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Astron. Astrophys. 319, 507-510 (1997)

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1. Introduction

The galactic center low-mass hard X-ray pulsar GX 1+4 was discovered in 1971 by Lewin et al. in a balloon observation with a pulse period of 135 seconds. The source X-ray luminosity with an assumed distance of 10 kpc, was estimated to be 1037 - 1038 erg s-1, very close to [FORMULA] for a small polar cap region. Follow up observations confirmed the pulsations and showed a very fast spin-up rate with a time scale of 40 years. In the conventional accretion disc theory such a fast spin up was difficult to explain. In the early 80's the source made a transition to a low intensity state and attempts to detect the source with EXOSAT failed. The source was observable again in 1987 with the GINGA satellite and by then the spin change had reversed from spin-up to spin-down. Since then, until a very recent increase in luminosity, all the observations gave the same spin down trend with [FORMULA] s yr-1. The spin-up and the spin-down episodes of GX 1+4 are explained by the disc accretion model developed by Ghosh and Lamb (1979). The total torque acting on the central object is resultant of three torques, 1) the torque carried by the in-falling matter, 2) positive torque acting through the magnetic lines by the disc inside the radius where the Keplerian frequency is the same as the rotation frequency of the core object and 3) negative torque acted through the magnetic lines by the disc outside the corotation radius. In the low luminosity state the third component can be dominant over the first two and a resultant negative torque will cause spin-down of the pulsar.

To improve the understanding of the neutron star magnetic field with an accretion disc and to verify the models of accretion torque on the neutron stars, periodic observations of GX 1+4 with a balloon-borne large area hard X-ray telescope were started in 1991. In four observations made so far the source was found in a high state during the last two times and the pulse period was determined accurately. Additionally, study of the X-ray spectrum and pulse phased spectroscopy was also carried out. These results will be reported in detail in a later publication. Any observation of change in the energy spectrum with the pulse phase will give interesting information about either the emission regions or the environment through which photons come out in different phases of its 122 s spin period. In this paper we report our results on a change in the hard X-ray pulse profile and its implications in terms of the current accretion models.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998