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Astron. Astrophys. 330, 181-188 (1998)

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1. Introduction

Cygnus X-1 is a well-known Galactic black hole candidate. It shows two distinct spectral states, `hard' and `soft', and it spends most of its time in the hard state. The soft X-ray spectral shape changes drastically between these two states, whereas the hard X-ray spectrum is described by a power law with photon index -1.5 [FORMULA] 0.2 at all times (Liang & Nolan 1984).

Rapid and chaotic intensity variations over time scales of milliseconds to several seconds have been seen from this source (Oda 1977; Liang & Nolan 1984) and such variations have been conventionally taken as one of the indicators of the existence of black holes. The power density spectrum obtained in the hard state (Nolan et al. 1981; Belloni & Hasinger 1990a) is flat below a certain frequency (about 0.1 Hz) and decreases above that value. The observed variability in Cyg X-1 is conventionally explained as random shots and the properties of these shots have been studied extensively (Weisskopf et al. 1978; Lochner et al. 1991; Negoro et al. 1994). Time delays between X-rays in different energy ranges have been detected by Miyamoto & Kitamoto (1989) and these delays manifest as double-peaked structure in the phase lag for different time scales and they have been interpreted as arising from clumps of matter having two preferred sizes. Though there are attempts to reproduce the power density spectrum from simple shot models (Belloni & Hasinger 1990a), the observed shape of the power density spectrum is more complex than a simple shot noise model (Belloni & Hasinger 1990b).

There have been attempts to model the wide-band X-ray spectrum of Cyg X-1 and interpret it in terms of more refined accretion theories. Using the simultaneous X-ray and gamma-ray data on Cyg X-1 obtained using Ginga and OSSE, respectively, Gierlinski et al. (1997) found that the energy spectrum requires Comptonization components along with additional components. Chitnis et al. (1997) used data obtained from EXOSAT, OSSE and balloon-borne detectors and concluded that two component thermal-Compton models are sufficient to explain the wide band data and the results are explained in terms of the accretion disk theory developed by Chakrabarti & Titarchuk (1995). The same data, however, can also be explained by a transition disk model (along with its reflection) with steeply varying temperature profiles (Misra et al. 1997). These results point towards the existence of Comptonizing plasma along with a reflecting material.

On the theoretical side, Chakrabarti & Titarchuk (1995) have taken a complete solution of viscous transonic equations and demonstrated that the accretion disk has a highly viscous Keplerian part which resides on the equatorial plane and a sub-Keplerian component which resides above and below it. The sub-Keplerian component can form a standing shock wave (or, more generally, a centrifugal barrier supported dense region) which heats up the disk to a high temperature. The wide-band X-ray spectrum of Cyg X-1 qualitatively agrees with this model (Chitnis et al. 1997). Narayan & Yi (1994) have examined advection dominated accretion flows in black holes and their model is also used to explain qualitatively the spectral behavior and spectral states of Cygnus X-1.

There are, however, no quantitative attempts to explain both the spectral and temporal properties of Cyg X-1 in its two states. Molteni et al. (1996) do give a qualitative picture of how quasi-periodic oscillations (QPO) can occur in black hole candidates. In recent times there are renewed attempts to give a complete physical picture of accretion onto black holes because of the theoretical work by Chakrabarti and collaborators and Narayan and co-workers. The transition of Cyg X-1 to a soft state in 1996 and its observation by several X-ray satellites currently in orbit have helped in providing valuable information for a proper understanding of black hole accretion.

Cyg X-1 entered a rarely observed soft state in 1996 May (Cui 1996; Zhang et al. 1996). The source remained in this state and started to go back to its normal hard state in 1996 July. The complete light curves in X-rays and gamma-rays have been tracked by the ASM on board the RXTE satellite and the BATSE on-board the CGRO satellite. A clear anti correlation was seen between the soft and hard X-rays (Cui et al. 1997a). Detailed spectral and temporal studies of the source were carried out periodically with the PCA on-board the RXTE (Cui et al. 1997a,b; Belloni et al. 1996). It was found that the spectral softening is associated with changes in the power density spectrum (PDS) and also that the average delay of hard photons relative to soft photons increases when the source makes a transition from the soft state to the hard state. These results show that the spectral and temporal parameters like the hardness ratio, time lag, PDS shape etc. change monotonically with time during the spectral transition, possibly indicating the existence of a single underlying mechanism responsible for both the spectral and temporal changes.

We have observed the source in both the states using the Pointed Proportional Counters (PPC) on-board the Indian X-ray Astronomy Experiment (IXAE). The soft-state observations are made during a period when there are no PCA observations and we present, for the first time, the timing observations carried out in the hard state just prior to the transition of the state. A preliminary report of these observations is given in Agrawal et al. (1996).

The paper is organized as follows. In Sect. 2 we describe the PPC observations and the details of the background modeling. The results obtained from a detailed temporal analysis of the source are given in the next section. In Sect. 4 we discuss the results in the light of the current understanding of black hole accretion and a brief summary of the main results of this work is given in the last section.

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998