THE RECORD BREAKING MAGNETIC WHITE DWARF RE J 0317-853

Matt Burleigh (Univ. of Leicester, UK)

&

Stefan Jordan (Univ. of Kiel, Germany)

RE J 0317-853 is a hydrogen rich strongly magnetic white dwarf discovered as an EUV source by the ROSAT Wide Field Camera (WFC). The average optical spectrum of this star could be fitted by a dipole model which is off-centered along the dipole axis by 0.2 stellar radii in the direction of the southern magnetic pole (Barstow, Jordan et al. 1995, MNRAS 277, 971). We concluded that the range of magnetic field strengths seen by an observer was about 170-660 MG, giving RE J 0317-853 one of the strongest magnetic fields ever detected on a white dwarf star.

High speed photometry has shown that the optical brightness of RE J 0317-853 varies almost sinusoidal with a period of 725 sec and an amplitude of more that 0.1 magnitudes. The only possible explanation is rotation, so that RE J 0317-853 is rotating faster than any other known isolated white dwarf. The photometric variation must be caused by differences in the brightness on various parts of the stellar surface.

Since RE J 0317-853 (very probably) has a DA white dwarf companion (LB 9802) 16” away, we could use its atmospheric parameters (Teff=16,030?230 K, log g=8.19?0.05) as an additional constraint for the parameters of RE J 0317-853, since both stars lie at the same distance (very likely). We concluded that RE J 0317-853 is the hottest (?50 000 K) known magnetic white dwarf and probably the most massive (? 1.35 solar masses) isolated white dwarf ever discovered.

In order to analyze the magnetic field of RE J 0317-853 in detail we have obtained time resolved spectropolarimetric data with the 3.9 m Anglo-Australian Telescope (shown in Fig. 1 and 2), phase resolved far-UV spectra with HST/FOS (Fig. 3 and 4), and additional data with the ROSAT, EUVE and ORFEUS satellites. We conclude that the magnetic field structure is extremely complicated and cannot be described by the offset dipole model used previously to fit the mean optical and UV spectrum. A preliminary analysis of the observations with theoretical models for the transfer of polarized radiation through a magnetized stellar atmosphere shows that the data can be best reproduced by a magnetic field with basically two different field strengths (?200MG and 500 MG) on different hemispheres, rather than a continuous variation by a factor of two expected for the dipole model (see Fig. 3 and 4). One intriguing feature at 1170 Å is a dipole forbidden component of Lyman ? (1s0->2s0), which is only possible due to the simultaneous presence of a magnetic and electric field.

Fig. 1: Variation of the flux spectrum of RE J 0317-853 with time. Both, the spectral features and the brightness (? m = ±0.1) change with phase.	Fig. 2: Variation of the wavelength dependent circular polarization of RE J 0317-853 shown as a surface and contour plot.

Fig. 3: Series of HST spectra of RE J 0317-853 showing the variation of the Lyman ? ? and ?+ components due to the change of the magnetic field. The fit with a theoretical model is plotted in red.

Observation	Theory

Fig. 4: Same HST spectra, but the variation with phase is shown as a contour plot.