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Astron. Astrophys. 320, L9-L12 (1997)

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1. Introduction

Period variations in X-ray binary pulsars are quite common and a number of pulsars show both spin-down and spin-up episodes over time scale of years or less. In binary systems with Roche-lobe overflow of the mass losing secondary, such variations are generally explained in terms of the conventional accretion disk theory where the spinning-up or spinning-down of a neutron star of a given magnetic moment, mass and period depends only on the X-ray luminosity. In binary systems containing massive early type secondaries, however, accretion onto the neutron star is mostly through strong stellar wind and conditions for forming stable accretion disks are generally not present. According to the numerical simulations of mass accretion onto such systems (Taam & Fryxell 1988; Blondin et al. 1990; Matsuda et al. 1991) the small accreted specific angular momentum can change sign in an erratic manner which may lead to alternating spin-up and spin-down episodes. Study of torque-luminosity relationships in X-ray binary systems can therefore, be very instructive in understanding the accretion process in them.

The luminous hard X-ray pulsar GX 1+4, first detected in 1970 (Lewin et al. 1971), has several characteristics which makes it an ideal source to test out the concepts of accretion powered X-ray pulsars. It has shown a continuous decrease of pulse period (spin-up) from about 135s in 1970 to about 110s in 1980 and it was included as one of the test sources in understanding the behaviors of disk-fed X-ray pulsars (Ghosh & Lamb 1979a, b). The source was below the detection limit of EXOSAT in 1983 (Hall & Davelaar 1983) and after its rediscovery by GINGA in 1987 (Makishima et al. 1988 ) it has been showing a monotonically increasing spin period (spin-down), except for a brief spin-up episode in between (Finger et al. 1993 ; Chakrabarty et al. 1994 ). GX 1+4 has been identified with a red giant M6III star V2116 Oph having an emission line spectrum that resembles a symbiotic star with a strong stellar wind (Davidsen et al. 1976). So far no binary period has been detected from this system, although optical pulsations with the same period as in X-rays have recently been reported (Jablowski et al. 1996). The presence of a giant companion and period change sign reversals could imply that GX 1+4 is a wind-fed system without a stable accretion disk. A correlation between spin-down and X-ray luminosity was pointed out by Chakrabarty (1996), which is in apparent contradiction with the general ideas of X-ray pulsars with accretion disks. To confirm the correlation between spin-down and X-ray luminosity found by Chakrabarty (1996) and to understand the torque-luminosity relation in greater detail, we have obtained the pulse period and luminosity history of GX 1+4 for about 1200 days from the Compton Gamma Ray Observatory Science Support Center (COSSC) BATSE archive and carried out our analysis. In the following sections, we present the analysis, results and its implications.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998