The X-ray properties of the magnetic cataclysmic variable UU Columbae
INAF–Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy e-mail: firstname.lastname@example.org
2 Dipartimento di Fisica, Universita' degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy e-mail: email@example.com
3 Laboratory for High Energy Astrophysics, NASA/GSFC, Code 662, Greenbelt, MD 20771, USA e-mail: firstname.lastname@example.org
4 Service d'Astrophysique, DSM/DAPNIA/SAp, CE Saclay, 91191 Gif-sur-Yvette Cedex, France e-mail: email@example.com
5 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, Postfach 1312, 85741 Garching, Germany e-mail: [burwitz;fwh]@mpe.mpg.de
6 Department of Physics, University of Warwick, Coventry CV4 7AL, UK e-mail: firstname.lastname@example.org
7 APC, UMR 7164, University Denis Diderot, 2 place Jussieu, 75005 Paris and LUTH, Observatoire de Paris, 92195 Meudon Cedex, France e-mail: email@example.com
Accepted: 11 March 2006
Aims.XMM-Newton observations to determine for the first time the broad-band X-ray properties of the faint, high galactic latitude intermediate polar UU Col are presented.
Methods.We performed X-ray timing analysis in different energy ranges of the EPIC cameras, which reveals the dominance of the 863 s white dwarf rotational period. The spin pulse is strongly energy dependent. Weak variabilities at the beat 935 s and at the 3.5 h orbital periods are also observed, but the orbital modulation is detected only below 0.5 keV. Simultaneous UV and optical photometry shows that the spin pulse is anti-phased with respect to the hard X-rays. Analysis of the EPIC and RGS spectra reveals the complexity of the X-ray emission, which is composed of a soft 50 eV black–body component and two optically thin emission components at 0.2 keV and 11 keV strongly absorbed by dense material with an equivalent hydrogen column density of that partially (50%) covers the X-ray source.
Results.The complex X-ray and UV/optical temporal behaviour indicates that accretion occurs predominantly (~) via a disc with a partial contribution (~) directly from the stream. The main accreting pole dominates at high energies whilst the secondary pole mainly contributes in the soft X-rays and at lower energies. The bolometric flux ratio of the soft-to-hard X-ray emissions is found to be consistent with the prediction of the standard accretion shock model. We find the white dwarf in UU Col accretes at a low rate and possesses a low magnetic moment. It is therefore unlikely that UU Col will evolve into a moderate field strength polar, so that the soft X-ray intermediate polars still remain an enigmatic small group of magnetic cataclysmic variables.
Key words: stars: binaries: close / stars: individual: RX J0512.2-3241 (UU Col) / stars: novae, cataclysmic variables / X-rays: binaries / accretion, accretion disks
© ESO, 2006