Evolution of hard X-ray spectra along the branches 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 spectra of the peculiar low-mass X-ray binary Circinus X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch, through the horizontal horizontal branch and normal branch, to the flaring branch. We detect a power-law hard component in the spectra. It is found that the derived photon indices (Gamma) of the power-law hard component are correlated with the position on the HID. The power-law component dominates the X-ray emission of Cir X-1 in the energy band higher than similar to 20 keV. The fluxes of the power-law component are compared with those of the bremsstrahlung component in the spectra. A possible origin of the power-law hard component is discussed.