V0332+53 in the outburst of 2004-2005: luminosity dependence of the cyclotron line and pulse profile

We present the results of observations of the transient X-ray pulsar V0332+53 performed during a very powerful outburst, from 2004 December to 2005 February, with the INTEGRAL and RXTE observatories in a wide (3-100 keV) energy band. A cyclotron resonance scattering line at an energy of similar to 26 keV has been detected in the source spectrum together with its two higher harmonics at similar to 50 and similar to 73 keV, respectively. We show that the energy of the line is not constant but changes linearly with the source luminosity. Strong pulse profile variations, especially near the cyclotron line, are revealed for different levels of source intensity. We discuss the obtained results in terms of the theoretical models of X-ray pulsars.