The close binary Algol system contains a radio-bright KIV subgiant star in a very close (0.062 astronomical units) and rapid (2.86 day) orbit with a main sequence B8 star. Because the rotation periods of the two stars are tidally locked to the orbital period, the rapid rotation drives a magnetic dynamo. A large body of evidence points to the existence of an extended, complex coronal magnetosphere originating at the cooler K subgiant1,2,3,4. The detailed morphology of the subgiant's corona and its possible interaction with its companion are unknown, though theory predicts that the coronal plasma should be confined in a magnetic loop structure5, as seen on the Sun. Here we report multi-epoch radio imaging of the Algol system, in which we see a large, persistent coronal loop approximately one subgiant diameter in height, whose base is straddling the subgiant and whose apex is oriented towards the B8 star. This suggests that a persistent asymmetric magnetic field structure is aligned between the two stars. The loop is larger than anticipated theoretically6,7, but the size may be the result of a magnetic interaction between the two stars.
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The NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. Support for this work was provided by the NSF through award GSSP08-0022 from the NRAO. R.L.M. is grateful to the Institute of Astronomy at ETH, Zurich, for logistic support during the preparation of this paper.
Author Contributions All authors contributed substantially to this work. R.L.M. and M.G. did the observation planning. W.M.P. and W.M.G. analysed the observations. R.L.M., M.G. and W.M.P. wrote and edited the manuscript.
The authors declare no competing financial interests.
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Peterson, W., Mutel, R., Güdel, M. et al. A large coronal loop in the Algol system. Nature 463, 207–209 (2010). https://doi.org/10.1038/nature08643
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