RS CVn versus ALGOL-type binaries: a comparative study of their X-ray emission

Abstract

We have compiled a list of 59 RS CVn binaries and 29 Algol-type binaries with well-known orbital parameters and which have been observed with the ROSAT PSPC detector in order to find out whether there is any detectable influence of mass transfer on the X-ray emission properties of these two types of active binaries. if there were, one might expect the RS CVn binaries (which typically are expected to have negligible mass exchange) and the semidetached "Algol" binaries (which have well-established mass exchange between the two components) to exhibit systematic differences in the X-ray region. We have studied the statistical correlations between (a) the X-ray luminosity (L_x) and the Roche-lobe filling fraction (Γ₂) for the active star, (b) L_x and the rotation period (P), and (c) the X-ray surface flux (S_x) and P. In general, we find that the RS CVn and Algol binaries do not have similar L_x distributions. There is a tendency for the Algol binaries to be ~3-4 times less luminous compared to RS CVns with the same period. This is the exact opposite of what might have been expected if mass transfer rates influenced X-ray emission levels, since in that case the Algol binaries would be more X-ray luminous than the RS CVn binaries. A highly significant correlation is found between log S_x and log P in both of the two types of binaries, while a weak correlation that is observed between log L_x and log Γ₂ in the RS CVn binaries is likely to be due to correlations of these quantities with the stellar radii.