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Introduction
EPIC background spectra are normally extracted from regions of the observation
field-of-view once contamination due to individual discrete celestial
sources has been removed. This technique is fully appropriate when analyzing
spectra of point-like sources in full window mode. However, there are cases
where the determination of the background from the same observation is
difficult due to the intrinsic extended nature of the celestial object
of interest, or because a bright source was observed using a partial window
mode. It is sometimes impossible to identify field-of-view
regions where the target source does not substantially contribute to the total
X-ray counts. It is recommended to have a look at the
XMM-EPIC Status of EPIC Calibration and Data Analysis where
the most up-to-date instrument behaviour is characterized, so it will help for the
selection of source and background extraction regions for spectral analysis.
In order to cope with this situation, blank sky event lists are available
through the XMM-Newton Science Operations Centre web site from the EPIC
Background Analysis web pages. This thread briefly explains how to make use
of these event files to create background spectra.
Expected Outcome
The expected outcome of this thread is a background spectrum file generated
from a blank sky event list to be used for the spectral analysis of an
extended diffuse source, or for those cases of a bright source observed
using a partial window mode.
SAS Tasks to be Used
Prerequisites
Before starting this thread, a blank sky event list should be obtained. To
receive a tailor made blank sky event list, ideally suited to a user's own
data, plus associated exposure maps, the user is invited to complete the
XMM-Newton EPIC Background Blank Sky Products Request Form via the
XMM-Newton EPIC 'Blank Sky' Background Web Page. The requested blank sky event list
is created in a semi-automatic manner. As soon as the files are available
for download, the user receives an email detailing the location of the files
and a summary of the request.
Useful Links
Caveats
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Procedure
- Set up your SAS environment (see Prerequisites for this thread at the
top of the page).
- Create an EPIC cleaned and filtered for particle background event file for your
observation (see Prerequisites for this thread at the
top of the page). Let the name of the filtered file be:
EPICclean.fits.
- Extract an image of the observation in detector coordinates:
evselect table=EPICclean.fits imagebinning=binSize imageset=EPICimage.fits withimageset=yes \
xcolumn=DETX ycolumn=DETY ximagebinsize=80 yimagebinsize=80
- Display the image:
imgdisplay withimagefile=true imagefile=EPICimage.fits
- Identify the center coordinates and radius of the source spectrum
extraction region. Let's assume that they are:
DETX(center) = 100
DETY(center) = -100
Radius = 6000
- Extract the source spectrum from the identified region:
For MOS:
evselect table=EPICclean.fits withspectrumset=yes spectrumset=EPICsource_spectrum.fits \
energycolumn=PI spectralbinsize=15 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
expression='((DETX,DETY) IN circle(100,-100,6000))'
For pn:
evselect table=EPICclean.fits withspectrumset=yes spectrumset=EPICsource_spectrum.fits \
energycolumn=PI spectralbinsize=5 withspecranges=yes specchannelmin=0 specchannelmax=20479 \
expression='((DETX,DETY) IN circle(100,-100,6000))'
- Let the name of the blank sky event list be (see Prerequisites for this thread at the
top of the page): blank_sky_event_lists.ds
- Add the keyword DATE-OBS to the primary and events extension of the blank
sky event file, for example utilising the FITS file editor fv and adding
the date value of the observation event file (see the watchout
notes of the blank sky web pages).
These keywords can also be added to the blank sky event file using FTOOLS
commands, for example:
fparkey '2004-07-12T12:30:00.0' blank_sky_event_lists.ds+0 DATE-OBS add=yes
fparkey '2004-07-12T12:30:00.0' blank_sky_event_lists.ds+1 DATE-OBS add=yes
- The blank sky event lists have been very conservatively
filtered, to allow leverage for a user to further filter the data to
the level that they desire. Therefore, it is strongly suggested that a user
filters their blank sky event list to a level suitable for their
observation and analysis using the same criteria applied for the
generation of the source spectrum (see Prerequisites for this
thread at the top of the page for filtering event files).
Let the name of the filtered blank sky event file be:
blank_sky_event_lists_clean.ds
- Perform the same operation as for the source extraction region and
extract the background spectrum from the same region in detector
coordinates as the source, using the same event selection
expression:
For MOS:
evselect table=blank_sky_event_lists_clean.ds withspectrumset=yes spectrumset=background_spectrum.fits \
energycolumn=PI spectralbinsize=15 withspecranges=yes specchannelmin=0 specchannelmax=11999 \
expression='((DETX,DETY) IN circle(100,-100,6000))'
For pn:
evselect table=blank_sky_event_lists_clean.ds withspectrumset=yes spectrumset=background_spectrum.fits \
energycolumn=PI spectralbinsize=5 withspecranges=yes specchannelmin=0 specchannelmax=20479 \
expression='((DETX,DETY) IN circle(100,-100,6000))'
- From this point onwards, with the source and background spectral files,
follow the thread for EPIC spectrum extraction to generate a redistribution
matrix, an ancillary file and to calculate the area of source and background
region (BACKSCALE) used to make the spectral files (see Prerequisites
for this thread at the top of the page and the watchout
notes of the blank sky background web pages). Please, note that the threads
for EPIC spectrum extraction is focused on the analysis of point-like
sources. In particular, the call to the SAS task arfgen
to
generate the ancillary file for extended sources, requires the use of
the parameter extendedsource=yes. It is also advised that
the call to arfgen
includes an externally-created detector map which is more suitable for
the analysis of extended sources (see for more details the documentation for arfgen).
It is also possible to extract a background spectrum from the blank sky event
lists using a spectral extraction region defined in sky coordinates. In this
case, one needs to recast the blank sky event list to mimic the nominal
pointing direction of the observer's field, using, for instance, the SAS task
attcalc:
attcalc eventset=blank_sky_event_lists.ds attitudelabel=fixed withatthkset=N \
refpointlabel=user nominalra=... nominaldec=...
where
nominalra and
nominaldec are the celestial
coordinates of the central reference point which can be user defined by setting
refpointlabl to
user. For the case where the blank sky
background field is matched to the source observation field, these values
should be
RA_NOM,
DEC_NOM as they appear in the header
of the source observation event file, which correspond to the
boresight RA/Dec values of the prime instrument (i.e. the proposed pointing).
Last Updated: 16 April 2010
Caveats
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This thread deals only with the blank sky data sets. There are other
background tools and files available. For example, the particle
background component is time variable and therefore one could imagine
the scenario where a particular blank sky file does not have enough
particle background included in it. A user may wish to understand more
fully the various components that contribute to make up the XMM-Newton background
by reading the relevant sections on the XMM-Newton EPIC Background
Analysis Web Pages.
- To scale the blank sky exposure maps (delivered with a
blank sky event list) to a particular energy band of choice, please see
the XMM-Newton EPIC 'Blank Sky' Background Web Page
Threads section for more details.
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