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XMM MONITORING OF THE ECLIPSING POLAR HU AQUARII

Robert Schwarz1, Axel D. Schwope1, Vikram S. Dhillon2, and Thomas R. Marsh3

1Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D–14482, Germany2Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

3Department of Physics & Astronomy, University of Sheffield,Sheffield, S3 7RH, UK

ABSTRACT

We present results of an ongoing XMM monitoring cam-paign of the bright eclipsing polar HU Aqr. During fourpointings performed between 2002 and 2005 the systemwas found in different accretion states ranging from in-termediate to very low mass-transfer rates. On the longrun these data uniquely constrain the distribution of thedifferent radiation components in the accretion zone (cy-clotron, bremsstrahlung, reprocessed soft X-ray emis-sion) as a function of the instantaneous, specific accre-tion rate. An interesting, first result is the equilibrium ofthe primary hard X-ray flux and the soft, blackbody-likecomponent during the intermediate state (Fig. 5). Thelast XMM pointing was accompanied by 4 hours of trulysimultaneous ULTRACAM high-speed photometry at theVLT providing additional sub-second variability informa-tion in three optical bands (Fig 1). These data resolve theeclipse ingress/egress with an unprecedented high S/Nand directly determine the location of the accretion spoton the white dwarf (Fig. 2). During all states the atmo-sphere of the white dwarf and the heated cap surroundingthe accretion spot is directly discernable from XMM-OMUV light curves and optical photometry. From our neweclipse timings we find a third case (after DP Leo and NNSer; Schwope et al. 2002 (A&A 392, 541); Brinkworth etal. 2005, astro-ph0510331) for a strong negative periodderivative in a CV (Fig. 3) ofP = −8.1 × 10

−12, whichindicates either a true decrease of the binary orbit or ismimicked by a third body.

Key words: Accretion – AM Herculis binaries – stars:binaries: eclipsing – stars: individual: HU Aqr – X-rays:stars.

ACKNOWLEDGMENTS

This work was supported by the DLR-Verbundforschungunder grant 50 OR 0404.

Figure 1. Simultaneous XMM and Ultracam high-speedphotometry at the VLT taken in May 2005 during anepisode of very low accretion activity. The top panelshows residual X-ray emission from the accretion plasma,while the UV light curve displayed in the middle panel isdominated from the heated photosphere below the acrre-tion shock. In the lowest panel ULTRACAM ugr high-speed data at 0.5 sec resolution are shown, which is sen-sitive to photospheric radiation (u-band) and additionalcyclotron emission (g- and r- band).

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Figure 2. A blow up of the ULTRACAM r-band lightcurves around eclipse phase. This data resolves theegress and ingress of the white dwarf (28 sec) and the ac-cretion spot (1.2 sec) with unrivaled high signal-to-noiseand will allow to discern the linear dimensions of theemission components directly. The dots represent datafrom four individual runs, while the line is the resultingphase-average light curve.

Figure 3. Monitoring of the eclipse egress of HU Aqrover a 12-years baseline using various space and groundbased facilities. The new optical and X-ray timings sug-gest a large period derivative with respect to a linearephemeris. This unexpected behaviour does either indi-cate the presence of a third body or a true period decreaseof the binary. For the latter case the angular momentumloss would be a factor of 100 higher than the value ex-pected for a short period CV where angular momentum-loss is thought to be only driven by gravitational radia-tion.

Figure 4. XMM EPIC light curve of HU Aqr taken duringthe intermediate state in May 2002. Count rates in thesoft (< 300eV, red) and hard (> 0.5keV) bands are showseparately. Aroundφ ∼ 0.65 and φ ∼ 0.9 broad andnarrow absortion features are evident, which also havean impact on the hardness ratio (lower panel).

Figure 5. X-ray spectrum of HU Aqr taken during the in-termediate accretion state in May 2002. Two-componentblackbody and thermal plasma model withTbb = 34eV

and Tmek = 39keV is required to roughly fit the data.The fit residuals show hints of line emission around0.6and6.4 keV, which correspond toOVII and the fluores-cence iron line. Both lines indicate presence of additionallow temperature plasma and a harder reflection compo-nents. The ratio between the soft and hard bolometricfluxes is only 2, and therefore strongly reduced when com-pared to a ROSAT high state observation.