XMM-Newton observations of EF Eridani: the textbook example of low-accretion rate polars^*

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: aschwope@aip.de
² Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany

Received: 13 December 2006
Accepted: 28 March 2007

Abstract

Archival X-ray observations of EF Eridani obtained in a low state revealed distinct X-ray detections at a luminosity 2 10²⁹ erg s^-1, three orders of magnitude below its high state value. The plasma temperature was found to be as low as  keV, a factor 10 below the high state. The X-ray/UV/IR spectral energy distribution suggests faint residual accretion rather than coronal emission as being responsible for the low-state X-ray emission. EF Eri thus showed a clear transition from being shock-dominated in the high state to be cyclotron-dominated in the low state. From the optical/UV spectral energy distribution we re-determine the photospheric temperature of the white dwarf to ~10 000 K. Contrary to earlier claims, WD model atmospheres produce sufficient UV flux to reproduce the published GALEX flux and orbital modulation.