XMM-Newton observations of AM CVn binaries

¹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK e-mail: gtbr@mssl.ucl.ac.uk
² Observatory, PO Box 14, 00014 University of Helsinki, Finland
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Institue of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵ Department of Astrophysics, Radboud University of Nijmegen, PO Box 9010, 6500 Nijmegen, The Netherlands
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-67, Cambridge, MA 02138, USA

Received: 28 February 2005
Accepted: 26 May 2005

Abstract

We present the results of XMM-Newton observations of four AM CVn systems – AM CVn, CR Boo, HP Lib and GP Com. Their light curves show very different characteristics. The X-ray light curves show no coherent pulsations, suggesting the accreting white dwarfs have relatively low magnetic field strengths. Their spectra were best modelled using a multi-temperature emission model and a strong UV component. We find that CR Boo and HP Lib have X-ray spectra with abundances consistent with relatively low temperature CNO processed material, while AM CVn and GP Com show an enhancement of nitrogen. A large fraction of the accretion luminosity is emitted in the UV. We determine accretion luminosities of ~ erg s^-1 and  erg s^-1 for AM CVn and GP Com respectively. Comparing the implied mass transfer rates with that derived using model fits to optical and UV spectra, we find evidence that in the case of AM CVn, we do not detect a significant proportion of the accretion energy. This missing component could be lost in the form of a wind.