New measurements of orbital period changes in Cygnus X-3

Abstract A nonlinear nature of the arrival time of the 4.8 hr binary modulation in the X-ray light curve of Cygnus X-3 indicates either a change in the orbital period or an apsidal motion of the orbit. We have made extended observations of Cygnus X-3 with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment during 1999 July 3–13 and and 199 October 11–24. Using these and archival data from ROSAT, ASCA, and RXTE we have extended the data base for this source. Adding these new arrival time measurements to the published results, we make a comparison between the various possibilities, (a) orbital expansion due to mass loss from the system, (b) mass transfer between the components, and (c) apsidal motion of the orbit due to gravitational interaction between the two components. Orbital expansion due to mass loss from the companion star seems to be the most probable scenario. Most possibly, the compact star in this system has an accretion disk with an extended corona.