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.

Orbital and spin phase-resolved spectroscopy of the intermediate polar EX Hya using XMM-Newton data

“…This broad modulation was well observed over the optical, UV and X‐ray regime and attributed to the bulge and/or hotspot on the disc (e.g. Belle et al 2005; Hoogerwerf et al 2005; Mhlahlo et al 2007a,b). The dip gets stronger and moves to phases closer to the eclipse as the wavelength increases due to the changes in the ionization state of the bulge.…”

The discovery of a persistent quasi-periodic oscillation in the intermediate polar TX Col

“…We suggest that the material that forms a ‘base excursion’ (Mhlahlo et al, in preparation, see also Hellier et al 1989; Mhlahlo et al 2007b) due to overflow stream falling near the magnetosphere/disc boundary, and the ‘blobs’ that drift from the outer disc towards this same shocked region, pile up near this region and are dumped on to the surface of the white dwarf via a mechanism similar to that of Spruit and Taam before the field lines snap to produce a prograde travelling wave (or ‘wall’) of Warner & Woudt (2002).…”

“…Spruit & Taam (1993) pointed out that their model could be applied to IPs to explain the QPO phenomena seen in these systems. This model was used recently by Mhlahlo et al (2007b) to describe the outburst of EX Hya.…”

Dwarf nova outbursts in intermediate polars

“…Such large-amplitude, short outbursts should also be present in non-magnetic systems -and they might be present; we refer to, for example, the case of V1316 Cyg (Shears et al 2006) which, in addition to standard dwarf nova outbursts, exhibits short (less than 1−2 days), faint (average amplitude of 1.4 mag), relatively frequent (6 outbursts recorded over 4 months in 2005) events. The suggestion by Mhlahlo et al (2007) that EX Hya outbursts could be due to the storage and release of matter outside the magnetosphere, with a similar mechanism to that put forward by Spruit & Taam (1993) to explain the Rapid Burster, is interesting, but it remains to be explained why the recurrence time between short DN outbursts in IPs is so long; longer than the viscous time. Another similar possibility is that the coupling of the magnetic field generated by the MRI with the magnetic field of the white dwarf can, under particular conditions, generate an instability.…”