Radio Emission from Algol

We report dual circular polarization VLBA observations of Algol made at orbital phases 0.22{0.30 using a diierential phase referencing technique. The ux density of Algol varied from 10{20 mJy during the observations. The radio maps show a double-lobe source separated by 1.6 mas (1.4 times the K star diameter). Although the total emission is only weakly circularly polarized, the individual lobes are strongly circularly polarized and of opposite helicity. Snapshot VLBI maps made at three hour intervals show variations in the ux density of both components, but no signiicant motions of the centroids. We also analyze Green Bank Interferometer (GBI) synoptic observations of RCP and LCP ux densities of Algol at 2.3 and 8.3 GHz several times per day from early 1995 to mid-1997. The resulting dataset, which consists of more than 2,500 observations over two years, is by far the most comprehensive available for any stellar system. In addition, we analyzed GBI observations of the very similar (but non-eclipsing) binary system HR1099 over the same time period in order to compare the two systems. We summarize the GBI observations using several statistical descriptions. We nd no phase dependence of either the radio luminosity or circular polarization for either system. The luminosity histograms for both systems are remarkably similar. The distribution functions are not well represented by exponentials as previously suggested, but can be represented by power-laws truncated at low luminosity. The cut-oo occurs at 20{30 mJy and may represent emission from a slowly-varying basal level which is always detected. We connrm several previous results, including the strong dependence { 2 { of spectral index on luminosity, the decrease of fractional circular polarization with luminosity, and the dependence of fractional circular polarization on orbital inclination angle. We suggest the the radio emission at 8.3 GHz is x-mode gyro-synchrotron emission from optically thin emission regions containing mildly relativistic electrons in a dipolar magnetic eld. There is no evidence for highly circularly polarized coherent ares at 8.3 GHz although it is possible that such ares occasionally occur at 2.25 GHz. The lack of orbital phase dependence in the GBI light curves, combined with the signiicant inclination of the VLBA structure with respect to the orbital plane, is inconsistent with previous models in which the radio lobes are located in the equatorial plane. The individual lobes seen in the VLBI maps may be associated with the polar regions, with the strong circular polarization …