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University of Southampton Research Repository Eprints Soton University of Southampton Research Repository ePrints Soton Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF PHYSICAL SCIENCES AND ENGINEERING Physics And Astronomy Fast Spectral Variability in the X-ray Emission of Accreting Black Holes by Chris J. Skipper Thesis for the degree of Doctor of Philosophy October 2013 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF PHYSICAL SCIENCES AND ENGINEERING Physics And Astronomy Doctor of Philosophy FAST SPECTRAL VARIABILITY IN THE X-RAY EMISSION OF ACCRETING BLACK HOLES by Chris J. Skipper The X-ray emission from accreting black holes provides the perfect probe for testing the geometry, behaviour and conditions present in the innermost regions of the accretion flow. In this thesis I use X-ray spectral analysis to investigate the properties of accret- ing black holes that extend over several orders of magnitude in accretion rate (m˙ E) and black hole mass (MBH), from the stellar mass black holes in X-ray binary systems (XRBs) to the supermassive black holes in active galactic nuclei (AGN). Firstly, through a survey of X-ray emission in the nuclei of nearby galaxies I show that the usefulness of the X-ray to optical line ratios as a Compton-thick diagnostic does not extend to low luminosity AGN, and instead these ratios may have more practical use in distinguishing between AGN and non-AGN emission processes. Secondly, and more importantly, the main focus of this thesis is upon the variability of the Comptonised power-law X-ray spectral com- ponent, and more specifically an examination of how the photon index (Γ) varies with accretion rate. By cross-correlating the count rate from an XRB source with the fitted Γ on time-scales as short as 16 ms I show the presence of separate components which I as- sociate with Comptonisation of disc seed photons, Comptonisation of cyclo-synchrotron seed photons in the hot inner accretion flow and asymmetry arising from the hard lags. I further demonstrate that, in Cygnus X-1, the anti-correlation between luminosity and Γ is associated only with the most rapid variability, and tends to be absent when the data are examined at lower time-resolution. Finally, I show that the relationship between lu- minosity and Γ in the hard state of two XRBs (XTE J1118+480 and GX 339-4) appears to be very similar on both short (seconds) and long (months) time-scales, and discuss whether the possibility of a fixed disc truncation radius on short time-scales can explain why this does not appear to be the case for Cygnus X-1. Contents Declaration of Authorship xiii Acknowledgements xv 1 Introduction 1 1.1 Active galactic nuclei .............................. 2 1.1.1 Overview ................................ 2 1.1.2 The unified model ........................... 3 1.2 X-ray binaries .................................. 5 1.3 Mass accretion onto black holes ........................ 5 1.3.1 Overview ................................ 5 1.3.2 Spherically symmetric accretion .................... 8 1.3.3 The thin-disc accretion model ..................... 9 1.3.4 Timescales in accretion disc theory .................. 10 1.3.5 Radiatively inefficient accretion flows ................ 11 1.4 The X-ray spectra of accreting black holes .................. 12 1.4.1 Overview ................................ 12 1.4.2 Inverse-Compton scattering ...................... 13 1.4.3 Spectral states in X-ray binaries .................... 16 1.4.4 The X-ray spectral components of XRBs and AGN .......... 18 1.4.5 Variability in black hole accretion ................... 20 1.5 The Chandra X-ray observatory ........................ 21 1.6 The Rossi X-ray Timing Explorer ........................ 22 1.7 Summary of this thesis ............................. 23 2 AGN: Absorption, spectral classification and the optical emission lines 25 2.1 Synopsis ..................................... 25 2.2 Introduction ................................... 26 2.3 The sample ................................... 27 2.4 The optical emission lines from AGN ..................... 28 v vi CONTENTS 2.5 Classifying low luminosity AGN ........................ 28 2.6 The optical lines as indicators of X-ray luminosity .............. 31 2.7 Data reduction ................................. 32 2.7.1 Observation selection ......................... 32 2.7.2 Image analysis ............................. 33 2.7.3 Spectral fitting ............................. 33 2.8 Distribution of MBH, Eddington ratio and NH for the Palomar sample ... 34 2.9 Identifying the Compton-thick AGN in our sample .............. 38 2.10 The relationship between L2−10 keV and L[O III] and its use as an absorp- tion indicator .................................. 39 2.11 A comparison of absorption in the X-ray and optical bands ......... 43 2.12 The unusual case of the Seyfert-2 galaxy NGC 3147 ............. 45 2.13 X-ray luminosity, radio luminosity and the fundamental plane ....... 46 2.14 Summary of the main observational results .................. 47 2.15 Discussion .................................... 49 3 Very Fast X-ray Spectral Variability in Cygnus X-1 51 3.1 Synopsis ..................................... 51 3.2 Introduction ................................... 52 3.3 Observations .................................. 53 3.4 Spectral fitting ................................. 54 3.4.1 Time-averaged fits to each complete observation .......... 54 3.4.2 Determining which model parameters to fix ............. 56 3.4.3 Fitting the spectra at 100 ms time resolution ............. 58 3.5 Cross-correlation of Γ and count rate on 100 millisecond time-scales ... 60 3.5.1 The hard states ............................. 60 3.5.2 The soft/intermediate states ...................... 62 3.6 Cross-correlation of Γ and count rate on 16 millisecond time-scales .... 62 3.6.1 A closer look at the soft state, bright hard state and faint hard state 62 3.6.2 A lag between the soft and hard counts ................ 65 3.7 A comparison with GX 339-4 ......................... 69 3.8 The effect of noise on my results ....................... 70 3.9 Summary of the main observational results .................. 71 3.10 Discussion .................................... 72 4 The m˙ Γ relationship in XRBs and AGN 77 E − 4.1 Synopsis ..................................... 77 4.2 Introduction ................................... 77 4.3 Observations and data reduction ....................... 80 4.3.1 The sample ............................... 80 4.3.2 Data reduction ............................. 81 4.3.3 Spectral fitting ............................. 82 4.4 The m˙ Γ relationship in X-ray binary systems ............... 83 E − 4.4.1 Short time-scale data .......................... 83 4.4.2 Long time-scale data .......................... 88 CONTENTS vii 4.4.3 A comparison of the m˙ Γ correlation on long and short time-scales 90 E− 4.4.4 Hardness-intensity diagrams ...................... 93 4.4.5 An alternative interpretation of the short time-scale anti-correlation in Cygnus X-1 .............................. 95 4.5 The m˙ Γ relationship in AGN ....................... 98 E − 4.5.1 NGC 1052 ................................ 98 4.5.2 NGC 3516 ................................100 4.5.3 NGC 3998 ................................100 4.5.4 NGC 5548 ................................103 4.5.5 NGC 7213 ................................104 4.5.6 Summary of AGN results ........................104 4.6 Summary of the main observational results ..................107 4.7 Discussion ....................................108 4.8 Acknowledgements ...............................110 5 Conclusions 111 5.1 Summary of findings ..............................111 5.1.1 The nature of compact emission regions in nearby galactic nuclei . 111 5.1.2 X-ray spectral variability in XRBs ...................112 5.1.3 A comparison with AGN ........................114 5.2 Future work ...................................115 A Spectra of Compton-thick AGN 117 B Spectra of Compton-thin AGN 123 References 147 List of Figures 1.1 The AGN unification model .......................... 4 1.2 Inverse-Compton scattering diagram ..................... 13 1.3 Inverse-Compton scattering of a black body seed spectrum. ......... 15 1.4 Diagram of the accretion flow in the soft and hard states .......... 17 1.5 The hardness-intensity diagram of GX 339-4 during its 2002/2003 outburst 17 1.6 The X-ray spectral components of Cygnus X-1 ................ 19 1.7 Chandra X-ray observatory ........................... 21 1.8 Rossi X-ray Timing Explorer .......................... 22 2.1 Optical AGN spectra from Ho et al. (1997a) for a selection of LINERs and Seyfert-1s .................................... 29 2.2 Three different BPT diagnostic diagrams (Kewley et al., 2006) ....... 30 2.3 BPT diagnostic diagram for the Palomar
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