4U 0115+63 from RXTE and INTEGRAL data: Pulse profile and cyclotron line energy

We analyze the observations of the transient X- ray pulsar 4U 0115+ 63 with the RXTE and INTEGRAL observatories in a wide X- ray ( 3 - 100 keV) energy band during its intense outbursts in 1999 and 2004. The energy of the fundamental harmonic of the cyclotron resonance absorption line near the maximum of the X- ray flux from the source ( luminosity range 5 x 10(37)- 2 x 10(38) erg s(-1)) is similar to 11 keV. When the pulsar luminosity falls below similar to 5 x 10(37) erg s(-1), the energy of the fundamental harmonic is displaced sharply toward the high energies, up to similar to 16 keV. Under the assumption of a dipole magnetic field configuration, this change in cyclotron harmonic energy corresponds to a decrease in the height of the emitting region by similar to 2 km, while other spectral parameters, in particular, the cutoff energy, remain essentially constant. At a luminosity similar to 7 x 10(37) erg s(-1), four almost equidistant cyclotron line harmonics are recorded in the spectrum. This suggests that either the region where the emission originates is compact or the emergent spectrum from different ( in height) segments of the accretion column is uniform. We have found signi. cant pulse pro. le variations with energy, luminosity, and time. In particular, we show that the pro. le variations from pulse to pulse are not reduced to a simple modulation of the accretion rate specified by external conditions.