Introduction

The X-ray pulsar GX 1+4 has been identified with the symbiotic system V2116 Oph believed to contain a neutron star accreting from an M giant companion (Davidsen et al. 1977). The high-energy X-ray spectrum of GX 1+4 is the hardest of all known X-ray binary systems (Mony et al. 1991). GX 1+4 is remarkable in other ways as well. It has the fastest known rate of period change of any pulsar. In the early 1980's the pulsar entered a low intensity state and this transition was associated with a change from spin-up to spin-down of the pulsar period. The subsequent rise in intensity since then has been accompanied by several changes from spin-up to spin-down and back again. Observations of GX 1+4 and theoretical considerations (Dotani et al. 1989, Mony et al. 1991, Greenhill et al. 1993) have provided strong evidence for an unusually strong magnetic field on GX 1+4, . This value is very large for an object that is thought to be very old if theories that magnetic fields in pulsars decay are correct. The high magnetic field derived implies a cyclotron energy near the stellar surface where the plasma deceleration region and the pulsed X-ray emission region is located.

GX 1+4 was observed with the ASCA satellite in September 1994, a month before a transition of GX 1+4 from a spin-down phase to a spin-up phase (Kotani, 1996). In this paper we compare the ASCA observations with model calculations of the beam shape from GX 1+4. In Section 2, we describe the model proposed for the emission from GX 1+4 . We derive expressions for the X-ray emission probability valid at the low frequencies of the ASCA observations and incorporate gravitational light bending into the calculation of the pulse shape. In Section 3 we compare model calculations of the pulse shape with the ASCA observations and other observations of GX 1+4 and our conclusions are presented in Sections 4 and 5.

© Copyright Astronomical Society of Australia 1997