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

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Schwarz, R. (Robert), Schwope, A. D. (Axel D.), Vogel, J., Dhillon, V. S., Marsh, T. R., Copperwheat, C. M., Littlefair, S. P. and Kanbach, G.. (2009) Hunting high and low : XMM monitoring of the eclipsing polar HU Aquarii. Astronomy & Astrophysics, Vol.496 (No.3). pp. 833-840. ISSN 0004-6361

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Official URL: http://dx.doi.org/10.1051/0004-6361/200811485

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 ULTRACAMinstruments. 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 LX = 4.7 × 1028 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 of ˙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.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science > Physics
Library of Congress Subject Headings (LCSH):	Eclipsing binaries, X-ray binaries, Cataclysmic variable stars, X-ray sources, Galactic -- Accretion
Journal or Publication Title:	Astronomy & Astrophysics
Publisher:	EDP Sciences
ISSN:	0004-6361
Date:	March 2009
Volume:	Vol.496
Number:	No.3
Number of Pages:	8
Page Range:	pp. 833-840
Identification Number:	10.1051/0004-6361/200811485
Status:	Peer Reviewed
Access rights to Published version:	Open Access
Funder:	Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR), Science and Technology Facilities Council (Great Britain) (STFC)
Grant number:	FKZ 50 OR 0206 (DLR), 50 OR 0404 (DLR), PP/D002370/1 (STFC)
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URI:	http://wrap.warwick.ac.uk/id/eprint/3431