Evolution of Hard X-Ray Spectra along the Orbital Phase in Circinus X-1

Using the data from the Proportional Counter Array and the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite, we investigate the evolution of the 3-200 keV spectrum of the peculiar low-mass X-ray binary Circinus X-1 along its orbital phase. A power-law hard component, dominating above 630 keV as well as contributing between 0.9% and 3.5% of the total flux, and the iron emission line are detected in the spectrum. It is found that the derived photon index () of the power-law hard component and the characteristics of the iron emission line are modulated by the orbital phase: the hardest hard tail, the lowest line energy, and the largest line width of the iron emission line are detected at the periastron. However, the softest hard tail, the highest line energy, and the lowest line width of the iron emission line are detected after the apastron. At the periastron the source evolves along a complete track in the color-color diagram. These behaviors of orbital modulation are discussed. A possible origin of the power-law hard component is also discussed.