Abstract
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.
Highlights
Long-term studies of cataclysmic variables and related objects have shown that variations in the orbital period occur in some of them, which are not understood or badly so (e.g. Beuermann & Pakull 1984; Pandel et al 2002; Schwope et al 2002; Brinkworth et al 2006; Schwarz et al 2009)
No optical photometry besides that in the discovery paper is in the literature, but an accurate ephemeris based on ROSAT X-ray data was derived by Schwope et al (1995)
The fifteen new MONET/N timings of 2009 combined with the 1985 timing of Morris et al (1987) define an alias-free ephemeris with an optical period Popt = 0.079544025(28) days, where the number in brackets refers to the uncertainty in the last digits
Summary
Long-term studies of cataclysmic variables and related objects have shown that variations in the orbital period occur in some of them, which are not understood or badly so (e.g. Beuermann & Pakull 1984; Pandel et al 2002; Schwope et al 2002; Brinkworth et al 2006; Schwarz et al 2009). Long-term studies of cataclysmic variables and related objects have shown that variations in the orbital period occur in some of them, which are not understood or badly so No optical photometry besides that in the discovery paper is in the literature, but an accurate ephemeris based on ROSAT X-ray data was derived by Schwope et al (1995). The combined data allow us to establish a common long-term ephemeris that extends 30 years back to the Einstein era and accounts for a systematic phase offset between the X-ray absorption dips and the optical cyclotron minima. No period change was found, but we derive a tight limit on O − C variations and associated period changes over the past 17 years
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