Thermal Cyclotron Reprocessing of Gammy-Ray Bursts - Theory and Model Spectra

We examine the generation of infrared, optical, and ultraviolet flashes from single, magnetized neutron stars are experiencing of gamma-ray burst. Cyclotron reprocessing of energetic gamma-ray burst photons in the neutron star magnetosphere is assumed to be the underlying mechanism reponsible for the display at longer wavelengths, and thermal equilibrium is assumed in order to calculate electron distribution function. It is shown that thesea good approximations for a wide range of conditions expected in neutron star magnetospheres. The thermal cycoltron model proves capable fo generating ptical outbursts similar to bright historical events. althrough opitcal transients most likely would be much fainter. For a wide range of conditions the model predicts bright, nondelayed flashes, extending in some cases even beyond the ultraviolet. Since the emission at long wavelengths is correlated with the gammar-rays down to time scales small compared with the burst duration, time-averaged spectra are calculated corresponding to the time-averaged gamma-ray burst spectrum. For flashes that do not exhibit a spectral turnover in the optical region, Lopt α Bas with α ~ 3/4, so that optical transients could be used to constrain the magnetic field strength and distance of gamma-ray burst sources. The long-wavelength fluxes for the recently discovered soft repearing source SGR 1806-20 are also estimated