Hot stars observed by XMM-Newton^{*,**,***,****}

I. The catalog and the properties of OB stars

GAPHE, Département AGO, Université de Liège, Allée du 6 Août 17, Bât. B5C, B4000 Liège, Belgium e-mail: naze@astro.ulg.ac.be

Received: 9 June 2009
Accepted: 6 July 2009

Abstract

Aims. Following the advent of increasingly sensitive X-ray observatories, deep observations of early-type stars became possible. However, the results for only a few objects or clusters have until now been reported and there has been no large survey comparable to that based upon the ROSAT All-Sky Survey (RASS).

Methods. A limited survey of X-ray sources, consisting of all public XMM-Newton observations (2XMMi) and slew survey data (XMMSL1), is now available. The X-ray counterparts to hot, massive stars have been searched for in these catalogs.

Results. About 300 OB stars were detected with XMM-Newton. Half of them were bright enough for a spectral analysis to be possible, and we make available the detailed spectral properties that were derived. The X-ray spectra of O stars are represented well by low (<1 keV) temperature components and seem to indicate that an absorption column is present in addition to the interstellar contribution. The X-ray fluxes are well correlated with the bolometric fluxes, with a scatter comparable to that of the RASS studies and thus larger than found previously with XMM-Newton for some individual clusters. These results contrast with those of B stars that exhibit a large scatter in the L_X – relation, no additional absorption being found, and the fits indicate a plasma at higher temperatures. Variability (either within one exposure or between multiple exposures) was also investigated whenever possible: short-term variations are far more rare than long-term ones (the former affects a few percent of the sample, while the latter concerns between one third and two thirds of the sources).

Conclusions. This paper presents the results of the first high-sensitivity investigation of the overall high-energy properties of a sizable sample of hot stars.