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We use Monte Carlo radiation-transfer calculations to fit dips seen at $\approx$ 20 keV in the 4 s spectrum S1, and at $\approx$ 20 and 40 keV in the 9 s spectrum S2, of the Ginga data for GB870303. These lines are thought to be due to cyclotron scattering in a cool scattering layer ($T \sim$ 5 keV) near the surface of a strongly magnetic ($B \approx$ 10$^{12}$~G) neutron star. Previous theoretical Monte Carlo calculations of cyclotron scattering lines (Wang et al. 1989) assumed a plane-parallel geometry with a constant $B$-field perpendicular to a slab with electron column density $N_e$. This assumption is only valid if the line-forming region is a small area at the magnetic pole. We generalize this approach by letting the $B$-field to lie at an arbitrary angle $\Psi$ relative to the slab normal, which allows us to explore line-emitting regions covering an arbitrary fraction of the neutron star surface. For several plane-parallel slab geometries with the continuum source either below or within the slab, we calculate best-fit values and confidence intervals for $B$, $\Psi$, $N_e$, and the viewing angles of the observer relative to the slab normal ($\theta$,$\phi$), for both GB870303 S1 and S2. The confidence intervals allow us to place limits on the size and location of the emitting region; also, analysis of the difference in fit parameters between S1 and S2 allows us to place constraints on the rotation period of the neutron star. \bigskip
This research has been supported in part by NASA Grants NGT-50778, NAGW-830, NAGW-1284, and NAG5-1454.
