Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-12-22
Astronomy and Astrophysics
Astrophysics
7 pages, to appear in "Active OB Stars: Laboratories for Stellar & Circumstellar Physics", eds. S. Stefl, S. P. Owocki & A. Ok
Scientific paper
A growing number of early Be stars discovered in X-ray surveys exhibit X-ray luminosities intermediate between those of normal stars and those of most Be/X-ray binaries in quiescence. Their X-ray spectra are also much harder than those of shocked wind OB stars and can be best fitted by a thin thermal plasma with T ~ 10^8 K, added to a cooler and much fainter thermal component. An iron line complex including a fluorescence component is detected in many cases. There is no evidence for coherent pulsations in any of these systems but strong variability on time scales as short as 100 s is usually observed. Large oscillations with quasi-periods of the order of one hour or more are detected in the X-ray light curves of several sources, but have so far failed to prove to be strictly periodic. The optical and X-ray properties of these new objects strikingly resemble those of the so far unique and enigmatic Be star Gamma-Cas and define a new class of X-ray emitters. We discuss the possible origin of the X-ray emission in the light of the models proposed for Gamma-Cas, magnetic disc-star interaction or accretion onto a compact companion object -- neutron star or white dwarf.
Haberl Frank
Janot-Pacheco Eduardo
Lopes de Oliveira Raimundo
Motch Christian
Negueruela Ignacio
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