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DOI: 10.1051/0004-6361/200912771
Research Note
A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris
K. Beuermann1, J. Diese2, S. Paik3, A. Ploch2, J. Zachmann2, A. D. Schwope4, and F. V. Hessman11 Institut für Astrophysik, Göttingen, Germany
e-mail: [beuermann;hessman]@astro.physik.uni-goettingen.de
2 Max-Planck-Gymnasium, Göttingen, Germany
e-mail: diese@gmx.de,
e-mail: icarebooten@web.de,
e-mail: jzachmann@t-online.de
3 Felix-Klein-Gymnasium, Göttingen, Germany
e-mail: swishfugu00@yahoo.co.kr
4 Astrophysikalisches Institut Potsdam, Potsdam, Germany
e-mail: aschwope@aip.de
Received 26 June 2009 / Accepted 15 August 2009
Abstract
Aims. We searched for long-term period changes in the polar EK UMa using
new optical data and archival X-ray/EUV data.
Methods. 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.
Results. 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 (
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 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° in azimuth, providing some insight into the accretion
geometry.
Key words: novae, cataclysmic variables -- stars: individual: EK UMa -- X-rays: stars
© ESO 2009