Cyclotron line strength variations in gamma-ray burst GB 870303: possible evidence of neutron star rotation.

An exhaustive search of the Ginga data on γ-ray burst GB 870303 reveals two separate time intervals during which statistically significant line features are evident. From model fits to the data, the authors find that both sets of lines are well-described by cyclotron resonant scattering in a magnetic field B ≈ 1.8×10¹²G, and that the differences in the line strengths between the two intervals are significant. The variations are qualitatively similar to those produced by a change in the viewing angle θ relative to the magnetic field. The authors conjecture that the change in θ is due to rotation of the neutron star, and derive limits 45 sec ⪉ P ⪉ 180 sec on the rotation period.