Evolution of Hard X-Ray Spectra Along the Branches in Cir X-1

Abstract

Using the data from the PCA and HEXTE on board the RXTE satellite, we investigate the evolution of the 3-200 keV spectra of the peculiar low mass X-ray binary (LMXB) Cir X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch (VHB), through the horizontal horizontal branch (HHB) and normal branch (NB), to the flaring branch (FB). 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 $\sim 20$ 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.

14 pages, 5 figures, ApJ Letter accepted

References in corpus (11)

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Cited by in corpus (8)

• Discovery of twin kHz QPOs in the peculiar X-ray binary Circinus X-1
• On the Soft Excess in the X-ray Spectrum of Cir X-1: Revisitation of the Distance to Cir X-1
• On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail
• Evolution of cross-correlation and time lag of Cyg X-2 along the branches
• Linking jet emission and X-ray properties in the peculiar neutron star X-ray binary Circinus X-1
• The hard X-ray emission along the "Z" track in GX 17+2
• Insight-HXMT Detections of Hard X-ray Tails in Scorpius X-1
• The evolution of inner disk radius with orbital phase in Circinus X-1