Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma‐Ray Burst GRB 870303

We demonstrate that models of resonant cyclotron radiation transfer in astrong field (i.e. cyclotron scattering) can account for spectral lines seen attwo epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using ageneralized version of the Monte Carlo code of Wang et al. (1988,1989b), wemodel line formation by injecting continuum photons into a staticplane-parallel slab of electrons threaded by a strong neutron star magneticfield (~ 10^12 G) which may be oriented at an arbitrary angle relative to theslab normal. We examine two source geometries, which we denote "1-0" and "1-1,"with the numbers representing the relative electron column densities above andbelow the continuum photon source plane. We compare azimuthally symmetricmodels, i.e. models in which the magnetic field is parallel to the slab normal,with models having more general magnetic field orientations. If the burstingsource has a simple dipole field, these two model classes represent lineformation at the magnetic pole, or elsewhere on the stellar surface. We findthat the data of S1 and S2, considered individually, are consistent with bothgeometries, and with all magnetic field orientations, with the exception thatthe S1 data clearly favor line formation away from a polar cap in the 1-1geometry, with the best-fit model placing the line-forming region at themagnetic equator. Within both geometries, fits to the combined (S1+S2) datamarginally favor models which feature equatorial line formation, and in whichthe observer's orientation with respect to the slab changes between the twoepochs. We interpret this change as being due to neutron star rotation, and weplace limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21 pages); accepted by ApJ (to be published 1 Nov 1999, v. 525