Cyclotron Resonance Effects in Two Binary X-Ray Pulsars and the Evolution of Neutron Star Magnetic Fields

The Ginga X-ray spectra of the two binary X-ray pulsars, 4U 1907+09 and Vela X-1, were analyzed for effects due to electron cyclotron resonance. For this purpose, a new continuum spectral model, called NPEX, was developed. The NPEX model, combined with the classical cyclotron scattering line profile, was first tested against the Ginga spectra (typically in 2-50 keV) of Her X-1, 4U 0115+63, 4U 1538-52, X0331+53, and Cep X-4 and was confirmed to reproduce successfully their overall spectra including the previously known cyclotron resonance features. Through application of the same model to the pulse-phase-averaged and phase-resolved Ginga spectra, it was confirmed that 4U 1907+09 and Vela X-1 exhibit fundamental cyclotron resonances at ~20 and ~25 keV, respectively. The data for both objects are also consistent with the presence of the second-harmonic resonances, which were discovered with hard X-ray experiments. Including these two examples, the cyclotron resonance effects are now established in about a dozen binary X-ray pulsars. Their surface magnetic field strengths, implied by their resonance energies, apparently distribute over a narrow range of (1-4)×10¹² G. Although the fewer number of higher field objects may be an instrumental selection effect, the lack of objects with magnetic fields of (0.2-1)×10¹² G is concluded to be real. A limited number of ASCA data are utilized to reinforce this conclusion. These results suggest that the magnetic field of binary X-ray pulsars do not decay significantly at least in ~10⁸ yr.