OBSERVATIONS AND MODELING OF THE HARD X-RAY-EMISSION FROM GX-1 + 4

The X-ray pulsar GX 1 + 4 was observed during a balloon flight from Alice springs, Australia on 1986 November 20. Significant flux was detected over the full energy range of the detector (15-114 keV). The spectrum is very hard with a power-law photon index of 1.85 +/- 0.25. The intensity at 30 keV is (5.1 +/- 0.5) x 10(-4) cm-2 s-1 keV-1, a factor of approximately 3 less than typical during the 1970s but orders of magnitude greater than during satellite measurements in 1984 and in early 1987, a few months after the flight. The pulse profiles below approximately 75 keV are very broad and possibly double-peaked. At higher energies there is a remarkable change, with a narrow pulse of phase width approximately 0.1 centred on the phase of the lower energy notch. 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 approximately 2 x 10(13) G - a value consistent with accretion models in which the system has a retrograde disc.