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curves of the magnetic cataclysmic variable DP Leo using recently performed XMM-Newton EPIC and archival published and unpublished ROSAT PSPC observations.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 P = -4.4 × 10-12 ss-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 ∼2.1° yr-1. Over the last 8.5 years the shift of the X-ray bright phase is ∼2.5° 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 Twd = 13 500 K, Mwd ≃ 0.6 M⊙, Tspot = 32 000 K, Aspot ≃ 0.1 Awd, and D = 400 pc, respectively. The implied inclination and mass ratio are i = 79.5° and Q = Mwd/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, Lx ≃ 2.5 × 1029 ergs s-1 (0.20-7.55 keV), at a distance of 400 pc.