Doppler Shifts and Broadening and the Structure of the X‐Ray Emission from Algol

In a study of Chandra High Energy Transmission Grating spectra of Algol, weclearly detect Doppler shifts caused by the orbital motion of Algol B. Thesedata provide the first definitive proof that the X-ray emission of Algol isdominated 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 Rsolarfor the Algol B center of mass. This could be caused by a small contribution ofX-ray flux from Algol A (10-15%), possibly through accretion. The more likelyexplanation is an asymmetric corona biased toward the system center of mass bythe tidal distortion of the surface of Algol B. Analysis of the strongest linesindicates excess line broadening of ~150 km/s above that expected from thermalmotion and surface rotation. Possible explanations include turbulence, flows orexplosive events, or rotational broadening from a radially extended corona. Wefavor the latter scenario and infer that a significant component of the coronaat temperatures <10^7 K has a scale height of order the stellar radius. This issupported by the shape of the X-ray lightcurve and the shallow dip at secondaryeclipse. We also examine the O VII intercombination and forbidden lines in aLow Energy Transmission Grating Spectrograph observation and find no change intheir relative line fluxes as the system goes from quadrature to primaryeclipse. Since these lines are strongly affected by UV irradiation from AlgolA, this supports the conjecture that the corona of Algol B at temperatures ofseveral million K must be significantly extended and/or located toward thepoles to avoid being shadowed from Algol A during primary eclipse.Comment: 36 pages, 10 figure