Detection of a very low hard X-ray pulse fraction in the bright state of GX 1+4

We have observed the Galactic center hard X-ray pulsar GX 1+4 on 1993 December 11, in the energy range of 20 - 100 keV with a balloon-borne Xenon filled Multi-cell Proportional Counter (XMPC) telescope. The source was detected in a bright state, the average observed source count rate being 10.1 ±0.2 s-1 per detector. X-ray pulsations with a period of 121.0 ± 0.4 s were detected in the source at a very high significance level. This, compared with the Burst and Transient Source Experiment (BATSE) observations, gives a spin down rate of dot-P = 1.6 ± 1.4 s yr -1. The pulse fraction in the hard X-ray range is low (35%), and the observed spectrum is very hard (power law energy index 0.54 ± 0.18). The observed source luminosity in the 20 - 100 keV range is 7.9 ± 0.3 x 1037 erg/s. These properties are similar to those seen during high luminosity spin up state of early 1970s and suggest that the source should revert back to a a spin-up episode