Cyclotron Line Formation in a Radiation-driven Outflow

Abstract

We calculate the properties of gamma-ray burst spectral lines formed by resonant cyclotron scattering in a radiation-driven outflow. Most previous models of line formation in gamma-ray bursts are appropriate at the polar cap of a neutron star located no further than several hundred parsecs away. However, the BATSE brightness and sky distributions indicate that, if bursters are galactic, they are located in a corona at distances greater than 100 kpc. At these distances the burst luminosity exceeds the Eddington luminosity and the plasma in a polar line-forming region is ejected along the field lines. The variation of the magnetic field strength and plasma velocity with altitude in such an outflow would seem to prevent the formation of narrow features like those observed by Ginga and other instruments. However, this is not the case because the majority of scatters occur close to the stellar surface, at altitudes below $z \approx r_h$, where $r_h \sim 10^{5}$ cm is the size of the photon source. Consequently, the interpretation of the observed features as cyclotron lines does not rule out burst sources in a galactic corona. The outflow model predicts both absorption-like and single-peaked emission-like features. The latter do not occur in models with static line-forming regions, but have been observed in gamma-ray bursts by the Konus and BATSE instruments.