XMM-Newton-NEWS  #45,    16-Dec-2004

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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.esac.esa.int/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