Magnetism and Rapid Photometric Variability

in Symbiotic Binary Stars

by

Jennifer Lynn Sokoloski

Ph.D. (University of California, Berkeley) 1999

Supervised by Professor Lars Bildsten

Abstract

We report on our survey for rapid (time scale of minutes) photometric variability in symbiotic binaries. These binaries are becoming an increasingly important place to study accretion onto white dwarfs due to their potential role as Type Ia supernova progenitors. Unlike in most cataclysmic variables, the white dwarfs in symbiotics typically accrete from a wind at rates greater than or equal to $10^{-9} \msyr$. In order to elucidate the differences between symbiotics and other white dwarf accretors, as well as search for magnetism in symbiotic white dwarfs, we have studied 35 symbiotic binaries via differential optical photometry. Included in our sample are all but one of the symbiotics from the lists of \citet{ken86} and \citet{dk88} with published $V$ magnitude less than 14 and declination greater than $-20^\circ$. Our study is the most comprehensive to date of rapid variability in symbiotic binaries.

Our survey has found one magnetic accretor, Z And. The discovery of a persistent oscillation at $P=1682.6\pm0.6$ s in the optical emission from Z And is the first of its kind for a symbiotic. The oscillation was detected on all 8 occasions on which the source was observed, over a time span of nearly a year. The amplitude was typically $2 - 5$ mmag, and it was correlated with the optical brightness during a relatively small outburst of the system. The most natural explanation is that the oscillation arises from the rotation of an accreting magnetic ($B_{\rm S} \simgt 10^5$G) white dwarf. This discovery constrains the outburst mechanisms, since the oscillation emission region near the surface of the white dwarf was visible during the outburst.

We have also confirmed that 5 symbiotics are strong ``flickerers'' on time scales as short as minutes, at levels which sometimes exceed 100 mmag. These are the 2 recurrent novae RS Oph and T CrB, plus CH Cyg, $o$ Ceti (Mira AB), and MWC 560. We do not confirm previous claims of periodicities in any of these systems, and we evaluate the magnetic propeller model in light of these results. Four other symbiotics show some evidence for flickering at a low level (EG And, BX Mon, CM Aql, and BF Cyg). These detections are marginal, however, and could be the result of systematic effects. For the other 25 systems, we place strong upper limits on both aperiodic variability (flickering) and periodic variability. We discuss the impact of our results on the ``standard'' picture of wind-fed accretion, and speculate on the causes of some of the outbursts typically observed in symbiotics.

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