stored in the les using the data-subspace. This explains the presence of the source
spectrum (i.e. the spectrum extracted from the source region, in channel and counts) as an input of
backcorr. Then backcorr takes the input background spectrum (also in channel and counts), the arf
les and makes the appropriate correction (in a task called make backcorr).
Before going into the details of the task description, it is worth stressing that backcorr is primarily
intended for use in the PPS. It could also be used in the IAS, but in many cases more sophisticated
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analysis will be required to get an accurate and reliable background spectrum. backcorr must be
robust, work for point sources and for observing modes which allow the extraction regions to be dened.
Its main input is a background spectrum which has corrections made to it. The details on how this input
is determined is beyond the scope of the task, and should be investigated separately.
3.1 Example
To create a corrected background spectrum, called outbkgspectrum.ds, from the source and background
spectra, spectrum.ds and background.ds, the following example can be used.
backcorr srcspectrumset="spectrum.ds" bkgspectrumset="background.ds"
outbkgspectrumset="outbkgspectrum.ds"
Alternatively, to create a corrected background spectrum using the position in detector co-ordinates, the
following could be used:
backcorr srcspectrumset="spec1.ds" bkgspectrumset="back1.ds"
outbkgspectrumset="outbkgspectrum.ds" withsourcepos=yes
sourcecoords="det" sourcex=-3917.0 sourcey=-3308.5 bkgdx=-2141.5 bkgdy=-2973.5
4 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
bkgspectrumset yes dataset bkgspectrum.ds Input dataset contain-
ing the spectrum accu-
mulated in the back-
ground region.
srcspectrumset yes dataset srcspectrum.ds Input dataset contain-
ing the spectrum accu-
mulated in the source
region.
outbkgspectrumset yes dataset outbkgspectrum.ds Output dataset con-
taining the corrected
background spectrum.
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withsourceposition no boolean no yes/no
Indicate whether you wish to specify the source position
sourcecoords no string eqpos eqpos/pos/det
Indicate the coordinate system for which the source position is specied
sourcex no real 0 0
Indicate the x-position of source centre, in terms of the coordinate system specied in sourcecoords.
sourcey no real 0 0
Indicate the y-position of source centre, in terms of the coordinate system specied in sourcecoords.
bkgdx no real 0 0
Indicate the x-position of the background region, in terms of the coordinate system specied in source-
coords.
bkgdy no real 0 0
Indicate the y-position of the background regions, in terms of the coordinate system specied in source-
coords.
5 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
Wrong instrument name (error)
Should be either EMOS1, EMOS2, EPN
WrongSourceCoordType (error)
Should be one of det, eqpos or pos
Input spectrum may be wrong (warning)
The input spectrum has a negative or null total number of counts
corrective action: none
Corrected spectrum may be wrong (warning)
The corrected background spectrum is negative everywhere
corrective action: none
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Corrected spectrum may be wrong (warning)
The corrected background spectrum has a negative or null total number of counts
corrective action: none
6 Input Files
1. Input EPIC background spectrum [1]
2. Input EPIC source spectrum [1]
7 Output File
1. EPIC backcorr corrected background spectrum [1]
8 Algorithm
subroutine backcorr
Arfgen computes the effective area curves for both the source
(arf_in_src.arf) and background (arf_in_bkg.arf) regions. The
extraction region is read directly in the input spectra through
the data sub space.
Calls the subroutine make_backcorr which :
Reads in the two temporary arf files: arf_in_src.arf & arf_in_bkg.arf
Reads in the input background spectrum (==> bkg_input_spectrum)
Makes the appropriate correction :
bkg_corrected_spectrum=(arf_src/arf_bkg)*bkg_input_spectrum
Writes out bkg_corrected_spectrum into a FITS file with the
same header of the input file + an updated history
End of make_backcorr
Delete temporary arf files
end subroutine backcorr
9 Comments
The various operations made on the spectra are done in units of counts. However, the
task supports the input spectrum in the form of \RATES"(to comply to the OGIP/92-07
standard).
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10 Future developments
So far, there are no corrections for the particle background component that may be dierent in the source
and background regions. This is due to an insuфcient knowledge of what the spatial distribution of the
particle background will be. Therefore it is likely that the task will evolve after the launch of XMM, once
the rst in- ight calibrations are performed. These calibrations, which are critically required, should tell
us about the most reliable way to estimate the particle component in a given region. Once this is known
with suфcient accuracy, it will be fairly easy to modify backcorr.
The most straightforward way will be to decompose the background input spectrum as the sum of a
\sky" component and a particle component computed in the background region. The correction will be
done in such a way:
Bsrc(E) = (Asrc(E)/Abkg(E)) x (Bbkg(E) - Cbkg(E)) + Csrc(E)
where we define:
Bsrc(E) is the estimated background spectrum in the source region (output)
Bbkg(E) is the background spectrum in the background region (input)
Asrc(E) is the effective area integrated over the source region
Abkg(E) is the effective area integrated over the background region
Csrc(E) is the particle background in the source region
Cbkg(E) is the particle background in the background region
E is the energy.
The present version assumes that Cbkg and Csrc are equal to zero at all energies.
References
[1] SSC. XMM Survey Science Centre to Science Operations ICD for SSC Products. Technical Report
XMM-SOC-ICD-0006-SSC Issue 2.1, SSC, Mar 2000.
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