Hot stars observed by XMM-Newton

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 − 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.

News on the X-ray emission from hot subdwarf stars

“…Both these findings strongly suggest that BD +37 ∘ 442 is a single star without any companion and that the observed X-ray emission originates from the sdO star itself. This hypothesis is supported by other data: the estimated X-ray luminosity implies an X-ray-to-bolometric ratio L X / L BOL ∼ 2 × 10 −7 , a value consistent with the average relation observed in normal O-type stars; we already know from UV and optical spectroscopy that this star is characterized by a significant stellar wind (Jeffery & Hamann 2010); finally, the estimated plasma temperature is comparable to the lowest values seen in the sample of O-type stars observed with XMM-Newton (Nazé 2009). Therefore, we conclude that the observed X-ray emission is most probably related to the shock-heated plasma produced by instabilities in the radiation-driven stellar wind of the sdO star.…”

“…For all stars the detected X-ray fluxes are very low (f X ∼ 10 −14 erg cm −2 s −1 ); they imply L X = 10 30−31 and 10 28−29 erg s −1 for the 'luminous' and 'compact' stars, respectively. Two results suggest that also for these subdwarf stars the observed X-ray emission originates in the stellar wind: in all cases the X-ray/bolometric flux ratio is ∼ 10 −7 , in agreement with the relation found for the massive early-type stars; both the spectra of BD +37 ∘ 442 (Mereghetti et al 2017) and BD +37 ∘ 1977(La Palombara et al 2015 were well described with the sum of two plasma emission models with different temperatures, as in the case of the massive stars (Nazé 2009). …”

“…After almost fourty years of studies with telescopes of increasing performances, now it is well known that these stars show X-ray luminosities up to L X ∼ 10 33 erg s −1 and spectra of thermal type, with plasma temperatures T ∼ 10 6 K; moreover, several surveys have shown that the X-ray and bolometric luminosities of these stars are related by the relation L X /L BOL = 10 −7±1 (Nazé 2009). These stars are characterized by strong stellar winds, with typical mass-loss ratesṀ W = 10 −7 − 10 −5 M ⊙ yr −1 and terminal velocities of a few thousands km s −1 .…”

The lack of X-ray pulsations in the extreme helium star BD+37°442 and its possible stellar wind X-ray emission

“…The temperature found in the spectral fits of BD +37 • 442 is in the lower range of the values seen in the sample of O-type stars observed with the XMM-Newton EPIC instrument, when a singletemperature plasma is sufficient to describe their low resolution Xray spectra (Nazé 2009). This might be related to the weaker wind of BD +37 • 442 compared to that of normal O-type stars.…”

“…X-rays are produced in the winds of single stars by shockheated plasma resulting from various instabilities (see, e.g., Oskinova 2016, and references therein). An empirical correlation has been found between the X-ray and bolometric luminosities of early type stars: L X ∼ 10 −7 L BOL (Pallavicini et al 1981;Nazé 2009). The X-ray luminosity we derived for BD +37 • 442 corresponds to a value of L X /L BOL ∼ 2 × 10 −7 , fully consistent with the observed dispersion around the above average relation.…”