Toward a clean sample of ultra-luminous X-ray sources

Instituto de Astrofísica de Canarias, C/.vía Láctea, s/n, 38200 La Laguna (S/C de Tenerife), Spain e-mail: martinlc@iac.es

Received: 19 December 2005
Accepted: 21 February 2006

Abstract

Context.Observational follow-up programmes for the characterization of ultra-luminous X-ray sources (ULXs) require the construction of clean samples of such sources in which the contamination by foreground/background sources is minimum.

Aims.We calculate the degree of foreground/background contaminants among the ULX sample candidates in a published catalogue and compare these computations with available spectroscopic identifications.

Methods.We use statistics based on known densities of X-ray sources and AGN/QSOs selected in the optical. The analysis is done individually for each parent galaxy. The existing identifications of the optical counterparts are compiled from the literature.

Results.More than a half of the ULXs, within twice the distance of the major axis of the 25 mag/arcsec² isophote from RC3 nearby galaxies and with X-ray luminosities L_X[ 2-10 keV erg/s, are expected to be high redshift background QSOs. A list of 25 objects (clean sample) confirmed to be real ULXs or to have a low probability of being contaminant foreground/background objects is provided.