On the X-ray spectra of soft gamma repeaters

Radiation transfer in a scattering medium in a superstrong magnetic field is considered. Because cross-sections depend on frequency, photons with different energies escape layers with different temperatures and therefore the spectrum of the outgoing radiation differs significantly from the equilibrium blackbody or Bose-Einstein spectrum. It is shown that the emergent spectrum (the photon flux per unit energy band) is flat at low energies. Applications of the result to soft gamma repeaters (SGRs) are discussed. Even though the spectrum is strongly distorted when the radiation propagates through the magnetosphere, a flat segment may be observed in the outgoing spectrum if the surface magnetic field of the neutron star is not too high, B < 10¹⁵ G.