Hunting high and low: XMM monitoring of the eclipsing polar HU Aquarii

Abstract

Aims. We want to study the temporal and spectral behaviour of HU Aqr in the X-ray domain during different accretion states. Methods. We obtained spectra and light curves from four different XMM-Newton pointings covering intermediate and low states. The X-ray observations were accompanied with high time resolution photometry obtained with the Optima and ULTRACAM instruments. Results. On two occasions in May 2002 and 2003 HU Aqr was found in an intermediate state with the accretion rate reduced by a factor of 50 compared to earlier high state measurements. X-ray spectra in the intermediate state can be described by a model containing a blackbody component and hot thermal plasma. Contrary to the high state the ratio between soft and hard X-ray flux is nearly balanced. In agreement with previous measurements we observed a migration of the accretion spot and stream towards the line connecting both stars. The brightness of HU Aqr was further reduced by a factor of 80 during two low states in October 2003 and May 2005, where it was detected at a luminosity of only L X = 4.7 × 10 28 erg s −1 . This luminosity would fit well with an active coronal emitter, but the relatively high plasma temperatures of 3.5 and 2.0 keV are more compatible with residual accretion. We updated the eclipse ephemeris of HU Aqr based on the eclipse egress of the accretion spot measured in various wavelength bands. The (O−C)-diagram of the observed accretion spot eclipse timings reveals complex deviations from a linear trend, which can be explained by a constant or cyclic period change or a combination thereof. The quadratic term implies a period decrease at a rate oḟ P orb = −7.. − 11 × 10 −12 s s −1 . In case the observed period change reflects a true angular momentum loss, this would be a factor of 30 larger than given by gravitational radiation.

The giant planet orbiting the cataclysmic binary DP Leonis

“…Brinkworth et al 2006;Chen 2009;Schwarz et al 2009). The detection of planets in HW Vir (Lee et al 2009) and NN Ser (Beuermann et al 2010), as well as the likely detection in HU Aqr (Schwarz et al 2009;Nasiroglu et al 2010) and QS Vir (Parsons et al 2010), suggests that the occurrence of planets or planetary systems in post-CE binaries may not be a rare incidence.…”

“…Apparent changes may be effected by apsidal motion or the presence of a third body. Eclipse-time variations in cataclysmic variables (CVs) have often been attributed to Applegate's (1992) mechanism of spin-orbit coupling, resulting from changes in the internal constitution of the secondary, but this process is generally too feeble to account for the observed amplitudes (Brinkworth et al 2006;Chen 2009;Schwarz et al 2009). Apsidal motion (Todoran 1972) is unlikely to be present in CVs, because tidal interaction is expected to circularize the orbits effectively.…”

“…DP Leo 1 belongs to the still small group of post-CE binaries known or suspected to possess planets, among them HW Vir (Lee et al 2009), NN Ser (Beuermann et al 2010), HU Aqr (Schwarz et al 2009;Nasiroglu et al 2010), and QS Vir (Parsons et al 2010). DP Leo ab is an 18.5 mag short-period (P orb = 89.9 min) polar, in which a synchronously rotating magnetic white dwarf accretes matter from its Roche-lobe filling companion (Schwope et al 2002;Pandel et al 2002).…”

The quest for companions to post-common envelope binaries

“…The often considered alternative of spin-orbit coupling, where star cycles change the spin of the secondary star and thereby the angular momentum of the binary orbit (Applegate 1992), remains a contender, but may be too feeble to account for the observed effects (e.g. Brinkworth et al 2006;Chen 2009;Schwarz et al 2009;Potter et al 2011). Apsidal motion, can be safely excluded, at least for the objects studied here.…”

“…With more data becoming available, it was realized, however, that internal processes might be too feeble to explain the observations (e.g. Brinkworth et al 2006;Chen 2009;Schwarz et al 2009;Potter et al 2011), but this notion was opposed by Wittenmyer et al (2012). It was also realized that the detached post-common envelope binaries (PCEBs) represent more suitable laboratories for studying eclipse time variations than the accreting cataclysmic variables, in particular, the variety to which UX UMa belongs.…”