ART-P/Granat observations of the X-ray pulsar Centaurus X-3

We present the results of our study of the X-ray pulsar Cen X-3 with the ART-P telescope onboard the Granat Orbiting Observatory. The source was observed twice in 1990 with an interval of six months (at orbital phases 0.54-0.91), and both times its X-ray luminosity was almost an order of magnitude lower than that in the standard (high) state. Its emission exhibited large variations, both regular (evolution of the intensity and the degree of low-frequency absorption with orbital phase, changes in the pulse profile in various energy bands) and irregular (bursts with a duration of hundreds and thousand of seconds, during which the flux from the source more than doubled). Bursts were accompanied by a change in hardness; the hardness slightly increased during long (similar to 3000 s) bursts and decreased during short (200-500 s) bursts. In August 1990, an eclipse ingress of the source was observed. At this time, a significant (13 sigma) persistent flux with a softer spectrum than that in the pre-eclipse state and with a strong iron emission line at energies similar to 6.7-6.9 keV was detected. Our analysis shows that the two ART-P observations were performed during the so-called pre-eclipse dip of the source, when its flux was strongly affected by scattering and absorption in a dense cloud of gas along the line of sight (apparently in the stream of matter flowing from the companion star into the accretion disk). The detected bursts could be attributed to a local increase in the transparency of the cloud (with a hole), while the slow variations in the intensity and the spectrum could be caused by a change in the orientation of the cloud, which was accompanied by a decrease in its visible surface area and an increase in its optical depth. The observed flux during the eclipse most likely consisted of several components. First, the pulsar emission scattered in the opaque cloud was observed (this component disappeared some time after the eclipse ingress). Second, we observed the emission that was scattered in the stellar wind from the optical star and that remained essentially unaffected by photoabsorption. Third, the hard X-ray emission heated the stellar wind in the vicinity of the pulsar to temperatures similar to 10(6)-10(7) K and produced its strong ionization. This region was responsible for the 6.7-keV line emission, while its thermal emission gave an additional contribution to the flux at low energies.