X-ray emission of massive stars and their winds

Abstract

Most types of massive stars display X-ray emission that is strongly affected by the properties of their stellar winds. Single non-magnetic OB stars have an X-ray luminosity that scales with their bolometric luminosity and their emission is thought to arise from a distribution of wind-embedded shocks. The lack of significant short-term stochastic variability indicates that the winds consist of a large number of independent fragments. Detailed investigations of temporal variability unveiled a connection between the photosphere and the wind: well-studied O-type stars exhibit a ∼ 10% modulation of their emission on timescales consistent with their rotation period, and a few early B-type pulsators display ∼ 10% modulations of their X-ray flux with the same period as their photospheric pulsations. Unlike OB stars, their evolved descendants (Wolf-Rayet stars and Luminous Blue Variables) lack a well-defined relation between their X-ray and bolometric luminosities, and several subcategories of objects remain undetected. These properties most likely stem from the combined effects of wind optical depth and wind velocity. Magnetic OB stars display an enhanced X-ray emission which is frequently modulated by the rotation of the star. These properties are well explained by the magnetically confined wind shock model and an oblique magnetic rotator configuration. Some massive binaries display phase-dependent excess emission arising from the collision between the winds of the binary components. Yet, the majority of the massive binaries do not show evidence for such an emission, probably as a consequence of radiative cooling of the shock-heated plasma. Finally, a growing subset of the Be stars, the so-called γ Cas stars, feature an unusually hard and strong thermal X-ray emission that varies in a complicated manner over a wide range of timescales. Several scenarios have been proposed to explain these properties, but the origin of the γ Cas phenomenon remains currently one of the major unsolved puzzles in stellar X-ray astrophysics.

X-ray properties of early-type stars in the Tarantula Nebula from T-ReX

“…X-ray variables are highlighted (with "var") on the basis of Gaussian or Poissonian statistics, although inevitably these are limited to the brightest sources with unattenuated X-ray luminosities of 𝐿 X ≥ 10 33 erg s −1 . Oskinova (2005) concluded that, in general, binaries follow the canonical X-ray relationship for single early-type stars, although occasionally colliding wind systems exhibit enhanced, harder X-ray emission, with respect to single stars (Pittard 2011;Gagné et al 2012;Rauw 2022). In general, establishing the single or binary nature of early-type stars is notoriously difficult.…”

“…It is now established that a high fraction of massive stars lie in binary systems with short periods (Sana et al 2012). A subset of binaries are known to exhibit enhanced, often harder (few keV), Xray emission (Pollock 1987;Chlebowski & Garmany 1991;Rauw 2022) attributed to colliding stellar winds, although the majority of binaries do not exhibit excess X-ray emission (Oskinova 2005). Further, stellar atmosphere improvements to our understanding of massive stars have resulted in revisions to the properties of early-★ paul.crowther@sheffield.ac.uk type stars including their temperatures and luminosities (Crowther et al 2002b;Repolust et al 2004) and a subset of early-type stars are known to possess kG-scale magnetic fields (Grunhut et al 2017), so historical calibrations should be used with caution.…”

Stars like gamma Cassiopeia: close to us and enigmatic

“…The presence of a non-thermal component in the X-ray spectra of 𝛾 Cas type stars remains an important problem. Rauw et al [11] believe that the X-ray spectrum of these stars is purely thermal, based on the results of Shrader et al [12]. These authors describe the X-ray spectrum of 𝛾 Cas itself in the region of 0.6-100 keV using the data from the Suzaku and Integral satellites and a thermal model with a temperature of 14 keV and with the addition of Gaussian profiles of the fluorescence…”

ULLYSES and Complementary Surveys of Massive Stars in the Magellanic Clouds

“…O vi λλ1031, 1037) N v λλ1238, 1242) are also sensitive to X-rays (e.g. Taresch et al 1997), which are known to originate in the winds of early-type stars (Rauw 2022). To date the majority of X-ray studies of OB stars have involved Milky Way stars, although deep Chandra X-ray observations of a few star-forming regions in the Magellanic Clouds have been obtained.…”