Swift J0503.7−2819: a Short-period Asynchronous Polar or Stream-fed Intermediate Polar

Abstract

We analyze a 7.4 hr XMM-Newton light curve of the cataclysmic variable Swift J0503.7−2819, previously classified using optical periods as an intermediate polar (IP) with an orbital period of 0.0567 days. A photometric signal at 975 s, previously suggested to be the spin period, is not present in X-rays and is readily understood as a quasiperiodic oscillation. The X-ray light curve instead shows clear behavior of a highly asynchronous polar or stream-fed IP. It can be described by either of two scenarios: one that switches between one-pole and two-pole accretion, and another in which accretion alternates fully between two poles. The spin periods in these two models are 0.0455 days and 0.0505 days, respectively. The spin frequency ω is thus either 24% faster or 12% faster than the orbital frequency Ω, and the corresponding beat period between spin and orbit is 0.231 days or 0.462 days. Brief absorption events seen in the light curve are spaced in a way that may favor the longer spin and beat periods. These periods are confirmed and refined using data from the Transiting Exoplanet Survey Satellite and the Asteroid Terrestrial-impact Last Alert System. The short beat cycle of Swift J0503.7−2819 makes it well-suited to resolving this common dilemma, which amounts to deciding whether the main signal in the power spectrum is ω or 2ω − Ω.