A multiwavelength timing analysis of the eclipsing polar DP Leo

Abstract

We present an analysis of the X-ray light curves of the magnetic cataclysmic variable DP Leo using recently performed XMM-Newton EPIC and archival published and unpublished ROSAT/ PSPC observations. We combine the timings of the X-ray eclipses with timings derived from archival HST-observations and new optical observations with the photon counting OPTIMA camera. We determine the eclipse length at X-ray wavelengths to be 235+/-5 s, slightly longer than at ultra-violet wavelengths, where it lasts 225 s. A new orbital ephemeris is derived which connects the more than 120 000 binary cycles covered since 1979. It has a highly significant quadratic term, implying an orbital period change of dot {P} = -4.4 x 10^-12 s s^-1, two orders of magnitude larger than being compatible with braking by gravitational radiation only. Over the last twenty years, the optical and X-ray bright phases display a continuous shift with respect to the eclipse center by ~ 2.1degr yr^-1. Over the last 8.5 years the shift of the X-ray bright phase is ~ 2.5degr yr^-1. We interpret this as evidence of an asynchronously rotating white dwarf although synchronization oscillations cannot be ruled out completely. If the observed phase shift continues, a fundamental rearrangement of the accretion geometry must occur on a time-scale of some ten years. Applying model atmosphere spectra to optical/UV eclipse light curves, we determine the temperature and mass of the white dwarf, the temperature and size of the optical/UV emitting spot and the distance to DP Leo to be T_wd = 13 500 K, M_wd =~ 0.6 M_sun, T_spot = 32 000 K, A_spot =~ 0.1 A_wd, and D = 400 pc, respectively. The implied inclination and mass ratio are i=79.5degr and Q = M_wd/M₂ = 6.7. DP Leo is marginally detected at eclipse phase in X-rays. The upper limit eclipse flux is consistent with an origin on the late-type secondary, L_X =~ 2.5 x 10²⁹ ergs s^-1 (0.20-7.55 keV), at a distance of 400 pc.