2004ApJ...610..390M

The X-ray binary pulsar 4U 0115+63 was observed twice with the Ginga Large Area Counter, in 1990 February and in 1991 March, when the luminosities in 2-60 keV were 1.4x10³⁸ and 2.0x10³⁷ ergs/s, respectively. In 1990, two harmonic cyclotron features were detected at energies of 11.3 and 22.1 keV, in agreement with previous observations. In 1991, the structure changed to a single absorption at an energy of 15.6 keV, which is interpreted as the fundamental resonance. All features were observed in absorption. The higher resonance energy observed in 1991 may be attributed to a decrease of the height of the accretion column, in response to the factor of ∼6.8 reduction in the mass accretion rate.