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We demonstrate that the cyclotron resonant scattering model can account for the spectral lines seen at two epochs, S1 and S2, in the Ginga data for GB870303. Using a Monte Carlo code, we model line formation by injecting continuum photons into a static plane‐parallel slab of electrons threaded by a superstrong magnetic field (B∼1012 G) oriented at an angle Ψ relative to the slab normal. We apply rigorous statistical inference to compare azimuthally‐symmetric models (Ψ=0°) with models having a more general magnetic field orientation. If the burst source is a neutron star with a simple dipole field, these two classes of magnetic field orientations are equivalent to models in which the line forming region lies at the magnetic pole, or elsewhere on the stellar surface. We present the results of separate fits to the spectra for epochs S1 and S2, as well as joint fits to both spectra. The joint fits marginally favor a line forming region with Ψ=90°; i.e., a line forming region at the magnetic equator, assuming ...

We provide an approximate prescription for determining the Bayesian odds favoring spectral models with lines over models without lines, using a new line parametrization which has several advantages over previous parametrizations. we use an exponentiated Gaussian line model, parameterized in terms of the equivalent width, WE, and the full width at half maximum, W1/2, of the line itself (not the full width of the Gaussian). Unlike other, equivalent, parameterizations, this parametrization yields Bayesian posterior probability distributions (and frequentist χ2 surfaces) which are approximately Gaussian near the maximum likelihood parameter values, and allows the formulation of prior probability distributions for the continuum and line parameters which are independent of each other. It has the additional advantage that it easily treats ‘‘saturated’’ lines, in which WE=W1/2. We use Bayesian inference and this parameterization to determine whether the Burst and Transient Source Experiment (BATSE) Spectroscopy D...

The Ginga Gamma‐ray burst detector has observed three gamma‐ray bursts with absorption lines yet BATSE has not reported any lines. If the lines are cyclotron lines then gamma‐ray bursts are almost certainly not at cosmological distances. Based on the Ginga data, we estimate the fraction of bursts that should contain lines if all bursts had lines like GB880205, GB870303, or GB890929 is consistent with roughly 35%. Using estimates of the steradian‐sec factors for Ginga and BATSE implies that BATSE should be seeing about 5.5 events with lines per year although that number is rather uncertain. The features Ginga detects are most certainly not statistical flukes yet it is not clear why Ginga has such strong lines and BATSE does not.