INTEGRAL observation of the accreting pulsar GX 1+4^*

¹ IASF – INAF, via Ugo la Malfa 153, 90136 Palermo, Italy e-mail: ferrigno@ifc.inaf.it
² IAAT, Abt. Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Dr. Remeis Sternwarte, Astronomisches Institut University of Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁵ Science Data Centre, 16 Ch. d'Écogia, 1290 Versoix, Switzerland
⁶ Space Research Center, Bartycka 18a, 00716 Warsaw, Poland

Received: 21 July 2005
Accepted: 20 September 2006

Abstract

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 , to 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 () 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  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  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.