EDP Sciences
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Volume 392, Number 2, September III 2002
Page(s) 541 - 551
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011651

A&A 392, 541-551 (2002)
DOI: 10.1051/0004-6361:20011651

A multiwavelength timing analysis of the eclipsing polar DP Leo

A. D. Schwope1, V. Hambaryan1, R. Schwarz1, G. Kanbach2 and B. T. Gänsicke3

1  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
3  Deptartment of Physics and Astronomy, University of Southampton, Hampshire, Southampton, SO17 1BJ, UK

(Received 3 August 2001 / Accepted 17 June 2002 )

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curves of the magnetic cataclysmic variable DP Leo using recently performed XMM-Newton EPIC and archival published and unpublished ROSAT PSPC observations.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 $235\pm5$ 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 $\dot{P} = -4.4 \times
10^{-12}$ 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 ~ $2.1\degr$ yr -1. Over the last 8.5 years the shift of the X-ray bright phase is ~ $2.5\degr$ 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 $T_{\rm wd} = 13\,500$ K, $M_{\rm wd} \simeq 0.6$  $M_\odot$, $T_{\rm spot} = 32\,000$ K, $A_{\rm spot} \simeq 0.1~A_{\rm wd}$, and D = 400 pc, respectively. The implied inclination and mass ratio are $i=79.5\degr$ and $Q = M_{\rm wd}/M_2 = 6.7$. 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, $L_{\rm X} \simeq 2.5 \times 10^{29}~{\rm ergs~s}^{-1}$ (0.20-7.55 keV), at a distance of 400 pc.

Key words: stars: binaries: eclipsing -- stars: novae, cataclysmic variables -- stars:individual: DP Leo -- X-rays: stars

Offprint request: A. D. Schwope, aschwope@aip.de

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