Resonant cyclotron radiation transfer model fits to spectra from gamma-rayburst GRB 870303

We demonstrate that models of resonant cyclotron radiation transfer in a st rong field (i.e., cyclotron scattering) can account for spectral lines seen at two epochs, denoted S1 and S2, in the Ginga data for GRB 870303. S1, wh ich extends 4 s, exhibits one line at approximate to 20 keV, while S2, whic h extends 9 s, exhibits harmonically spaced lines at approximate to 20 and 40 keV. The midpoints of SI and S2 are separated by 22.5 s. Using a general ized version of the Monte Carlo code of Wang et al., we model line formatio n by injecting continuum photons into a static plane-parallel slab of elect rons threaded by a strong neutron star magnetic field (similar to 10(12) G) that may be oriented at an arbitrary angle relative to the slab normal. We examine two source geometries, which we denote "1-0" and "1-1," with the n umbers representing the relative electron column densities above and below the continuum photon source plane. The 1-0 geometry may represent, e.g., a line formation region levitating above the surface of the neutron star, or possibly a plasma-filled flux tube illuminated from below. The 1-1 geometry , on the other hand, corresponds to line formation in a semi-infinite atmos phere at the surface of a neutron star. We apply rigorous statistical infer ence to compare azimuthally symmetric models, i.e., models in which the mag netic field is parallel to the slab normal, with models having more general magnetic held orientations. If the bursting source has a simple dipole hel d, these two model classes represent line formation at the magnetic pole, o r elsewhere on the stellar surface. We find that the data of S1 and S2, con sidered individually, are consistent with both geometries, and with all mag netic field orientations, with the exception that the S1 data clearly favor s line formation away from a polar cap in the 1-1 geometry, with the best-f it model placing the line-forming region at the magnetic equator. Within bo th geometries, fits to the combined (S1 + S2) data marginally favor models that feature equatorial line formation, and in which the observer's orienta tion with respect to the slab changes between the two epochs. We interpret this change as being due to neutron star rotation, and we place limits on t he rotation period.