A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris

Details A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris

2009-11-06

arxiv.org/abs/0911.1270v1

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

4 pages, 2 Postscript figures, accepted for publication in Astronomy & Astrophysics

Scientific paper

We searched for long-term period changes in the polar EK UMa using new optical data and archival X-ray/EUV data. An optical ephemeris was derived from data taken remotely with the MONET/N telescope and compared with the X-ray ephemeris based on Einstein, Rosat, and EUVE data. A three-parameter fit to the combined data sets yields the epoch, the period, and the phase offset between the optical minima and the X-ray absorption dips. An added quadratic term is insignificant and sets a limit to the period change. The derived linear ephemeris is valid over 30 years and the common optical and X-ray period is P=0.0795440225(24) days. There is no evidence of long-term O-C variations or a period change over the past 17 years Delta P = -0.14+-0.50 ms. We suggest that the observed period is the orbital period and that the system is tightly synchronized. The limit on Delta P and the phase constancy of the bright part of the light curve indicate that O-C variations of the type seen in the polars DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The X-ray dips lag the optical minima by 9.5+-0.7 deg in azimuth, providing some insight into the accretion geometry.

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