A Further Study of the Luminosity-Dependent Cyclotron Resonance Energies of the Binary X-ray Pulsar 4U 0115+63 with RXTE

The present paper reports 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 the cyclotron resonance features. When the 3−50 keV luminosity at an assumed distance of 7 kpc was in the range (5−13)×10 erg s, 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 ∼5×10 erg s, the second resonance disappeared, and the fundamental resonance energy gradually increased, up to ∼16 keV at 0.16×10 erg s. These results reconfirm the report by Mihara et al. (2004) using Ginga, who observed a single absorption at ∼16 keV in a minor (∼ 10 erg s) outburst of this object. The luminosity-dependent cyclotron resonance energy may be understood as a result of a decrease in the accretion column height, in response to a decrease in the mass accretion rate. Subject headings: pulsars: binary, magnetic fields, cyclotron —X-ray: pulsars