paper

Doppler Shifts and Broadening and the Structure of the X-ray Emission from Algol

arXiv:astro-ph/0401583 · doi:10.1086/383195

Abstract

In a study of Chandra High Energy Transmission Grating spectra of Algol, we clearly detect Doppler shifts caused by the orbital motion of Algol B. These data provide the first definitive proof that the X-ray emission of Algol is dominated by the secondary, in concordance with expectations that Algol A (B8) is X-ray dark. The measured Doppler shifts are slightly smaller than expected, implying an effective orbital radius of about 10 Rsolar, instead of 11.5 Rsolar for the Algol B center of mass. This could be caused by a small contribution of X-ray flux from Algol A (10-15%), possibly through accretion. The more likely explanation is an asymmetric corona biased toward the system center of mass by the tidal distortion of the surface of Algol B. Analysis of the strongest lines indicates excess line broadening of ~150 km/s above that expected from thermal motion and surface rotation. Possible explanations include turbulence, flows or explosive events, or rotational broadening from a radially extended corona. We favor the latter scenario and infer that a significant component of the corona at temperatures <10^7 K has a scale height of order the stellar radius. This is supported by the shape of the X-ray lightcurve and the shallow dip at secondary eclipse. We also examine the O VII intercombination and forbidden lines in a Low Energy Transmission Grating Spectrograph observation and find no change in their relative line fluxes as the system goes from quadrature to primary eclipse. Since these lines are strongly affected by UV irradiation from Algol A, this supports the conjecture that the corona of Algol B at temperatures of several million K must be significantly extended and/or located toward the poles to avoid being shadowed from Algol A during primary eclipse.

arXiv comment36 pages, 10 figures

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