Spin period evolution of GX 1+4^⋆

¹ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, PO Box 99 03080 Alicante, España
e-mail: anagonzalez@ua.es
² European Space Astronomy Centre (ESA/ESAC), Science Operations Department, Villanueva de la Cañada ( Madrid), Spain
³ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
⁴ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
⁵ Stenberg Astronomical Institute, Moscow MV Lomonosov State University, 119992 Moscow, Russia
⁶ National Space Science and Technology Center, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁷ Universities Space Research Association, 6767 Old Madison Pike, Suite 450, Huntsville, AL 35806, USA

Received: 16 August 2011
Accepted: 5 November 2011

Abstract

Aims. We aim both to complement the existing data on the spin history of the peculiar accreting X-ray pulsar GX 1+4 with more past and current data from BeppoSAX, INTEGRAL, and Fermi and to interpret the evolution in the framework of accretion theory.

Methods. We used source light curves obtained from BeppoSAX/WFC and INTEGRAL/ISGRI to derive pulse periods using an epoch-folding analysis. Fermi/GBM data were analysed by fitting a constant plus a Fourier expansion to background-subtracted rates, and maximizing the Y₂ statistic. We completed the sample with hard X-ray light curves from Swift/BAT. The data were checked for correlations between flux and changes of the pulsar spin on different timescales.

Results. The spin-down of the pulsar continues with a constant change in frequency, i.e., an apparently accelerating change in the period. Over the past three decades, the pulse period has increased by about  ~50%. Short-term fluctuations on top of this long-term trend do show anti-correlation with the source flux. Possible explanations of the observed long-term frequency and its dependence on flux are discussed.