Cyclotron line strength variations in gamma-ray burst GB870303 - Possible evidence of neutron star rotation

An exhaustive search of the Ginga data on gamma-ray burst GB870303 reveals two separate time intervals during which statistically significant line features are evident. One interval shows a single prominent line feature at about 20 keV; a second, shows two line features at about 20 and 40 keV. From model fits to the data, we find that both sets of lines are well-described by cyclotron resonant scattering in a magnetic field B about 1.8 x 10 exp 12 G, 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 theta relative to the magnetic field. We conjecture that the change in theta is due to rotation of the neutron star, and derive limits P = 45-180 sec on the rotation period P.