Hot stars observed by XMM-Newton I. The catalog and the properties of OB stars

TL;DR

This study presents a comprehensive catalog of X-ray observations of about 300 OB stars from XMM-Newton data, analyzing their spectral properties, flux correlations, and variability to enhance understanding of their high-energy phenomena.

Contribution

It provides the first large-scale, high-sensitivity survey of X-ray properties of OB stars, including spectral analysis, flux relations, and variability insights.

Findings

X-ray spectra of O stars are dominated by low-temperature components with additional absorption.

X-ray flux correlates well with bolometric flux, with some scatter.

B stars show higher temperature plasma and no additional absorption, with more variability.

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 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. 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 (<1keV) 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 for some individual clusters. These results contrast with those of B stars that exhibit a large scatter in the L_X-L_BOL 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.