A detailed analytical study of the relativistic proÐle for magnetic Compton scattering is performed and use is made of Monte Carlo methods to construct a numerical scheme for the production of relativistic theoretical cyclotron spectra from internally irradiated scattering atmospheres in the low-density/highÐeld regime. The cross section includes natural line widths, and the Monte Carlo implementation allows for spatial di †usion of photons in arbitrary geometries while accounting for relativistic angular redistribution. Detailed consideration of "" Raman-Landau ÏÏ scattering permits photon spawning from up to fourth harmonic photons. This scheme is utilized to make speciÐc predictions on the e †ect of geometry and optical depth on the formation of cyclotron lines in hard incident continua. Even if the injection is isotropic, a strongly anisotropic radiation Ðeld ensues at relatively low optical depths. The scattering proÐles at large angles and low Ðelds are highly asymmetric and peak at the relativistic energy cuto †, not necessarily at the resonant energy. Higher harmonic features in general bear little resemblance to the fundamental even at high Ðelds (B D 10 TG), and slabs yield the deepest features, while cylinders present the widest lines at small angles. Moreover, the fundamental line photons in slab scattering regions tend to redistribute to the line wings from all angles, producing fairly broad emission line wings at small angles. Higher optical depths at those angles hampers such redistribution in cylindrical geometries.

A recent high-resolution measurement of an absorption line at 3 110 keV in the phase-averaged spectrum of A0535ϩ26 (Grove et al.) and the conspicuous absence of a previously reported harmonic feature at 1 50 keV (Kendziorra et al.) are indicative of cyclotron scattering in a magnetic field of about 10 13 G. However, controversy has risen over whether a lower field scenario may account for the alleged absence or weakness of a lower energy fundamental harmonic feature. This work explores these two alternatives through the generation of theoretical cyclotron spectra. For lower field models, a cylindrical geometry of the line-forming region and the strong angle dependence of the cross section for resonant scattering conspire to fill in a first harmonic at 3 57 keV. Higher field models (B 3 10.75 TG), however, yield significantly better fits ( low 2 / high 2 1 71), thus strengthening the case for a near-critical field (B crit 3 44 TG). Phase-resolved OSSE spectra are not yet available but would greatly help in resolving this issue.

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