Determination of magnetic field strength on compact thermal stellar sources from their linear-polarization spectrum.

The integrated radiation scattered in an optically thin, spherically symmetric, magnetoactive stellar envelope will be linearly polarized. The spectrum of the percentage polarization with respect to frequency is calculated. Polarization arises because of the differing contribution of the Faraday depolarization effect from parts of the envelope where the magnetic field lines run in different directions. The polarization spectrum is also calculated for an aspherical magnetoactive emission region. On the basis of X-ray polarimetry of Scorpius X-1, the magnetic field strength in the polarized-radiation site is estimated to be approximately 30 million gauss. Scattering in an optically thin envelope might explain the polarization data obtained for Be stars if they have a surface field strength of a few tens of gauss.