Flare Activity in Algol-type Binaries

Two sites of flare activity have been identified in Algol-type binaiies. One is associated with the shock region from the accretion on B-A type primary, another, predominant one is apparently connected with coronal mass ejections (CME) from the late-type, Roche lobe filling secondary. The relative contribution from both sources of activity is still to be found. According to Peters & Polidan (1984), a moderately hot (105 K) and low density ne ≃ 109 cm−3 high turbulent accretion region (HTAR) surrounding the hot component has been discovered. Typical dimensions of HTAR agree with the scale height of the X-ray source HX ≃ 1.2 · 1011 · T7 cm (Harnden et al 1977), where T7 is the temperature in units of 107 K. But the decay time of the HTAR, comparable to the orbital period, more favourably agrees with the idea of CME as deduced from Ginga observations of X-ray flares on Algol (Stern et al. 1992). One solution of the controversy as to the contribution from HTAR would be to concentrate on X-ray observations of systems like u Her and U CrB, where the secondary is possibly earlier than FO (thus, is not expected to possess an extensive convection zone) and on the other hand on systems like U Cep, where during periods of high activity Ṁ ≃ 10−6M⊙ per year. Thus, the accretion luminosity must be at least by two orders of magnitude higher than in Algol. At present the analysis of X-ray flares is based upon the solar type CME assuming different scenarios of radiative and/or conductive cooling (see Stern et al. 1992). However, the current models of CME do not take into account the binary nature of the flaring object.