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There is mounting evidence that, if gamma-ray bursters are Galactic inorigin, they are located in a Galactic corona at distances greater than 100kpc. This has created a need to explore new models of cyclotron line formation.In most previous calculations the line-forming region was modeled as a staticslab of plasma, optically thin to continuum scattering, and threaded by amagnetic field of the order $10^{12}$ gauss oriented normal to the slab. Such amodel is appropriate, for example, for the magnetic polar cap of a neutron starwith a dipole field. However, if bursters lie at distances farther than severalhundred parsecs, the burst luminosity exceeds the magnetic Eddingtonluminosity, and the plasma in a line-forming region at the magnetic polar capwould be ejected relativistically along the field lines. Mitrofanov and Tsyganhave modeled the dynamics of such an outflow, and Miller {\it et al.} havecalculated the properties of the cyclotron second and third harmonics,approximating them as due to cyclotron absorption. Here we describe Monte Carlocalculations of cyclotron resonant scattering at the first three harmonics in arelativistic outflow from the magnetic polar cap, and show that such scatteringcan produce narrow lines like those observed by Ginga.

Cyclotron line formation in a relativistic outflow