XMM-Newton observations of AM CVn binaries: V396 Hya and SDSS J1240–01

¹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK e-mail: gtbr@mssl.ucl.ac.uk
² Department of Astrophysics, IMAPP, Radboud University of Nijmegen, PO Box 9010, 6500 Nijmegen, The Netherlands
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-67, Cambridge, MA 02138, USA
⁵ Observatory, PO Box 14, 00014 University of Helsinki, Finland
⁶ Tuorla Observatory, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 25 April 2006
Accepted: 1 July 2006

Abstract

We present the results of XMM-Newton observations of two AM CVn systems – V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions.