Energy Spectrum and Pulse Profiles of the Binary Pulsar GX 1+4

The X-ray pulsar GX 1+4 was observed during a balloon flight from Alice Springs, Australia on November 20, 1986. Significant flux was detected over the full energy range of the detector (15 - 114 keV). The spectrum was very hard with a photon power law index of 1.9 ± 0.2. The intensity at 30 keV was (5.1±0.2) × 10-4 cm-2s-1keV-1; a factor of ∼ 3 less than typical during the 1970's but orders of magnitude greater than during recent satellite measurements. The pulse profiles below ∼ 75 keV were very broad with indications of a notch at the peak. At higher energies there was a remarkable change with a narrow pulse of phase width ∼ 0.2 centred on the phase of the lower energy notch. We believe this to be the first observation of phase reversals in pulse profiles in the hard X-ray energy range. The observational evidence can be explained within the framework of a model based on two-photon emission as the dominant continuum source. This requires a magnetic field ∼ 2 × 1013 gauss - a value consistent with accretion models in which the system has a retrograde disc. © 1991.