THE SIGMA-PULSE PROFILES OF GX-1- 4 YEARS OF MONITORING(4 )

The soft gamma-ray source GX 1+4 has been regularly monitored by the S IGMA telescope on board GRANAT since 1990. The accumulated data provid es both period and spectral information on this source in the 35-150 k eV range. The SIGMA telescope also allows a determination of a pulsed fraction. We report here on the last six years of the SIGMA data on GX 1+4. The general spin down trend is confirmed and the derived periods are consistent with those obtained at other energies. The pulse profi les are shown to be roughly divided into two groups. One including tho se instances when GX 1+4 presents a relatively high pulsed fraction (> 50%), the other containing those when the source exhibits a low pulse d fraction (< 50%). In an effort to understand this result, we constru ct an emission model which is based nearly entirely on the geometry of the system to describe the pulse profiles. It is found that the pulse d fraction might possibly be linked to the height of the accretion col umn.