On UV and IR spectra of magnetic white dwarfs having hot plasma envelopes

Cyclotron radiation is shown to be much more sensitive indicator of plasma envelopes of isolated, magnetic white dwarfs than their X-ray emission. Thermal cyclotron radiation by hot plasma on magnetic white dwarf GR 290 is calculated. The star has been probably detected by Einstein but not seen in more sensitive ROSAT observations. We predict that an envelope with density > 10^10 cm^-3, far below the detectability level of recent X-ray facilities, will exhibit itself through the significant excess over the photospheric blackbody at wavelengths lambda ~ 4-8 μm corresponding to the cyclotron fundamental in the dipole field of the star. The IR observations rather than X-ray ones would be experimentum crucis as far as the existence of hot corona in GR 290 is concerned. Besides coronas similar to those in ordinary stars, another type of envelopes - radiation-driven ones - may arise in magnetospheres of sufficiently hot, strongly magnetic dwarfs. The model of such an object, radiation-driven diskon, is applied to interpret the unidentified depression band at 2000-3000 Angstroms in the UV spectrum of another magnetic white dwarf, GD 229. This feature is explained as a result of cyclotron scattering by optically thick, radiation-driven envelope in the inhomogeneous magnetic field of the star. Spectral and temporal variability within the observed depression is predicted.