INTEGRAL observation of the accreting pulsar GX 1+4

Aims.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. Methods.The source was observed in different luminosity states ranging from $L_{20-40\,\mathrm{keV}}= 1.7 \times 10^{-10}~\mathrm{erg\,cm^{-2}\,s^{-1}}$, to $L_{20-40\,\mathrm{keV}}= 10.5 \times 10^{-10}~\mathrm{erg\,cm^{-2}\,s^{-1}}$ for about 779 ks from March 2003 until October 2004. Results.Our observations confirm the secular spin down of GX 1+4 with the spin period ($P_\mathrm{s}$) varying from 139.63 s to 141.56 s. 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 110 keV. At highest luminosity, the source is found to pulsate up to 130 keV. Phase resolved spectroscopy reveals a phase-dependent continuum and marginal evidence for an absorption feature at $34\pm2$ keV 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 $(2.9\pm0.2) \times 10^{12} (1+z)$ 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 15-100 keV energy range