Flare activity in Algol-type binaries

Abstract

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 (10 5 K) and low density n e ≃ 10 9 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 H X ≃ 1.2 · 10 11 · T 7 cm (Harnden et al 1977), where T 7 is the temperature in units of 10 7 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 −6 M⊙ 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.