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

Archival X-ray observations of EF Eridani obtained in a low state revealed distinct X-ray detections at a luminosity L X ≃ 2 × 10 29 ergs −1 , three orders of magnitude below its high state value. The plasma temperature was found to be as low as kT < ≈ 2 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 ∼10000 K. Contrary to earlier claims, WD model atmospheres produce sufficient UV flux to reproduce the published GALEX flux and orbital modulation.