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

Abstract

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 erg s −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 ∼10 000 K. Contrary to earlier claims, WD model atmospheres produce sufficient UV flux to reproduce the published GALEX flux and orbital modulation.

X-Shooting EF Eridani: further evidence for a massive white dwarf and a sub-stellar secondary

“…Beuermann et al 1987). The inclination was further constrained by cyclotron models applied to photoelectric polarimetry (Cropper 1995;Piirola et al 1987), and by modeling X-ray and UV light curves (Beuermann et al 1987;Schwope et al 2007). All authors agree that the inclination is most probably between 50 • and 65 • , although values as high as i = 75 • cannot be completely excluded.…”

“…The effective temperature of the white dwarf is likely below 10 000 K (Schwope et al 2007;Szkody et al 2010, submitted), although a temperature as high as 11 000 K is not completely ruled out. The implied distances for the two assumed temperatures are illustrated in Fig.…”

“…6 we show an updated version of the ultraviolet/optical/IR spectral energy distribution of the binary (update with respect to Schwope et al 2007). Apart from an optical lowstate spectrum adapted from Harrison et al (2004) it comprises GALEX photometry at orbital minimum and maximum (Szkody et al 2006), XMM-OM optical and visual photometry (Schwope et al 2007, UV red dots), orbital phase-resolved infra-red photometry in JHK (Harrison et al 2003) and SPITZER infrared data from Brinkworth et al (2007) (for a Spitzer spectrum and a further discussion of the mid-IR spectral energy distribution see Hoard et al 2007, data points shown as blue dots). The green lines illustrate the combined white-dwarf/accretion spot model adapted from Schwope et al (2007).…”

“…During high and low states EF Eri behaved as the textbook example of AM Herculis binaries (Beuermann et al 1987;Schwope et al 2007), at least as far as energy conversion and release in the accretion column was concerned. It changed from shock-heating/bremsstrahlung cooling in the high state to particle heating/cyclotron cooling in the low state, when the overall accretion rate is reduced by a factor 100 or more.…”

“…Likely spectroscopic and photometric signatures were reported by Howell & Ciardi (2001) and Harrison et al (2003) but the claims of the detected secondary remained unconfirmed. Infrared cyclotron emission from a tenuous accretion plasma is definitely contributing to the excess infrared emission, the contribution of the low mass donor is still uncertain (Harrison et al 2004;Schwope et al 2007;Campbell et al 2008).…”