EDP Sciences
Free access
Volume 462, Number 3, February II 2007
Page(s) 995 - 1005
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20053878

A&A 462, 995-1005 (2007)
DOI: 10.1051/0004-6361:20053878

INTEGRAL observation of the accreting pulsar GX 1+4

C. Ferrigno1, A. Segreto1, A. Santangelo2, J. Wilms3, 4, I. Kreykenbohm2, 5, M. Denis6, and R. Staubert2

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)

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</I>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.

Key words: X-rays: stars -- stars: pulsars: individual: GX 1+4

© ESO 2007