X-ray Spectral & Timing Properties of Narrow Line Seyfert 1 Galaxies

Transcription

1 X-ray Spectral & Timing Properties of Narrow Line Seyfert 1 Galaxies Abderahmen Zoghbi Institute of Astronomy and Clare Hall This dissertation is submitted for the degree of Doctor of Philosophy. May 2011

2

3 Declaration I hereby declare that my thesis entitled X-ray Spectral and Timing Properties of Narrow Line Seyfert 1 Galaxies, is not substantially the same as any I have submitted for a degree or diploma or other qualification at any other University. I further declare that no part of my thesis has already been, or is being concurrently submitted, for any such degree, diploma or other qualification. This dissertation is my own work and contains nothing which is the outcome of work done in collaboration with others, except as specifically indicated in the text and acknowledgements. Parts of the thesis that have been published are as follows. Chapter 2 was published as: A. Zoghbi, A. C. Fabian, and L. C. Gallo, Reection-dominated X-ray spectra of narrow- line Seyfert 1 galaxies: Mrk 478 and EXO , MNRAS, 391 (2008), Chapter 3 was published as: A. Zoghbi, A. C. Fabian, P. Uttley, G. Miniutti, L. C. Gallo, C. S. Reynolds, J. M. Miller, and G. Ponti, Broad iron L line and X-ray reverberation in 1H , MNRAS, 401 (2010), Chapter 4 was published as: A. Zoghbi, P. Uttley, and A. C. Fabian, Understanding Reverberation Lags in 1H , MNRAS, 412 (2011), Various figures in the thesis are reproduced from the work of other authors for discussion. Such figures are credited are clearly credited in the text. This thesis contains less than the word limit of 60,000 words. A. Zoghbi Cambridge, 05 May 2011

4

5 Summary Understanding the physics of accreting black holes and their role in the universe is a major aim of modern astrophysics. In this thesis, I will present several studies conducted with the aim of improving our understanding of how black holes work. I will concentrate on supermassive black holes at the centres of active galaxies, particularly those of the Narrow Line Seyfert 1 class. We use recent high quality X-ray observations of several objects, and employ a combination of spectral fitting, spectral variability and timing analyses to obtain a complete observational picture of the behaviour of these accreting black holes. The thesis has four main science chapters, with an introductory chapter at the start and a concluding note on the prospects for future similar studies. In chapter 2, an analysis of several X-ray observations of two objects, Mrk 478 and EXO , is presented. The main conclusion is that the two objects are reflectiondominated, where the reflected emission originates close to the black hole where light bending due to the strong gravity is significant. This forces the direct illuminating continuum out of our line of sight. In chapter 3, a detailed analysis of a long observation of the object 1H is presented. The chapter discusses the discovery of a broad iron-l line and reverberation seen for the first time. Further modelling of the origin and interpretation of the reverberation delay in 1H is presented in chapter 4. I specifically compare between reverberation from inner and outer reflections, favouring the former. In chapter 5, the study of the previous two chapters is extended to another object, RE J , where we show that the reverberations delays discovered in 1H are not unique, and are also present in other objects. Looking into the future, the last chapter discusses the capabilities of current and future X-ray missions in studying X-ray reverberation from black holes. The chapter concludes with new results on lags in NGC 4151.

6

7 Acknowledgements First of all, I have to thank my supervisor Andy Fabian for his immense support during the PhD. His insights and guidance made this work possible. I have also to thank Phil Uttley for his discussions and ideas throughout the second half of my PhD. Roderick Johnstone deserves a special thanks for his help with computers and softwares. I also extend my thanks to members of the X-ray group: Jeremy Saunders (and his plotting package veusz), Sandra Raimundo, Becky Canning, Julie Hlavacek-Larrondo, Ranjan Vasudevan, Helen Russell and especially Rubens Reis and Dom Walton for the many useful discussions. I also acknowledge my collaborators: Luigi Gallo, Giovanni Miniutti, Chris Reynolds and others. This work would not have been possible without the financial support from the Algerian Ministry of Higher Education, the Dorothy Hodgkin fund and the Cambridge Overseas trust. I have also to mentioned those who made Cambridge such an enjoyable place: Samir Rihani, EL Habib Sahraoui and Sofiance Naci. Also I thank members of the Cambridge University Islamic Society for their intellectual and social hospitality. Before and after all these, I thank the One, my parents for their continuous love and support, my whole family, and special thanks to Karmel Al-Khaldi for her patience, love and support.

8 Contents DECLARATION SUMMARY ACKNOWLEDGEMENTS LIST OF TABLES LIST OF FIGURES iii v vii x xi OUTLINE 1 1 Introduction Black holes in the universe A historical perspective Current understanding Accretion onto black holes X-ray emission from black hole systems X-ray spectra of Galactic black holes X-ray spectra of Active galaxies X-ray reflection X-ray Variability Black hole binaries Active Galaxies Reflection-dominated NLS1: Mrk 479 & EXO Introduction Observations & Data Reduction Spectral Analysis Mrk EXO Spectral Variability Discussion Broad iron L-line and X-ray reverberation in 1H Introduction Observations & Data Reduction viii

9 CONTENTS 3.3 Spectral Analysis Hard Spectrum Soft Spectrum Spectral Variability Time-resolved spectra Flux-resolved spectra Difference Spectrum & the Nature of the Spectral Components Timing Analysis Coherence Time Lags Lag Significance Alternative Approach: Time domain Discussion Time Lag and Reverberation Understanding Reverberation Lags in 1H Introduction Modelling the lag Lag vs Energy Lag vs Frequency Discussion Summary X-ray Reverberation Lags in RE J Introduction Observations & data reduction Time Lags Lag measurements Lag significance Energy Spectrum Spectral fitting Modelling the lag Discussion The future of X-ray reverberation Introduction Lag uncertainties XMM-Newton observation of NGC Lag measurement in NGC Lag interpretation The Final Word BIBLIOGRAPHY 90 ix

10 List of Tables 2.1 XMM-Newton observation log The parameters of the best-fit reflection models The best fitting model parameters x

11 List of Figures 1.1 BHB spectral states Relativistic line in MCG Absorption-based model for MCG Reflection from an X-ray illuminated slab Reflection spectra for different parameters Data ratio to a simple power-law model Best fit reflection and absorption models for Mrk Contour plot for absorption model parameters Best fit reflection model for EXO The light curves for the two sources RMS variability for Mrk Hardness ratios for Mrk RMS variability for EXO Unfolded spectra for 1H for O1 and O4 observations Residuals from a power-law + edge fit to the hard band The expected line if the drop is due to an edge Best fit models for the hard band Fit residuals for several phenomenological models The photoionisation absorption model fitted to the data A ratio plot of the data to a simple power-law The best fitting reflection model The light curves for all previous XMM observations Time-resolved spectra for 10ks segments Time-resolved spectra for 20ks segments Flux-resolved spectra for 10 flux segments Ratios of spectra from flux segments to a power-law The difference spectrum between high and low states Reflection flux versus power-law flux Ionisation of the reflector vs power-law flux Coherence function between soft and hard bands Time lag as a function of Fourier frequency Time lags for constant lag simulations Time lags for functional lag simulations Filtered segments of the light curve xi

12 LIST OF FIGURES 3.22 CCF of filtered light curves Time lag measured using different methods A Contour plot for the spin of 1H Energy-dependence of the lag at high and low frequencies The best fitting model to the energy spectra A distant reflector model with two constant transfer functions The lag spectrum of 1H with a two component model fit The power spectra at different energies D plot of the lag as a function of energy and frequency Lag dependence on energy for the first observation Lag as a function of energy: Pileup tests The spectra from the two XMM observations RE J spectra with the best fitting models Lag vs energy plot with model Lag uncertainties and telescope performance Unfolded spectra of NGC The 2 10 kev light curves of NGC Lag-energy spectrum using all the light curves Lag-energy spectrum for low and high flux states Lag-energy and difference spectra for NGC The spectra and model for the high and low flux states xii

13 Outline This thesis has six chapters. The content of each chapter is as follows. 1- Introduction & background information The first chapter will be an introduction to the subject. Properties of accreting black holes relevant to this work up to the start of the current work is reviewed, along with a highlight of the necessary physics that will be discussed in the main thesis. The chapter begins with concept of black holes in astrophysics, then a very brief discussion on the accretion phenomena. Then, a quick review of X-ray emission mechanisms and X-ray variability is presented for both stellar-mass and super-massive black holes. 2- Reflection dominated NLS1: Mrk 478 & EXO In these two objects, the X-ray spectrum is dominated by reflection, with the primary emission hidden from view. These are strong signatures of light-bending effects caused by the strong gravity the two black holes. Data analysis and description of the model used to interpret several XMM-Newton observations are presented. The work in this chapter is is published in Zoghbi et al. (2008). 3- Broad Fe L-line and reverberation in 1H H is an extreme, highly variable, NLS1. In this chapter, I present a detailed analysis of data from long XMM-Newton observations of the object. Detailed spectral and timing analyses are presented. The main result is the discovery of a broad iron L line in addition to the usual K line, and the detection of the first reverberation signature from within few gravitational radii of the black hole. This work is published in Zoghbi et al. (2010). 1

14 OUTLINE 4- Understanding reverberation lags in 1H0707 The importance of the lag detected in 1H requires detailed modelling, which ultimately may allow the geometry of these systems to be studied. In this chapter, I explore models that can explain the observed lag spectrum. Particularly, I draw a comparison between reverberation from a distant, outer reflector and a close, inner relector. Using several argument, the second is preferred. This work is published in Zoghbi et al. (2011). 5- Reverberation lags in RE J This chapter is on the discovery and study of reverberation delays in RE J , the only AGN known to show quasi-periodic oscillations in its light curve. The work shows interesting links between the lag, QPO and the energy spectrum that allows us to put constraints on the geometry and their origins. This chapter is published in Zoghbi et al. (2011b, submitted). 6- The future of X-ray reverberation In this chapter, I discuss some of the prospects and possibilities of reverberation studies with current and future X-ray missions. This work is part of the contribution to the science working teams for the X-ray missions Gravitas and Athena. While working on calculating performance of future missions in lag measurements, we identified few objects that would be the best target for currents and near future observations. Notably, NGC 4151, is found to be a source where reverberation studies with the Fe K line can be achieved with XMM, and the current observations are very promising and showing very exciting results. Some of the initial work on this object is presented in this chapter and will be published soon. 2

15 1 Introduction The work presented in this thesis is part of a huge effort aiming to understand the physics of accretion into black holes and the role they play in the universe. In this introductory chapter, I briefly review the observational and theoretical framework that has developed over the decades, paving the way for the detailed work that will be presented in later chapters. I start with a brief historical note and the most direct evidence for the existence of black holes, then I highlight the current understanding of the their role as a class of objects (Sec ). A brief note on accretion discs and the energy release process is then presented. In Sec. 1.3, I summarise the spectral properties of accreting black holes, including X-ray reflection (Sec ), then I will review their variability properties (1.4). Although supermassive black holes are the main subject of this thesis, I will also discuss stellar mass black holes as they share many similarities that will be useful in the discussion of later chapters. 1.1 Black holes in the universe A historical perspective Black holes in the centres of active galaxies are recognised as the most powerful persistent individual objects in the universe. Their concept was first discussed by Michell (1784), then properly established with Einstein s theory of general relativity (Einstein 1916), they were invoked in astronomy in the 1960s and 1970s to explain the energy output of quasars (Lynden-Bell 1969). They are now thought to be present in all galaxies, but 3

16 1.1. BLACK HOLES IN THE UNIVERSE not always active. Two distinct black hole classes have been identified based on their mass ranges, stellar Galactic black holes (GBH) with masses between 4 20 solar masses ( M ), and supermassive black holes (SMBH) with masses in the range M. Stellar mass black holes are the end result of the catastrophic death of massive stars. They are observed in binary systems, where the radiated energy of the black hole is the result of accreting matter from a companion star. The origin of supermassive black holes on the other hand is still not well understood. They could be the result of old massive stellar populations formed in haloes in the early universe, that have subsequently grown through accretion. Alternative scenarios include the direct collapse of large amounts of gas in quasi-stars. Their growth could be enhanced if two or more black holes of low mass coalesce following a merger between their host galaxies, events that were very common in the early universe (Rees 1978; Madau & Rees 2001; Loeb & Rasio 1994; Volonteri & Begelman 2010). Evidence for the existence of black holes has grown substantially in the last few decades (Kormendy & Richstone 1995; Begelman 2003; McClintock & Remillard 2006). For the stellar mass case, dynamical mass measurements, along with the short time scale variability in X-rays, provided the first strong evidence that Cyg-X1 hosts a black hole of mass no less than 6 M (Webster & Murdin 1972; Giacconi 1974). Today, around 25 X-ray black hole binaries (BHB) are known to host a compact object that is too massive (M > 3 M ) to be a neutron star or a white dwarf (Özel et al. 2010). For supermassive black holes, early studies concentrated on some nearby galaxies where high resolution data can be obtained, particularly those showing the potential for hosting a SMBH through the presence of jets or broad optical mission lines. Tracking the motion of the gas in the central regions shows that the central mass in the centres of some galaxies exceeds 10 9 M (Dressler & Richstone 1988). However the most direct evidence has come from studying the detailed proper motion of stars in our Galaxy. The relative close proximity of the central regions in our Galaxy makes it the best laboratory for stellar dynamics analysis. Tracking the motions of stars in the Galactic centre for almost two decades has revealed the presence of an unresolved central mass around which stars orbit. The central density is so high that alternative explanations are ruled out (in excess of M pc 3, or a central pass of M within the orbit of the closest observed star 125 AU; Eckart & Genzel 1997; Ghez et al. 2008; Genzel et al. 2010). Other strong evidence comes from VLBI observations of water masers in the galaxy NGC Line emission from masers are very strong and intrinsically narrow, rendering them an excellent tool for tracking the motion of gas in the centres of some galaxies. This was done in detail for NGC 4258 (M106), and indicates the presence of a central mass in excess of M in a region less than 0.13 pc in radius (Miyoshi et al. 1995). In these cases, the exceedingly high densities found in the central regions of galaxies, rule out alternative explanations such as compact star clusters. The high densities makes the clusters unstable on long time scales. They either collapse or evaporate. The age of these clusters may be made arbitrarily large if the objects in the cluster are made small 4

17 1.1. BLACK HOLES IN THE UNIVERSE enough (e.g. white and brown dwarfs with masses < 0.1 M ), however such objects would rapidly collide and merge (Maoz 1995; Richstone et al. 1998). Other lines of evidence comes from other, less direct, methods such as reverberation mapping. The method works for type 1 active galactic nuclei (AGN) characterised by broad optical emission lines. The lines are emitted from high velocity clouds surrounding the central source (known as the Broad Line Region, BLR). The width of the lines gives an estimate of the clouds velocities, while the distance to the centre is inferred from the time delay in the clouds response to variability in the illuminating central source. These two quantities are used to estimate the mass of the central object, and it is found to be in the range of M (Netzer & Peterson 1997; Peterson & Bentz 2006). Further evidence comes from X-ray observations of small scale variability in AGN (Mushotzky et al. 1993; Edelson et al. 1996). The size of the emitting region is limited to few light minutes across, as an object cannot vary faster than its light crossing time. Additional direct evidence comes from the detection of broad and skewed spectral lines from partially ionised iron in the X-ray spectra of AGN (Tanaka et al. 1995; Nandra et al. 2007) and X-ray binaries (Miller 2007). They are the result of iron fluorescence in partially-ionised matter in the accretion disc when illuminated by hard X-ray radiation. Because of the close proximity of the reflecting surface to the black hole, space-time distortions change the shape of the otherwise symmetric emission line. Radial Doppler shifts boost the blue side (or wing) of the line so its intensity increases, while a combination of the strong gravitational and transverse Doppler redshifts produces a broad red wing (Fabian et al. 1989). Such remarkable line structures were first detected in 1995 in the active galaxy MCG using observations from the ASCA space mission (Tanaka et al. 1995). Later observations by satellites like XMM-Newton and Suzaku confirmed it in this object and many others Current understanding Understanding black holes as extreme objects in the universe is not just important in its own sake. It has many implications on wider subjects in physics, astronomy and cosmology. Light emitted from accretion flows in the vicinity of black holes carries with it information on the nature and structure of space-time distortions, making these environments the best laboratories to probe Einstein s General Relativity theory in the strong field limit (e.g. Cunningham 1975). Most galaxies are now thought to host supermassive black holes at their centres (Magorrian et al. 1998). For those with mass measurements, some remarkable correlations with galaxy properties have been discovered: (1) a correlation between the black hole mass and the bulge luminosity M BH -L bulge (Kormendy & Richstone 1995).(2) a correlation between the black hole mass and the velocity dispersion of their host bulges, known as the M BH -σ relation (Ferrarese & Merritt 2000; Gebhardt et al. 2000). These correlations between the supermassive black hole at sub-parsec scales and stars at kpc scales indicate that the growth of black holes and the assembly of galaxies take place together and influence each other in what is known as AGN feedback (Silk & Rees 1998; 5

18 1.2. ACCRETION ONTO BLACK HOLES Hopkins et al. 2006; Croton et al. 2006). The general picture is that mergers of small gas-rich proto-galaxies provide a source of fuel (gas) that drives black hole growth and star formation. The energetic activity of the black hole then terminates the star formation through radiative and/or mechanical feedback. This process is directly observed locally in the cores of some clusters of galaxies, where jets emitted close to the black hole inflate bubbles that push the gas away. These bubbles are seen in emission in the radio and as cavities in the X-ray band (McNamara et al. 2000; Fabian et al. 2003). In terms of the demographics, theoretical modelling and population synthesis for the Cosmic X-ray Background (CXB) predict that most of the black holes in the universe are obscured (column densities > cm 2 ) with a significant fraction being Compton-thick (N H > cm 2 ; Comastri et al. 1995; Treister & Urry 2005; Gilli et al. 2007). In the local universe, about 75% of AGN are obscured (Risaliti et al. 1999). The populations at higher redshift (z > 1) are not well known as their high obscuration makes them mostly undetectable in current X-ray surveys (e.g. Brandt & Hasinger 2005; Treister et al. 2009). These populations appear to be hosted in infra-red-bright, optically-faint galaxies, where the mid-ir excess is thought to originate in the thick gas obscuring the AGN (Daddi et al. 2007; Alexander et al. 2010). For stellar-mass black holes in binary systems, there are so far around 25 well-studied individual objects (e.g. Dunn et al. 2010) out of 10 8 thought to exist in the Galaxy (e.g. Remillard & McClintock 2006 and references therein). The gravitational collapse of a star (with mass M ) produces a degeneracy-pressure-supported compact white dwarf (for M < 8 M ) or a neutron star (NS for 8 < M < 30 M ), or if the mass of the star exceeds 30 M, a black hole is formed (Rhoades & Ruffini 1974). A compact object with mass > 3 M cannot be supported by degeneracy pressure and must collapse into a black hole. Therefore, a compact object with measured mass above the limit is considered a black hole. There are about 55 candidates and around 25 confirmed black holes (Remillard & McClintock 2006). The masses of the confirmed objects are measured using different methods that mostly depend on measuring the dynamics of the compact object-companion binary (Orosz 2003; Zió lkowski 2008). As well as being laboratories for strong gravity, stellar-mass black holes represent the last stage in stellar evolution, so they are tools for understanding supernova explosions and the evolution of the binary system. The detection of supernovae-like spectra in some Gamma Ray Burst (GRB) afterglows also means that BHB (spin in particular) could be important in understanding long duration GRB (Bloom et al. 1999; MacFadyen & Woosley 1999; see for example Miller et al. 2011). 1.2 Accretion onto black holes It was realised from early observations of quasars and X-ray binaries that the release of gravitational energy through accretion is the only process efficient enough to produce the required energy. Gas flowing into the compact object is not expected to fall directly to the centre, but rather and to conserve angular momentum, it forms a disk structure 6

19 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS that transports angular momentum out allowing matter to flow inward, which requires some form of viscous dissipation (Pringle 1981). Henceforth, most of the gas moves inward, while a small fraction moves out transporting angular momentum to outer radii (Frank et al. 2002). The details of the dissipation process are not fully understood. Standard molecular viscosity is too small for the size of these systems. Quantitatively, the Reynold s number, measuring the relative contribution of inertial to viscous forces, is too large (Re ) for molecular viscosity to operate (Re 10 3 for laboratory fluids; Frank et al. 2002). Fluids with such high Reynold s number are expected to be unstable to turbulence, the details of which have remained unknown. Shakura & Sunyaev (1973) introduced the α-prescription for the viscosity so that ν = αc s H, where ν is the viscosity, c s is the sound speed and H is the vertical scale height of the disc. α is essentially a parameter describing our ignorance of the exact mechanism producing the shear stress. Further progress was made when it was realised that magnetic fields render differential rotation in discs unstable to the magneto-rotational instability (MRI). Turbulence from such an instability can provide the necessary stress that allows angular momentum transport and accretion to take place (Balbus & Hawley 1991). Most of the work since then has relied on MHD simulations of these discs and it is an active area of research (Balbus & Hawley 2003; Stone 2009). Discs are thought to be present in many astrophysical systems, from proto-planetary discs to binaries with a compact object to active galactic nuclei. Evidence for their existence and their description with the thin-disc model of Shakura & Sunyaev (1973) have grown substantially over the past decades. The first evidence comparing observation with theory came from using techniques such as eclipse mapping and Doppler tomography to study the structures of discs in Cataclysmic Variables (CVs: binary systems where the primary is a white dwarf). These, along with the modelling of transient outbursts caused by disc instabilities, provided the strongest evidence for Keplerian thin discs (e.g. see review by Rutten 1996). Masers in AGN (see Sec ), their line and broadband spectroscopy and polarimetry all gave further support for the picture (Lin & Papaloizou 1996). There is additional evidence that is related to the X-ray spectra and the observations of thermal blackbody emission and are discussed in Sec X-ray emission from black hole systems In the steady thin disc model (Shakura & Sunyaev 1973), viscous stresses convert gravitational energy to heat. Under the assumption that the heat released is radiated locally after being thermalised (e.g. no convection), each elemental area in the disc emits a blackbody-like spectrum. The total spectrum is a multi-colour blackbody produced by the integrated sum across the whole disc. The temperature of the optically-thick accretion disc as a function of radius in the standard model is (e.g. Frank et al. 2002): ( 3GMM T(r) ) 1/4 8πσr 3 (1.1) 7

20 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS where Ṁ is the mass accretion rate for a black hole of mass M, and σ is the Stefan- Boltzmann constant. This gives an integrated luminosity of L 4πrin 2 σt in 4 (using r out r in ), where T in is the temperature at the inner radius of the disc r 1 in. This is equivalent roughly to T 1.0M L 0.25 edd kev, where M 10 is the mass of the black hole in unites of 10 M, and L edd is the luminosity in units of the Eddington luminosity ( M/M erg s 1 ). Therefore, for Galactic black holes, the blackbody emission peaks at soft X-ray energies while for an AGN with M 10 8 M, the peak is in the ultraviolet, producing what is known as the big blue bump of quasars. Another useful temperature scale is that accelerated material would reach if all the gravitational energy is converted to thermal energy, kt th GMm p /r, where m p is the mass of the particle (proton or election). This would be the temperature of an optically-thin environment (Frank et al. 2002). It is independent of mass and its value is around 30 kev for electrons and 50 MeV for protons. Therefore, if there significant emission from optically-thin clouds close to the black hole, such characteristic temperatures would have signatures in the observed spectra (e.g. Gilfanov 2010; Frank et al. 2002) X-ray spectra of Galactic black holes Due mainly to their brightness, this class of accreting black holes have been studied extensively since the launch of the first X-ray satellites. The blackbody component from an optically thick accretion flow was observed and it is well-described by the thin disc standard model (Mitsuda et al. 1984; Makishima et al. 1986). This component dominates the spectra of black hole binaries in what is known as the soft (thermal) state (also called the high state because the 2-10 kev flux is higher than the so-called low hard state). Improvements on the original disc models, which include proper treatment of disc atmospheres and relativistic effects (e.g. Li et al. 2005; Davis et al. 2005), are now being used to constrain the inner radius of the accretion disc and provide measures on the spin of the black holes (Orosz et al. 2007; Steiner et al. 2010). In addition to the thermal component, spectra in the soft state also show a hard power-law component with a photon index of that extends to high energies (beyond 500 kev; e.g. Gierliński et al. 1999). This is clearly not thermal emission, and is generally explained as being due to Comptonization (Sunyaev & Truemper 1979) in hybrid populations of thermal and non-thermal electron populations (Coppi 1999). However, and despite extending to very high energies, this component carries little energy in the soft state and most of the spectral power is radiated by the thermal disc component. In the hard state (also known as the low state), the luminosity of the thermal disc drops and the spectrum is dominated by the hard Comptonization tail which accounts for most of the source luminosity (Fig. 1.1). This hard component, unlike in the soft state, has an energy turnover at 100 kev (e.g. Gierliński et al. 1999). The hard tail is thought to be produced by thermal Comptonzation (see review by Gilfanov 1 There is a correction factor to equation 1.1 when applied to radii < 20r g, but it is not important for this rough estimate. 8

21 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS Figure 1.1: High energy spectrum of the black hole binary Cyg X-1 showing the two spectral states: the soft in red and hard in blue. The plot (Gierliński et al. 1999) is presented in an EF(E) format so that a peak in the spectrum represent the energy with maximum power. 2010). The geometry of the Comptonizing clouds (corona) is not well-known. The optically-thick, geometrically-thin disc appears to extend down to the inner most stable circular orbit (ISCO) in the soft state but the picture is less clear in the hard state (Remillard & McClintock 2006). Data prior to XMM-Newton covering energies down to 2 kev suggested that disc inner radius may change with state transitions, but recent data from XMM-Newton and Suzaku extending down to 0.3 kev show that the disc temperature is consistent with the L T 4 relation extending to the soft state, indicating that the geometry (and hence the inner radius) does not change. This is also supported by the observation of relativistically broadened iron lines in the hard state (Miller 2007; Reis et al. 2010; see Sec ) X-ray spectra of Active galaxies Before discussing the details of X-ray emission, it is worth mentioning that AGN are classified into several types based on their broadband spectral properties. There are the radio-quiet and radio-loud AGN. The spectral energy distribution (SED) of radio-loud object have significant contribution from synchrotron radiation from relativistic jets, while the contribution of the accretion disc is more important in radio-quiet sources. Type 1 and 2 is another classification based on the presence (type 1) or absence (type 2) of broad optical/uv emission lines in the spectra. In the standard unified picture (Antonucci 1993), the two types are a consequence of our viewing angle. A dusty torus obscures the Broad Line Region (BLR) in the type 2 objects viewed at low inclination, while for high viewing angles the inner region is seen directly. This is also the reason for the X-ray classifications of obscured and un-obscured AGN. Further classification into quasars and Seyferts is solely based on luminosity. Quasars represent the high luminosity, high redshift, populations while Seyfert galaxies are those in the local universe. Narrow Line Seyfert 1 (NLS1) galaxies are a subclass of type 1 Seyfert galaxies where the width of the broad emission lines is smaller than usual (< 2000 km s 1 ) 2, compared to See Sec. 2.1 for further defining properties for NLS1 9

22 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS Figure 1.2: The relativistic broad iron line in MCG as revealed by XMM- Newton (red) and Suzaku (black). The plot is a ratio to a power-law fitted to the energies excluding the line energies (from Miniutti et al. 2007). km s 1 in Broad Line Seyfert 1 galaxies (BLS1). A simple scaling of the disc temperature from a stellar-mass to a supermassive black hole (M 10 7 M ) shifts the blackbody temperature from soft X-rays to the UV or extreme UV. Quasars and Seyfert galaxies are very bright in this band producing what is known as the Big Blue Bump (BBB). This component typically carries about half the bolometric luminosity and there are strong evidence that it is the thermal emission from the optically-thick accretion disc (e.g. review by Koratkar & Blaes 1999). In X- rays, early observations showed that the hard band (> 2 kev) can be modelled with a power-law with a photon index of Γ 1.7 (e.g. Mushotzky 1984). This power-law is now thought to be the result of Comptonization of optical/uv photon in a Comptonthin thermal plasma (Haardt & Maraschi 1991; Zdziarski et al. 1994). With advances in instrumentation, it was realised that there are other components present in the spectra, namely an iron Kα emission line at 6.4 kev and a reflection hump above 10 kev (Pounds et al. 1990). These feature are thought to be due to fluorescence and Compton reflection in cold material as suggested by Guilbert & Rees (1988) and calculated by Lightman & White (1988), possibly in the accretion disc itself (see also Sec ). In the soft band (< 1 kev), with the launch of EXOSAT and ROSAT, it was realised that many AGN show an excess above the extrapolation of the hard power-law (Arnaud et al. 1985; Turner & Pounds 1989). This soft excess has since been the subject of many discussions and it is probably the least understood component of the AGN spectral energy distribution. Some sources showed signs of absorption at 0.7 kev indicating the presence of ionised absorbers in the line of sight, mostly from OVII (0.739 kev) and OVIII (0.871 kev) (Nandra & Pounds 1992). 10

23 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS The Fe Kα band The improved spectral resolution of ASCA enabled the iron Kα line in MCG to be resolved. It showed an intrinsically-broad line with an extended red wing (Tanaka et al. 1995). Its shape is well-modelled with a line originating in an accretion disc (see also Sec ) and broadened by Doppler and relativistic effects (Fabian et al. 1989; Laor 1991). The later missions like XMM-Newton, Chandra and Suzaku did not just confirmed that the sight-lines to several AGN have evidence for ionised (warm) absorbers (e.g. Young et al. 2005; Blustin et al. 2007), but also confirmed that the underlying continuum has the broad iron line features. MCG , as a prototype for broad lines, was observed many times. The nature of the line was confirmed with XMM-Newton and Suzaku and other missions. The quality of the data has allowed the spin of the its black hole to be measured to a high accuracy (Fabian et al. 2002; Brenneman & Reynolds 2006; Miniutti et al. 2007; Fig. 1.2). Interpreting the red-wing in MCG as being due to strong gravity was challenged by Inoue & Matsumoto (2003). They pointed out that if the excess at 4 5 kev is the broad wing of a reflected iron line, it should vary in phase with the continuum, contrary to observation. Also, Miller et al. (2008) argued that the reflection fraction (ratio of reflected to direct emission) implied at energies > 20 kev in the Suzaku PIN data cannot be explained by a simple illuminated disc picture. Instead, an apparent red-wing can be produced by absorbing material partially-covering the source (Fig. 1.3). This partial-covering model however has many difficulties as will be discussed in this thesis, particularly when detailed variability studies are included. Furthermore, the model is complex enough with many degrees of freedom that it can essentially fit any spectrum if enough complex absorbing zones are added. Fabian & Vaughan (2003) suggested that the lack of variability of the broad wing of the Fe Kα and the high reflection fractions could be a natural consequence of the strong gravity environment through light-bending effects. Miniutti & Fabian (2004) modelled such behaviour and showed that the variability in the reflection is suppressed if the illuminating source changes height above the reflecting disc. The strong gravity effect on light rays magnifies changes in the primary continuum while suppressing those of the reflected emission. This can also explain the high reflection fraction seen in some sources. Light rays from the illuminating source are bent down towards the disc and less of them reach the observer, while the reflected rays can easily escape. Therefore, the reflection fraction is expected to be higher than for the simple isotropy case (Martocchia et al. 2002). Many objects show broad features around at 4 5 kev. In a survey of 26 Seyfert galaxies from 37 XMM-Newton observations, Nandra et al. (2007) showed that at least 50% have excess emission below 6 kev consistent with a broad iron line emitted very close to the black hole. Also, about half the sample shows absorption signatures from ionised material, and for a third of the sample, the excess at 4 5 kev cannot be fitted by absorption only and a broad iron line is required. 11

24 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS b a c b a d c Figure 1.3: The spectral model for MCG that does not require Fe Kα lines broadened by relativistic effects, but does require five absorbing zones in the line-ofsight. The data from Suzaku XIS and PIN are shown in black plotted in an unfolded EF(E) form. The emission components are: (a) primary, directly-viewed, power-law, absorbed by zones 1 and 2; (b) partially-covered power-law, absorbed by zones 1, 2, 3 and 5; (c) distant reflection, absorbed by zones 1, 2, 3 and 4. (d) The expected X-ray background contribution for Suzaku PIN. (from Miller et al. 2008). The Soft excess Since the discovery of the excess of emission when a power-law fitted to energies > 2 kev is extrapolated to lower energies (Turner & Pounds 1989), its interpretation has remained unclear. The spectrum is smooth with a blackbody-like shape, however fits with thermal blackbody models or low-temperature Comptonization both give temperatures independent of black hole mass, luminosity and accretion rate. The fact it is observed at the same energy is more likely a consequence of atomic features, either through emission or absorption (Gierliński & Done 2004). Ross & Fabian (1993) presented the spectra expected from ionised material when illuminated by an X-ray source, and showed that one of its signatures is the soft emission between kev owing to lines and bremsstrahlung from the hot layers of the illuminated surface. The model predicts sharp features which have to be relativistically blurred to match to the observation, requiring the emitting region to be very close to the black hole. This interpretation was applied by Crummy et al. (2006) to a sample of AGN spectra who showed that it describe their spectra very well. Alternatively, an excess can appear in the spectrum if absorption from Fe, C, and O is also smoothed by relativistic motion, the absorption troughs can merge into each other producing a steep excess (Gierliński & Done 2004; Schurch & Done 2006). Although this has the advantages of producing a soft excess with different strengths as observed, matching the observed smoothness of the soft excess requires the absorbing matter to be moving too close to the speed of light, and the acceleration zone would produce deep spectral features, which are not observed (Schurch & Done 2008). 12

25 1.3. X-RAY EMISSION FROM BLACK HOLE SYSTEMS Figure 1.4: The reflected spectrum (solid) from a cold slab when illuminated with a power-law source (dashed). In the simplest case, the observed spectrum is the sum of the incident and reflected spectra (from Reynolds & Nowak 2003) X-ray reflection The hard X-rays seen from black holes are expected to irradiate the accretion flow. A combination of backscattering, fluorescence and recombination constitute what is described here as reflection (Guilbert & Rees 1988; Lightman & White 1988). If the reflecting medium is cold, the hard illuminating X-ray is absorbed by the ejection of K-shell electrons from iron and other elements. The K-shell can then be filled by an outer electron. The energy of the incident photon is dissipated by the release of an Auger electron or a fluorescent K emission line. Due to its fluorescence yield, emission from iron is particularly strong (George & Fabian 1991; Fig. 1.4). As the X-ray illumination of the medium becomes more intense near the black hole, the matter becomes ionised thereby softening the reflection spectrum (Ross & Fabian 1993). The ionisation is parametrised with ξ = 4πF/n H where F is the incident flux and n H is the number density of hydrogen nuclei. The emerging spectrum requires detailed treatment of Comptonization and ionisation structure of the disc atmosphere. Many calculation have been done in the last two decades, for constant density atmospheres (Ross & Fabian 1993; Zycki & Czerny 1994; García & Kallman 2010) and for density structures in hydrostatic equilibrium (Nayakshin et al. 2000; Ballantyne et al. 2001). The work presented in this thesis uses mainly calculations for a constant-density disc (using the table model reflionx of Ross & Fabian 2005). Calculations without this assumption give similar results but with diluted spectral features (Ballantyne et al. 2001; Fabian & Ross 2010). The input parameters for the XSPEC model reflionx (Ross & Fabian 2005) are the iron abundance, the incident power-law photon index Γ and the ionisation parameter ξ. 13

26 1.4. X-RAY VARIABILITY Figure 1.5: The reflection spectra from a partially-ionised disc computed for different input parameters. The effect of relativistic blurring is also shown in the bottom right panel (from Ross & Fabian 2005). If this spectrum is emitted close to the black hole, the spectral features are blurred by relativistic and Doppler effects. Calculation of the disc line emerging close to a black hole were done by Fabian et al. (1989) for the case of a non-rotating Schwarzschild black hole and by Laor (1991) for the Kerr metric. The latter is implemented as a blurring kernel in XSPEC in the form of kdblur and kdblur2 which are used in this work. A sample of the spectra from the model is shown in Fig X-ray Variability Light curves from black holes exhibit variability on all time scales. In black hole binaries, state transition occur on scales of few months to years. Active galaxies too show significant flux changes with significant spectral changes. PHL 1092 is such a recent example, where its X-ray flux dropped by a factor 200 in 4 years (Miniutti et al. 2009). The match however between the well-studied state transitions in BHB with similar transitions in AGN is not clear Black hole binaries Variability within a single state is usually characterised by power spectral density (PSD) of the light curve. In the soft state, the energy spectrum is dominated by the thermal disc emission that is mostly constant (rms 10%). The PSD is characterised by a simple bending power-law, P(f) f α with α 2 at high frequencies breaking to α 1 below a characteristic frequency (e.g. van der Klis 2004; Remillard & McClintock 2006). In 14

27 1.4. X-RAY VARIABILITY the hard state, the energy spectrum is dominated by the more variable Comptonization emission. The PSD can also be approximated with a bending power-law with an extra bend at low frequencies to an α 0. High quality data however shows that the PSD is better described by several peaked noise components (usually Lorentzian-shaped; Nowak 2000). On top of this broadband noise, BHB show quasi-periodic oscillations in the hard state and during state transitions. Although their origin is not well understood, they have proved useful in tracking characteristic time-scale changes between the states (e.g. see review by Belloni 2010). They can potentially be powerful tools in studying strong gravity when their origin is better understood. Other information in the variability can be extracted using other techniques such as the coherence and time/phase delays (van der Klis 1989). The coherence function γ 2 (f) (Vaughan & Nowak 1997) is a Fourier-frequency-dependent measure of the degree of linear correlation between two light curves. If s(t) and h(t) are two light curves, γ 2 (f) is a measures the fraction of the variance in h(t) that can be predicted from knowing s(t). Although the use of coherence remained limited, it can provide constraints on the emission region and mechanism. High coherence (γ 2 1) is usually observed in BHB (Vaughan & Nowak 1997; Nowak et al. 1999) in the hard state below f 10 Hz. A high coherence is produced if the source (and/or response) is localised or uniform. The coherence in the hard state is lost at frequencies > 10 Hz. This, along with the hardening of the PSD with energy at these frequencies, possibly indicate that the variability below and above 10 Hz is produced by two different mechanisms. Time lags on the other hand have been explored in more detail. BHBs show hard lags where the broadband noise at harder energies always lag behind softer energies in the kev band (Miyamoto & Kitamoto 1989). Knowing that the spectra in the hard state are dominated by Comptonised emission (Sec ), the simplest interpretation as suggested then, is that the lags are due to Comptonisation delays, hard photons scatter more and therefore are expected to be delayed with respect to soft photons. This predicts a lag that is nearly independent of Fourier frequencies contrary to the observation (where the lag τ(f) f 1 ; Miyamoto & Kitamoto 1989; Cui et al. 1997; Nowak et al. 1999). It also requires the Comptonising corona to be as large as 10 4 gravitational radii to produce the largest lags, which is inconsistent with other timing and spectral properties. This lead Miller (1995) to suggest that the delays are intrinsic to the seed disc emission that are preserved in the Comptonisation. Nowak et al. (1999) showed that such a model fails to produce large enough lags to match the observation, and also does not produce the logarithmic dependence of lag on energy as observed. Propagation models appear to provide a better explanation for these lags and other variability properties. In these models, the variability does not have to be produced in the emission region but can modulate it. Lyubarskii (1997) proposed a model where the variability at different time scales is produced as accretion rate fluctuations at different radii. Long time scale random fluctuations originate at large radii before they propagate inward as matter is accreted. The fluctuations then hit and modulate a small emitting region in the centre. The effect is that fluctuations at each radius are the product of 15