2007A&A...462..995F

We present the results of the INTEGRAL monitoring campaign on the accreting low mass X-ray binary pulsar GX 1+4 performed during the Galactic plane scan of the INTEGRAL Core Programme. The source was observed in different luminosity states ranging from L_20–40keV=1.7x10^–10erg/cm2/s, to L_20–40keV=10.5x10^–10erg/cm2/s for about 779ks from March 2003 until October 2004. Our observations confirm the secular spin down of GX 1+4 with the spin period (P_s) varying from 139.63s to 141.56s. In the highest luminosity state, a spin-up phase is observed. The phase-averaged spectrum of the source was modelled either with an absorbed cut-off power law or with a Comptonization model with significantly different parameters in the two brightest luminosity states. No evidence of any absorption-like feature is observed in the phase averaged spectrum up to 110keV. At highest luminosity, the source is found to pulsate up to 130keV. Phase resolved spectroscopy reveals a phase-dependent continuum and marginal evidence for an absorption feature at 34±2keV in the descending part of the pulse. If interpreted as due to electron resonant cyclotron scattering, the magnetic field in the emitting region would be (2.9±0.2)x10¹² (1+z)G where z is the gravitational red shift of the emitting region. We also observed a very low luminosity state, typical of this source, which lasted for about two days during which the source spectrum was modelled by a simple power law, and a pulsed signal was still detectable in the 15-100keV energy range.