Abstract
Stellar black holes and neutron stars, so-called compact objects, are the final stages in the evolution of massive stars with more than ~8 solar masses. If the mass of a compact object is greater than the theoretical mass limit of a neutron star, the existence of a black hole is the logical consequence within the framework of the general relativity theory. Cygnus X-1 is an X-ray binary which consists of an O-type supergiant and a black hole accreting mass from its companion. One characteristic of such systems is the powerful emission of temporally and spectrally variable X-ray radiation. Basically, the system can be found in either of two states, referred to as hard state and soft state. Although Cygnus X-1 is to be counted among the best studied black hole X-ray binaries, its exact geometry is not clear, just as for similar sources. A definite constituent of all models is a mass accretion disk releasing soft X-rays. Less agreement exists with regard to where the hard X-rays originate during the hard state. Most models assume regions where soft disk photons are energized by inverse Compton scattering to hard X-rays. In recent years, several X-ray binaries have been detected to possess matter outflows, so-called jets. Apart from open questions concerning their formation, the contribution of jets in the X-ray regime remains to be clarified. The subject of this thesis is the variable emission of Cygnus X-1 in the X-ray and in the radio band. The analyzed data span several years and were taken in an X-ray monitoring with the satellite RXTE and simultaneous radio observations with the Ryle Telescope at Cambridge (UK). After an introductory description of the basic issues in chapter 1, the following chapter 2 summarizes the mathematics to describe variability by Fourier analysis as well as theoretical models of variability and of the underlying physical mechanisms. Apart from phenomenological approaches like the shot noise model, a recently discovered linear relationship between the rms variability and the flux of X-ray lightcurves, the so-called rms-flux relation, is outlined. In chapter 3 this relation is investigated using the lightcurves of Cygnus X-1. It is shown that the relationship is valid on all time scales > 5 s, both in the hard state and in the soft state. The existence of a non-zero intercept C on the flux axis argues for two lightcurve components, for example, one variable and one non-variable component, or a possible constant variability component. The slope k of the linear rms-flux relation mirrors the fractional variability. The relationship between these two hypothesized components can be described by a fundamental dependence of k and C with long term averages of the flux. The two following chapters deal with radio-X-ray correlations as they are observed in many black hole X-ray binaries with jets, the so-called microquasars. Chapter 4 reviews basics of radio astronomy, gives a summary of the radio emission from the jets of microquasars, and introduces possible physical scenarios which could cause the observed radio-X-ray correlations. In chapter 5 simultaneous X-ray and radio observations of Cygnus X-1 are studied with regard to correlations on short (= minutes to ~ 5 hours) and long time scales (= days to months). The existence of correlations on long time scales, as observed in many microquasars, can be confirmed also with the used database of Cygnus X-1, particularly at hard energies. On the other hand, there is no indication for correlations on short time scales, as they are known, e.g., for GRS 1915+105, another prominent black hole X-ray binary. Apparent correlations on short time scales in the lightcurves of Cygnus X-1 are rather the coincidental outcome of white noise statistics. Interpreting this result as a breakdown of radio-X-ray correlations on shorter time scales, this sets a limit to the speed of the jet of Cygnus X-1. The last chapter 6 summarizes the main issues of this thesis and gives a brief outlook.
Author and committee
dc:creator, dc:contributor.*- Author
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- Gleißner, Thomas
Identifiers
dc:identifier.*- Identifier
- hdl:10900/48612