CygX-1/A candidate of the black hole

Abstract

Abstract. Among discrete galactic X-ray sources, Cyg X-1 has been noted for its peculiar features in several respects. It is one of the few sources with a hard power law spectrum extending beyond several hundred keV. Cyg X-1 also distinguishes itself by its profound time variability over a wide range of time scales. The most remarkable incident was that its optical identification with a spectroscopic binary HDE226868 has led to a presumption that it is a black hole. This possibility has induced continuous interests in the physical character of this source in conjunction with the nature of the black hole. The purpose of this paper is to summarize presently available pieces of knowledge on this source to help the design of future experimental and theoretical works, while the complexity of the source characteristics still has prevented us to construct a clear, coherent picture of this source in spite of the fact that numerous observational facts have been accumulated. Contents:

An Intermediate State of Cygnus X-1

“…In earlier observations Cyg X-1 may have been encountered in this intermediate state as well (Canizares & Oda 1977), and it is possible that the earlier reports of state transitions (Oda 1977;Liang & Nolan 1984) also involved such an intermediate state.…”

“…Although these variations are uncorrelated with most other source parameters like 1-20 keV flux or spectral shape, Crary et al (1996aCrary et al ( , 1996b found that they depend on spectral slope in the 45-140 keV band. Cyg X-1 sometimes switches to a different ("high") state, where the X-ray spectrum is much softer in the 1-10 keV band (Tananbaum et al 1972) and the time variability is suppressed (Oda 1977). Unfortunately, no such state was reported in the last 16 years.…”

“…In its low state, the energy spectrum in the 1-200 keV band is a power law with photon index 11.7 (Tananbaum et al 1972;Bałucińska-Church et al 1996;Oda 1977). The power spectral distribution is flat below a first characteristic ("break") frequency 1 , has a slope of 1 between 1 and a second frequency 2 , then steepens to a slope of 2 (Makishima 1988;Belloni & Hasinger 1990a, hereafter BH90a).…”

X‐Ray Shots of Cygnus X‐1

“…The studies of temporal properties of the rapid variability in the hard X-rays should be a hopeful method of understanding the properties of the compact object. In the low state (LS), the variability of X-rays could be described approximately as the superposition of individual shots (Terrel 1972 ;Oda 1977 ;Sutherland, Weisskopf, & Kahn 1978 ;Miyamoto et al 1988 ;Miyamoto et al 1992). The properties of the rapid Ñuctuation or shot had been studied from the autocorrelation function (ACF) and the power density spectrum (PDS) based on the supposition of randomly occurring shots (Nolan et al 1981 ;Lochner, Swank, & Szymkowiak 1991).…”

Cygnus X-1 revisited

“…Oda etal., 1976;Oda, 1977;Eardley et al, 1978), the present article will concentrate on the more recent developments since the HEAO launches. We summarize the X-and gamma-ray data and their physical interpretations, and try to compare them with the new flux of AGN data.…”