EX Hydrae in outburst

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Abstract
Photometry and red spectroscopy of the intermediate polar EX Hya in its rare outburst state are presented. Photometry during the declining phase of the 1986 July–August outburst shows the 67-min (spin) modulation to be present with similar characteristics to that in quiescence. In contrast, photometry from near the peak of the 1987 May outburst shows little evidence of the 67-min modulation, whilst spectroscopy obtained nearly simultaneously is similarly lacking in such evidence, despite its presence in quiescent spectroscopic data. Near the beginning of the 1987 May outburst the Hα emission line develops a broad, high velocity base component whose velocity is modulated with the orbital cycle. The velocity and phase of the broad base component suggest that it is produced near the magnetosphere of the white dwarf at a point along the projected trajectory of the gas stream from the companion. The feature disappears later in the outburst and is not present during quiescence. We suggest that the outbursts in EX Hya are caused by an increase in the rate of mass transfer from the companion, and that part of this enhanced mass-transfer stream skims over the top of the accretion disc to strike the magnetosphere directly. The interaction of the stream with the magnetosphere gives rise to the broad-base component we see.

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