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GSFC XMM-Newton GOF Status Report #098: XMM-NEWTON NEWS #45

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_/            XMM-NEWTON NEWS #45   ---    16-Dec-2004            _/
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              XMM-Newton Science Operations Centre at
               ESA's European Space Astronomy Centre,
            P.O. Box - Apdo. 50727, 28080 Madrid, Spain


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         - Empirical correction to the positional errors in the
           1XMM catalogue

Empirical correction to the positional errors in the 1XMM catalogue

A significant fraction of the statistical errors on the source
positions in the 1XMM catalogue are underestimated, as is noted in
the original 1XMM documentation (ref.1). New documentation has been
produced to accompany the new release (ref.2).

This problem obviously has some impact on the cross-correlation
with other astronomical catalogues. Although a systematic error was
incorporated in the calculation of the cross-correlation radius for
1XMM, the XMM-Newton Survey Science Centre (SSC) has discovered
that a significant number of potential source identifications were
nevertheless missed, thus rendering the catalogue less suitable for
statistical identification.

A correction algorithm based on simple statistical considerations
has been designed by O. Herent and A. Martocchia (Strasbourg) to
make an empirical correction to the existing positional errors. The
correction particularly concerns the faintest sources. Details of
the procedure applied are described in the technical note
SSC-OAS-TN-21 (ref.3).

SSC has now produced a new set of "PPS catalogue products" (lists
of correlations with archival astronomical catalogues, finding
charts and other graphical products) for the 1XMM catalogue. This
has been done at Strasbourg using the original EPIC source lists,
but using the new empirical positional error estimate to determine
the cross-correlation radius. SSC has also taken the opportunity to
expand the list of queried catalogues to include several that were
not used for 1XMM.

The new release of the Xcat-DB

The revised catalogue products have been ingested in a new release
of the Xcat-DB (http://xcatdb.u-strasbg.fr) and also into the XSA
at the XMM-Newton SOC (http://xmm.vilspa.esa.es/xsa).

The new position errors appear in the new version of the FITS
catalogue file in the column named RADEC_ERR. The original
positional errors are retained in the RADEC_ERR_ORG column.


1: http://xmmssc-www.star.le.ac.uk/newpages/UserGuide_1xmm.html;
   section 6.4
2: http://xmmssc-www.star.le.ac.uk/newpages/UserGuide_1.1xmm.html
3: http://xmmssc-www.star.le.ac.uk/ftp/public/SSC-OAS-TN-21.ps.gz

Yours sincerely,
XMM-Newton SOC
XMM-NEWS mailing list

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