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XMM-Newton CCF Release Note
XMM-CCF-REL-217 RGS Background Sp ectra Templates
R. Gonzґ alez-Riestra June 28, 2006

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CCF comp onents

Name of CCF RGS1 TEMPLATEBCKGND 0003 RGS2 TEMPLATEBCKGND 0003

VALDATE 2000-01-01T00:00:00 2000-01-01T00:00:00

EVALDATE ­ ­

Blocks changed X100 P090 1 0.00... X100 P095 2 8.00 X100 P090 1 0.00... X100 P095 2 8.00

XSCS flag NO NO

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Changes

This is the second release of the RGS background sp ectra templates CCFs to b e used by the SAS task rgsbkgmodel. Each of these calibration files contain 64 extensions: 32 for first-order and 32 for second-order background sp ectra (note that the CCFs in the first release only had 32 [16+16] extensions). The first 32 extensions in the file corresp ond to an extraction region of 90% in PI (16 for first order, 16 for second order). The last 32 extensions corresp ond to an extraction region of 95% in PI (16 for first order, 16 for second order). Each of the 16 extensions groups contain the sp ectra derived from RGS blank fields within each of the 16 defined levels of background. The levels are defined by the count rate in an offset region of CCD 9. In all cases the sp ectra have b een extracted from the full field of view (i.e. 100% of the cross disp ersion PSF). This choice improves the signal-to-noise ratio of the templates, while the background can b e prop erly scaled to the extraction area of any sp ectrum through the BACKSCAL column. The names of the extensions in the file have b een changed as to reflect the corresp onding extraction regions used. They are now named Xsss Pppp o r.rr, with sss b eing the size of the extraction

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region in cross disp ersion (100 in all cases), ppp the size of the extraction region in PI (090 or 095), o the order and r.rr the level (from 0.00 to 8.00). These new background template sp ectra have b een extracted with version 2.6 of the SAS task rgsspectrum. The new templates only differ from the previous ones in the values in the BACKSCAL and QUALITY columns. While in the previous versions of this task a single value of BACKSCAL was assigned to all the channels of each CCD no de, the new version makes a more rigorous computation, deriving this quantity channel by channel. As a consequence of this, with the new version of rgsspectrum more pixels have a low value of BACKSCAL and are flagged as 'bad' in the extracted sp ectrum. Some analysis software (e.g. XSPEC) do es not take into account the quality flags of the background, and this gives rise to a significant numb er of outliers in the background-subtracted sp ectrum. This problem has b een solved by replacing the value of BACKSCAL in the channels with bad quality with the median for all the valid channels in the corresp onding no de.

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Scientific Impact of this Up date

The use of the background sp ectra templates is an alternative to the conventional analysis, in which the background is derived from the offset region in the spatial image. The metho d is fully describ ed in in XMM-SOC-CAL-TN-0058. This new way of determining the background is esp ecially imp ortant for analysing:

· extended ob jects leaving little or no background regions in the RGS spatial image, · weak sources, for which an improvement of the signal to noise ratio could b e essential. The need for a new set of templates is twofold:

· For consistency, to use templates extracted with the last available version of rgsspectrum · To include templates extracted with a PI extraction region of 95%, which will b e the default as of SAS 7.0.

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Estimated Scientific Quality

With the exception of pathological cases (less than a few p ercent) the establishment of the overall background sp ectrum in an observation using the background sp ectra templates (as done by the SAS task rgsbkgmodel) works very well, constituting a valid alternative for ANY observation, and the ONLY one for largely extended sources. A very go o d repro duction of the background obtained with the conventional analysis could b e achieved in all the control cases tested so far, using different typ es of ob ject with different levels of background.


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Exp ected Up dates

Further up dates can b e made in the future to improve the signal-to-noise ratio of the templates by adding more blank fields and/or to cop e with software up dates.

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Test pro cedures

General checks:

· use fv (or another FITS viewer) for file insp ection. It should contain 64 binary extensions, each with five columns: CHANNEL, RATE, QUALITY, BACKSCAL and AREASCAL. · use the SAS task cifbuild to see if the CAL digests and creates correctly the calibration index file.

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Summary of the test results

A full description of the metho d and results obtained, also using even these CCF files with the new SAS task rgsbkgmo del is given in the Calibration Technical Note XMM-SOC-CAL-TN-0058 "Templates for the RGS Background" [1]. General checks: The fits viewer fv was used to insp ect the two CCF files, wrt their structure and validity dates. Everything was OK. The SAS task cifbuild was run successfully in order to check the ingestion of the files into the calibration index file.

References
[1] "Templates for the RGS Background", R. Gonzґ alez-Riestra, XMM-SOC-CAL-TN-0058, Octob er 2004