EVIDENCE FOR SUPERHUMPS IN THE RADIO LIGHT CURVE OF ALGOL AND A NEW MODEL FOR MAGNETIC ACTIVITY IN ALGOL SYSTEMS

摘要

Extensive radio data of two Algol systems and two RS CVn binaries were reanalyzed. We found evidence for a new periodicity that we interpret as a superhump in β Per (Algol), for which it may have been expected according to its semidetached nature and low binary mass ratio. The concluded presence of an accretion disk (or an annulus) is consistent with previous studies of optical data and numerical simulations of β Per. In our model, the 50 day period, previously found in the radio data of β Per, is explained as the apsidal precession of the elliptical accretion disk or annulus. If our interpretation is correct, this is the first detection of the superhump phenomenon in the radio and the first observation of superhumps in Algol systems. According to our result, the accretion disk or annulus in β Per processes in spite of its non-Keplerian nature, and therefore, this phenomenon is not restricted to the classical Keplerian accretion disks in compact binaries. We propose that in Algol systems with short orbital periods, which have accretion disks or annuli, the disk is magnetically active in addition to the cool secondary star. The magnetic field in the disk originates from amplification of the seed field in the magnetized material transferred from the secondary. The disk and stellar fields interact with each other, with reconnection of the field lines causing flares and particle acceleration. Relativistic particles are trapped in the field and directed toward the polar regions of the secondary star because of the dipole structure of its magnetic field. These systems are, therefore, somewhat analogous to the RS CVn systems, which have two magnetically interacting stars. Our proposed model for the magnetic activity in Algol systems provides a simple explanation for the observed properties of β Per in the radio wavelengths, and in particular, for the presence of quiescent gyrosynchrotron emission near the polar region of the secondary star, where electrons are difficult to confine if the field lines are open as in normal, single, magnetic stars. It further explains the recent discovery that the Doppler shifts of the emission lines in the X-ray data of β Per are somewhat lower than what is expected from the orbital motion of the secondary star. We propose that the superhump variation in the radio is generated by enhanced reconnection when the elongated side of the elliptic accretion disk is the closest to the cool star. This leads to flares and enhancement in particle acceleration and is manifested as stronger gyrosynchrotron radiation. The observed superhump period, at 3.037 ± 0.013 days (～6% longer than the orbital period), was used to deduce a binary mass ratio of ～0.23 in β Per, which is consistent with previous studies. Our finding opens the possibility to extend the superhump phenomenon to Algol systems and to test the theories of precessing accretion disks in various types of interacting binaries. It provides a new method to estimate the mass ratios in these binaries. It also offers new insights and improves our understanding of the complicated magnetic interaction and feedback between mass transfer dynamics, time-dependent disk accretion, and induced magnetic activity in the Algol and related systems.