Institut für Astronomie und Astrophysik

Abteilung Astronomie

Preprint 07/97

Periodic Changes of the Accretion Geometry in the Nearly-Synchronous Polar RX J1940.1-1025

Ralf D. Geckeler and Rüdiger Staubert

Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany

accepted by Astronomy & Astrophysics

Abstract. The magnetic Cataclysmic Variable (mCV) RX J1940.1-1025 belongs to the three-member subclass of polars with a slight non-synchronism (<2%) between the spin of the white dwarf and the orbital rotation. In these systems the accretion geometry changes periodically with phase of the beat frequency. We have now found in RX J1940.1-1025 a periodic shift in the timings of the `troughs' in the X-ray and optical pulse profiles with beat phase. We interpret this to be due to a longitudinal displacement of the accretion region relative to the magnetic pole and to the varying angle between the field line and the white dwarf surface, caused by the asynchronous rotation. Such an effect is expected, because the accreting matter is progressively threaded by different field lines during the beat period and therefore produces a hot spot at varying positions relative to the magnetic pole.
The application of a dipole accretion model to the data of RX J1940.1-1025 yields a refined value for the spin period of 12150.40 +/- 0.08 s (epoch 1994.5), with a resulting beat period of 50.0 +/- 0.3 d (1994.4). The fit includes a quadratic term in P_spin, leading to a value for a secular decrease of the spin period by -(9.8 +/- 2.9) * 10^-9 s/s, marginally significant. This would correspond to a synchronization time scale of 100 y, comparable to the 170 y seen in V1500 Cyg.
In addition, further constraints on the accretion geometry in RX J1940.1-1025 are given.

Key words: cataclysmic variables - stars: individual: RX J1940.1-1025 - stars: magnetic fields - X-rays: stars

Paper (90k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de) Last updated 04-Aug-1997