PHOTON SPECTRUM AND PERIOD EVOLUTION OF GX 1-RAY-ENERGIES BY SIGMA(4 AS OBSERVED AT HARD X)

The galactic pulsar GX 1+4 has been detected by the coded-aperture har d X-ray/soft gamma-ray SIGMA telescope during the 1991 observations of the Galactic Center (GC) region. The source, whose emission varied du ring the survey by more than 40%, reached a maximum luminosity in the 40-77 keV range of 5.0 10(36) erg s-1 (d = 8.5 kpc), thus approaching the emission level of the 1970-1980 high state. Periodic flux variatio ns (2 min. period) have been detected in Spring and Fall of 1991. Comp arison with the periods measured in the eighties shows that the spin-d own behaviour of GX 1+4 is rather complex.