Study of the luminosity-dependent cyclotron resonance energies of the binary X-ray pulsar 4U 0115+63 with RXTE Original Research Article

We report on the luminosity dependent change of the cyclotron resonance energy obtained from a transient X-ray pulsar, 4U 0115+63. Using RXTE data observed on 1999 March, we found that the fundamental resonance energy stayed constant (∼11 keV) when the source luminosity was above ∼5 × 1037 erg s−1. As the luminosity decreased below ∼5 × 1037 erg s−1, the fundamental resonance energy gradually increased up to ∼16 keV at 0.16 × 1037 erg s−1. The luminosity dependence of the resonance energy can be understood by the change of the accretion column height.