Outbursts of EX Hydrae revisited

Abstract

We present optical spectroscopy of EX Hya during its 1991 outburst. This outburst is characterized by strong irradiation of the front face of the secondary star by the white dwarf, an overflowing stream which is seen strongly in He IIλ4686 and by a dip in the light curves, which extends from 0.1 to 0.6 in the binary and spin phases. Strong irradiation of the accretion curtain and that of the inner regions of the disc led to strong emission of He IIλ4686 and to the suppression of the Hγ and Hβ emission. Disc overflow was observed in quiescence in earlier studies, where the overflow stream material was modulated at high velocities close to 1000 km s−1. In outburst, the overflowing material is modulated at even higher velocities (∼1500 km s−1). These are streaming velocities down the field lines close to the white dwarf. Evidence for material collecting near the outer edge of the disc and corotating with the accretion curtain was observed. In decline, this material and the accretion curtain obscured almost all the emission near binary phase 0.4, causing a dip. The dip minimum nearly corresponds with spin pulse minimum. This has provided additional evidence for an extended accretion curtain, and for the corotation of material with the accretion curtain at the outer edge of the disc. From these observations we suggest that a mechanism similar to that of Spruit and Taam, where outbursts result due to the storage and release of matter outside the magnetosphere, triggers the outbursts of EX Hya. This is followed by the irradiation of the secondary star due to accretion induced radiation.