Institut für Astronomie und Astrophysik
Abteilung AstronomieSand 1, D-72076 Tübingen, Germany
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In this thesis I present long-term X-ray variability studies of sources belonging to two of the big groups of objects present in the X-ray universe: the Active Galactic Nuclei (AGN) and the X-ray Binaries (XRBs).
After an introduction to X-ray astronomy in chapter 1, a description of the classification, observational properties, and variability characteristics of AGN and XRBs are presented in chapter 2.
Chapter 3 refers to mathematical methods for the time series analysis. Currently, two methods for the variability analysis of astronomical sources, especially in searching for periodic signals, are common in astronomy: periodogram analysis and epoch-folding. Other methods, as the dynamical power spectra or the wavelet decomposition of the light curve, can give information on the possible variable spectral properties of the data (i.e., change of periods, amplitude, and phases). Methods to examine the X-ray variability directly in the time domain, as the Structure Function (SF) and Excess Pair Fraction (EPF), are also commonly used in time-series analysis in astronomy. An alternative analysis method in the time domain is the Linear State Space Model (LSSM) which explicitly models the noise of astronomical data and allows to estimate the hidden stochastic process. For the detection of periodic signals, it is worth scrutinizing the methods used to determine their significance. Methods to compute significance levels for the existence of QPOs in X-ray light curves which take the red-noise character of the astronomical objects into account are discussed in chapter 4.
The rest of the thesis contains the results of long-term variability studies applying the methods and theory described in the previous chapters. In chapter 5 two variability studies of AGN are presented. I report on the analysis from 110 ks of X-ray observations of the AGN Cen A carried out with the Rossi X-ray Timing Explorer (RXTE) between 1996 and 2000. Spectral and temporal variability on time scales from years to tens of minutes are present. The X-ray spectrum can be well described by a heavily absorbed power-law with photon index 1.8 and a narrow iron line due to fluorescence of cold matter. The measured column depth decreased by about 30% between 1996 and 2000, while the detected 2-10 keV continuum flux remained constant between 1996 and 1998, but increased by 60% in 2000. Since in all observations the iron line flux did not vary, a corresponding decrease in equivalent width was noted. No appreciable evidence for a reflection continuum in the spectrum was detected. We present the interpretation of the iron line strength through Monte Carlo computations of various geometries. Applying epoch-folding and periodogram analysis to the XMM-Newton observation of the Seyfert galaxy Mrk 766, a possible QPO at a time scale of 4200 s has been reported. Our computation of the significance of this QPO, however, shows that the 4200 s peak is not significant at the 95% level. We conclude that the 4200 s feature is an artifact of the red-noise process and not the result of a physical process within the AGN.
In chapter 6 temporal variability behavior from long-term light curves obtained with the All-Sky Monitor (ASM) on board the RXTE is reviewed. Results of searches for periodicities in the ASM light curves from X-ray binaries are presented. The ASM light curves show a wide variety of phenomena. In conclusion, chapter 7 presents a summary and outlook on future work.
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Last modified 05 Nov 2010