Step-like variations in the optical spectrum of the magnetic cataclysmic variable EU UMA (RE1149+28)

We present the results of optical time-resolved spectroscopy of the magnetic cataclysmic variable EU UMA (RE1149+28) obtained with the help of the scanner of the 6 m telescope on April 3 and 4, 1994. Analysis of the behaviour of the parameters of Balmer and HeII emission lines in the spectra of EU UMA has revealed

-- dips in the behavior of equivalent widths of the lines repeatable with the orbital period. Considering the dips as a result of eclipse of the most part of the accretion stream by the secondary we have improved the value of the orbital period of 90.1441 ±0.0007 min and estimated the inclination of the system as ≈ 70°;

-- the radial velocity variations over the orbital period and the oscillations of the velocities of the center of gravity and the peak of the lines with periods of 45 min and 37 min, respectively. We associate the 45 min oscillations with the radiation from the accretion curtain or the beginning of the magnetic part of the accretion stream and the 37 min variations with the radiation from the regions with strong resonance magnetic fields, which was observed as monochromatic pulsations in the spectra of intermediate polars (Somov et al. 1997, 1998a, 1998b, 2000, 2001). The presence of the 45 min oscillations is an argument in favor of synchronous rotation of the accretion curtain or diskless accretion in the system. The 37 min variations point to asynchronous rotation of the white dwarf over a spin period of ≈74 min.

Spectral light curves (integrated flux in the range of wavelengths 4000-5000 Å) have shown significant changes of the mean fluxes (≈ 4 times) and of the composition of quasi-periodic oscillations in the range of periods 20-120 min on a time-scale of 1 day. In the high state of brightness on April 3, 1994 the spectral light curve showed a hump which corresponds to the hot spot in the treading region, where the ballistic stream is redirected to follow the magnetic field lines of the white dwarf.

In the low accretion state on April 4, 1994 the spectral light curve showed a step-like jump when the mean brightness of the object increased ≈ 2 times. Such a behaviour of the system can be the result of the change of the magnetic part of the accretion trajectory due to asynchronism. We found a relationship between the jump of brightness and the jump of the phase and amplitude of the 37 min oscillations in the emission lines. On the basis of all the detected properties we conclude that the system manifests the properties of both a polar and an intermediate polar and is asynchronous magnetic rotator with a polar-like magnetic field and with the magnetic and rotation poles close to each other.