Comprehensive Spectral Analysis of Cyg X-1 using RXTE Data

Abstract

We analyse a large number ($> 500$) pointed RXTE observations of Cyg X-1 and model the spectrum of each one. A subset of the observations for which there is simultaneous reliable measure of the hardness ratio by the All Sky Monitor, shows that the sample covers nearly all the spectral shapes of Cyg X-1. The relative strength, width of the Iron line and the reflection parameter are in general correlated with the high energy photon spectral index $Γ$. This is broadly consistent with a geometry where for the hard state (low $Γ\sim 1.7$) there is a hot inner Comptonizing region surrounded by a truncated cold disk. The inner edge of the disk moves inwards as the source becomes softer till finally in the soft state (high $Γ> 2.2$) the disk fills the inner region and active regions above the disk produce the Comptonized component. However, the reflection parameter shows non-monotonic behaviour near the transition region ($Γ\sim 2$), suggestive of a more complex geometry or physical state of the reflector. Additionally, the inner disk temperature, during the hard state, is on the average higher than in the soft one, albeit with large scatter. These inconsistencies could be due to limitations in the data and the empirical model used to fit them. The flux of each spectral component is well correlated with $Γ$ which shows that unlike some other black hole systems, Cyg X-1 does not show any hysteresis behaviour. In the soft state, the flux of the Comptonized component is always similar to the disk one, which confirms that the ultra-soft state (seen in other brighter black hole systems) is not exhibited by Cyg X-1. The rapid variation of the Compton Amplification factor with $Γ$, naturally explains the absence of spectra with $Γ< 1.6$, despite a large number having $Γ\sim 1.65$.