Is T Leonis a superoutbursting intermediate polar?

Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg e-mail: [svrielmann, jness, jschmitt]@hs.uni-hamburg.de

Corresponding author: S. Vrielmann, svrielmann@hs.uni-hamburg.de

Received: 11 December 2003
Accepted: 13 February 2004

Abstract

We present an XMM-Newton analysis of the cataclysmic variable T Leo. The X-ray light curve shows sinusoidal variation on a period P_x equal to times the previously spectroscopically determined orbital period. Furthermore, we find a signal in the power spectrum at 414 s that could be attributed to the spin period of the white dwarf. If true, T Leo would be the first confirmed superoutbursting intermediate polar (IP). The spin profile is double-peaked with a peak separation of about 1/3 spin phases. This appears to be a typical feature for IPs with a small magnetic field and fast white dwarf rotation.
An alternative explanation is that the 414 s signal is a Quasi-periodic Oscillation (QPO) that is caused by mass transfer variation from the secondary, a bright region (“blob”) rotating in the disc at a radius of approximately 9R_wd or – more likely – a travelling wave close to the inner disc edge of a dwarf nova with a low field white dwarf.
The XMM-Newton RGS spectra reveal double peaked emission for the O VIII Ly α line. Scenarios in the IP and dwarf nova model are discussed (an emitting ring in the disc, bright X-ray spot on disc edge, or emitting accretion funnels), but the intermediate polar model is favoured. Supported is this idea by the finding that only the red peak appears to be shifted and the “blue” peak is compatible with the rest wavelength. The red peak thus is caused by emission from the northern accretion spot when it faces the observer. Instead, the peak at the rest wavelength is caused when the southern accretion funnel is visible just on the lower edge of the white dwarf – with the velocity of the accreting material being perpendicular to the line of sight.