The SIGMA pulse profiles of GX 1+4: four years of monitoring

The soft γ–ray source GX 1+4 has been regularly monitored by the SIGMA telescope on board GRANAT since 1990. The accumulated data provides both period and spectral information on this source in the 35–150 keV 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 profiles are shown to be roughly divided into two groups. One including those instances when GX 1+4 presents a relatively high pulsed fraction (> 50%), the other containing those when the source exhibits a low pulsed fraction (< 50%). In an effort to understand this result, we construct an emission model which is based nearly entirely on the geometry of the system to describe the pulse profiles. It is found that the pulsed fraction might possibly be linked to the height of the accretion column.