2006ApJ...646.1125N

We report on the RXTE observations of the binary X-ray pulsar 4U 0115+63, covering an outburst in 1999 March-April with 44 pointings. The 3-30 keV PCA spectra and the 15-50 keV HEXTE spectra were analyzed jointly for cyclotron resonance features. When the 3-50 keV luminosity at an assumed distance of 7 kpc was in the range (5-13)x10³⁷ ergs.s^–1, harmonic double cyclotron features were observed in absorption at ∼11 and ∼22 keV, as was measured previously during typical outbursts. As the luminosity decreased below ∼5x10³⁷ ergs.s^–1, the second resonance disappeared, and the fundamental resonance energy gradually increased, up to ∼16 keV at 0.16x10³⁷ ergs.s^–1. These results reconfirm the report by Mihara et al. using Ginga, who observed a single absorption at ∼16 keV in a minor (∼10³⁷ ergs.s^–1) outburst of this object. The luminosity-dependent cyclotron resonance energy might possibly be understood as a result of a decrease in the accretion column height, in response to a decrease in the mass accretion rate.