NuSTAR view of Active Galactic Nuclei: Probing the reflected emission

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Thesis

Reference

NuSTAR view of Active Galactic Nuclei: Probing the reflected emission

PANAGIOTOU, Christos

Abstract

Active Galactic Nuclei (AGN) are among the most luminous and mysterious celestial sources, thought to be powered by the accretion of matter onto a supermassive black hole. In this thesis, I investigated the hard X-ray radiation of AGN, with a main focus on the X-ray reflected emission, in order to obtain a better insight into the geometry and physics of these sources.

This was achieved by studying the NuSTAR observations of a large sample of nearby AGN.

Local AGN were found to feature a large range of spectral shapes above 10 keV. In unabsorbed AGN, the hard X-ray spectrum is shaped by the dynamics of the X-ray corona, while the spectrum above 10 keV of absorbed sources is shaped by the torus' characteristics.

Moreover, the continuum X-ray emission was found to differ between absorbed and unabsorbed objects, which might hint the existence of intrinsic differences between the two groups.

PANAGIOTOU, Christos. NuSTAR view of Active Galactic Nuclei: Probing the reflected emission. Thèse de doctorat : Univ. Genève, 2021, no. Sc. 5547

DOI : 10.13097/archive-ouverte/unige:151429 URN : urn:nbn:ch:unige-1514292

Available at:

http://archive-ouverte.unige.ch/unige:151429

Disclaimer: layout of this document may differ from the published version.

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Département d’ astronomie Docteur Roland Walter

NuSTAR view of Active Galactic Nuclei: Probing the reflected emission

THÈSE

Présentée à la Faculté des sciences de l’Université de Genève Pour obtenir le grade de Docteur ès sciences,

mention astronomie et astrophysique

par

Christos Panagiotou

de

Preveza (Grèce)

Thèse N^o 5547

GENÈVE

Atelier d’impression ReproMail 2021

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DE GENEVE

FACUTTT DES SCIENCES

DOCTORAT ES SCIENCES, MENTION ASTRONOMIE ET ASTROPHYSIQUE

Thdse de Monsieur Ghristos PANAGIOTOU

intitul6e

<<NUSTAR View of Active Galactic Nuclei: Probing the Reflected Emission>>

La Facult6 des sciences, sur le pr6avis de Monsieur R. WALTER, docteur et directeur de

thdse

(D6partement d'astronomie),

Madame E. KARA,

professeure (Department of Physics, Massachusetts Institute

of

Technology, Cambridge, United States

of

America), Monsieur

S.

PALTANI, professeur

ordinaire

(D6partement d'astronomie)

et

^Monsieur

A. FRAGKOS, professeur assistant (D6partement d'astronomie), autorise I'impression de la pr6sente thdse, sans exprimer d'opinion sur les propositions qui y sont 6nonc6es.

Gendve, le 22flvrier 2021

Thdse

-5547 -

Le Doyen

N.B.

-

La thdse doit porter la d6claration pr6c6dente et remplir les conditions 6num6r6es dans les "lnformations relatives aux thdses de doctorat d I'Universit6 de Gendve".

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Résumé

Les noyaux actifs de galaxies (AGN) font partie des sources célestes les plus lu- mineuses et les plus mystérieuses. Le terme “galaxies actives" est utilisé pour décrire une sous-catégorie de galaxies qui présentent une activité, qui ne peut être attribuée à des processus stellaires. Cette activité se produit dans la région centrale de la galaxie, appelée AGN. Cependant, et malgré des décennies de recherche scientifique, notre compréhension de la physique de l’AGN est loin d’être parfaite.

Selon la vision la plus largement répandue, un trou noir supermassif, possédant une masse de plus d’un million de fois la masse du soleil, se trouve au centre de chaque AGN et est alimenté par la matière environnante. Au fur et à mesure que la matiàre est accrétée sur le trou noir sous la forme d’un disque optiquement épais et géométriquement mince, l’énergie gravitationnelle est libérée. Une partie de cette énergie est ensuite rayonnée par le disque sous forme d’émission thermique, princi- palement dans les longueurs d’ondes ultraviolettes et visible. Plus près du trou noir on trouve la couronne à rayons X. Cette région compacte est remplie d’électrons à haute énergie, qui transforment les photons du disque en rayons X via une diffusion Compton inverse. Le système disque/couronne est entouré d’un milieu poussiéreux et gazeux, souvent appelé tore, qui absorbe l’émission centrale.

L’objectif principal de cette thèse était d’étudier le rayonnement X dur de l’AGN avec des observations, en mettant l’accent sur l’émission réfléchie des rayons X, afin d’obtenir une meilleure compréhension de la géométrie et de la physique de ces sources. J’ai cherché à contraindre en détail la forme des spectres d’AGN dans ce régime d’énergie et à étudier comment les propriétés spectrales varient entre dif- férentes sources.

Ceci a été rendu possible par l’étude d’observations d’un large échantillon d’AGN proche avec NuSTAR. Étant le premier observatoire en orbite capable de focaliser des rayons X au-dessus de 10 keV, NuSTAR offre une occasion unique de résoudre l’émission de rayons X durs d’AGN avec des détails sans précédent.

J’ai découvert que les AGN locaux présentent une large gamme de formes spectrales dans ce régime d’énergie. Cette forme est régie par une propriété physique différente selon l’obscurcissement de la source. Pour les AGN non absorbés, le spectre des rayons X durs est façonné par la dynamique de la couronne de rayons X. En revanche, le spectre au-dessus de 10 keV des sources absorbées est façonné par les caractéristiques du tore.

De manière plus précise, mon analyse spectrale a révélé l’existence de deux cor- rélations principales. Dans le cas des objets non absorbés, l’intensité de l’émission réfléchie s’est révélée être positivement corrélée avec l’indice de photon des rayons X, ce qui suggère une forte interaction entre la couronne et le disque. Les capacités avancées de NuSTAR ont permis d’étudier cette corrélation avec des détails sans précédent. En examinant différents modèles possibles, j’ai conclu que la corrélation

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est mieux expliquée lorsqu’on suppose un mouvement modérément relativiste de la couronne par rapport au disque.

Une telle corrélation n’était cependant pas évidente dans le cas des AGN ab- sorbés étudiés. Au contraire, l’émission réfléchie dans ces objets s’est avérée être en corrélation avec l’absorption de la densité de la colonne, ce qui indique une variation des propriétés du tore selon les sources. En simulant les effets d’un tore grumeleux, j’ai pu montrer que la corrélation observée est bien reproduite grâce à l’augmentation du facteur de recouvrement du tore.

De plus, il a été constaté que l’émission continue de rayons X différait également en fonction de l’obscurcissement. Il a été montré que les AGN absorbés présentaient des spectres plus durs que les objets non absorbés. Bien que ce résultat puisse laisser supposer l’existence de différences intrinsèques entre les deux groupes, l’hypothèse d’une géométrie de dalle pour la couronne semble permettre de reproduire l’écart observé.

En plus des travaux ci-dessus, ce manuscrit présente les résultats de deux autres projets que j’ai menés pendant mes études de doctorat. Pour le premier projet, j’ai étudié l’émission de rayons X durs de η Carinae, un système stellaire binaire extrême. Son émission a été jugée trés variable au cours de son dernier périastre, ce qui indique une perturbation partielle de la source de rayons X dans cet objet. Dans le cadre du second projet, j’ai utilisé une nouvelle approche pour étudier la variabilité ultraviolette et optique de NGC 5548, un AGN archétypique. J’ai pu reproduire la dépendance énergétique des amplitudes de variabilité suite au retraitement des rayons X par le disque.

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Abstract

Active Galactic Nuclei (AGN) are among the most luminous and mysterious celestial sources. The term “active galaxies" is used to describe a subclass of galaxies that feature energetic activity, which cannot be attributed to stellar processes. This activity occurs in the galactic central region, which is called AGN. However, and despite the decades of scientific research, our understanding of the physics of AGN remains far from complete.

According to the broadly accepted view, a supermassive black hole, with a mass of more than a million times the mass of the sun, lies in the centre of every AGN and is fed with matter from its surroundings. As the matter accretes onto the black hole in the form of an optically thick and geometrically thin disc, gravitational energy is liberated. Part of this energy is then radiated away by the disc as thermal emission, mainly in the ultraviolet and optical wavebands. Closer to the black hole is positioned the X-ray corona. This compact region is filled with high energy electrons, which upscatter the disc photons to X-rays via inverse Compton. The disc/corona system is surrounded by a dusty and gaseous medium, frequently called as the torus, which absorbs the central emission.

The primary objective of this thesis was to study observationally the hard X-ray radiation of AGN, with a main focus on the X-ray reflected emission, in order to obtain a better insight into the geometry and physics of these sources. I aimed at constraining in detail the spectral shape of individual AGN in this energy regime and to investigate how the spectral properties vary between different sources.

This was achieved by studying the NuSTAR observations of a large sample of nearby AGN. Being the first observatory in orbit able to focus X-rays above 10 keV, NuSTAR offers a unique opportunity to resolve the hard X-ray emission of AGN in unprecedented detail.

I found that the local AGN feature a large range of spectral shapes in this energy regime. This shape is driven by a different physical property depending on the obscuration of the source. In unabsorbed AGN, the hard X-ray spectrum is shaped by the dynamics of the X-ray corona. On the other hand, the spectrum above 10 keV of absorbed sources is shaped by the torus’ characteristics.

More precisely, my spectral analysis revealed the existence of two main corre- lations. In the case of unabsorbed objects, the strength of the reflected emission was inferred to be positively correlated with the X-ray photon index, which suggests a strong interplay between the corona and the disc. The advanced NuSTAR capabilities allowed the in-depth investigation of this correlation. Examining different potential models, I concluded that the correlation is better explained when a moderately relativistic motion of the corona with respect to the disc is assumed.

Such correlation, though, was not evident in the case of the absorbed AGN under study. On the contrary, the reflected emission in these objects was found to correlate

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with the absorption’s column density, indicating a variation of the torus’ properties among the sources. Simulating the effects of a clumpy torus, I was able to show that the observed correlation is well reproduced as a result of increasing the covering factor of the torus.

Moreover, the continuum X-ray emission was found to differ with obscuration, as well. Absorbed AGN were shown to feature harder spectra than unabsorbed objects.

Although this result might hint the existence of intrinsic differences between the two groups, the assumption of a slab geometry for the corona seems to be able to explain the observed discrepancy.

In addition to the above work, this manuscript presents the results of two more projects I conducted during my PhD studies. During the first one, I studied the hard X-ray emission of η Carinae, an extreme binary stellar system. Its emission was concluded to vary significantly during its last periastron, indicating a partial disruption of the X-ray source in this object. As part of the second project, I employed a novel approach to study the ultraviolet and optical variability of NGC 5548, an archetypical AGN. I was able to reproduce the energy dependence of the variability amplitudes as the result of X-ray reprocessing by the disc.

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Acknowledgments

As probably many before me found out, conducting a PhD is not an easy trip.

Thankfully, there are some people who by offering their assist and support make it feel less intense or who help you realise it is worth the effort. I wish to use this space to express my gratitude to them.

First, I would like to thank my supervisor, Dr. Roland Walter, for giving me the opportunity to conduct my PhD within his group, for his supervision, for sharing his knowledge with me, and for always finding the time to discuss with me. I also wish to thank for his support Prof. Iossif Papadakis, who was the first person to introduce me to the mysteries of AGN and X-ray astronomy, and with whom I shared several fruitful and stimulating discussions in the last years.

There are few more people from the department’s faculty that I want to mention here. First of all, I wish to give my sincere thanks to Prof. Daniel Schaerer, who has readily offered me his advices and support over the last 4 years. PhD students in the astronomy department are very lucky that he is the coordinator of the doctorate program. Special thanks are awarded to Prof. Tassos Fragos for his invaluable advices, for introducing me to the secrets of academic life, and for the nice time we spent outside the work environment. Finally, I would like to thank Prof. Stephane Paltani for his help with RefleX and for always being available to discuss with me.

I also wish to thank the numerous colleagues who have been part of my PhD journey, especially Gozde, Guillaume, Sotiria, Jean-Gabriel, and Manos. In partic- ular, I would like to mention Dr. Matteo Balbo. Matteo was one of the first people I met when I arrived to Geneva and his advices and information were invaluable in making my transition to PhD life easier. Many thanks are also awarded to our secretary Marie-Claude, who was always willing to help me with any work related or not, bureaucratic issue I had during the last years.

Thanks should also be given to many friends for their support, including, but not limited to, Alexis, Andreas, and Sakis. I would especially like to mention Rene- gade Saints for having me in their amazing group and for all the nice moments we experienced over the last years.

Moreover, I wish to express my gratitude to two little guys, whom interacting with or watching them grow was always enough to put a smile on my face and offer some relaxed time. Leonidas and Daphne, I wish you both all the best for the adventures waiting ahead for you.

There are two people who made it possible for me to be here today and to be able to present this thesis. These people are no other than my parents. Thank you both for everything.

Last, but definitely not least, I wish to thank a person who made the last years in Geneva feel more colourful. Thank you, Dr J.

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List of my publications

1. Panagiotou C., Walter R. & Paltani S. subm. to A&A, “NuSTAR view of heavily absorbed AGN: TheR−N_H correlation”

2. Panagiotou C., Papadakis I., Kammoun E. & Dovčiak M. 2020, MNRAS, 499, 1998, “Multiwavelength power-spectrum analysis of NGC 5548” (arxiv:

https://arxiv.org/abs/2009.09693)

3. Panagiotou C. & Walter R.2020,A&A, 640, 31, “NuSTAR view of Swift/BAT AGN: The R−Γ correlation” (arxiv: https://arxiv.org/abs/2006.04441) 4. Panagiotou C. & Walter R.2019,A&A, 626, 40, “Reflection geometries in ab-

sorbed and unabsorbed AGN” (arxiv: https://arxiv.org/abs/1907.02523) 5. Panagiotou C. & Walter R. 2018, A&A, 610, 37, “The environment of the wind-wind collision region in eta Carinae.” (arxiv: https://arxiv.org/abs/

1712.01382)

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Introduction

1.1 Observational characteristics

The term “active galaxies" is used to describe a group of galaxies featuring energetic activity that cannot be directly attributed to stars. Recent studies suggest that around 10% of the galaxies in the Universe are active (Xue et al.2010). The central region of these galaxies, where this activity takes place, is called Active Galactic Nucleus (AGN, hereafter).

Historically, Seyfert (1943) was, probably, the first one to observe a group of AGN and denote that they form a special group of galaxies; although there was already evidence of individual sources showing special spectral characteristics (e.g.

Fath 1909). Studying the optical spectrum of six nearby galaxies¹, Seyfert noticed that, in contrast to the spectra of normal galaxies, they feature high excitation emission lines in their core, most of which correspond to forbidden transitions. In addition, some sources featured broad wings in their hydrogen lines.

At the time of that study, twelve galaxies in total were known to exhibit similar nuclear emission lines. Many more were to be identified in later sky surveys in the second half of the twentieth century. It is now well established that strong emission lines are a ubiquitous feature of the majority of AGN and the width of Balmer lines with respect to the width of the observed forbidden lines serves as a criterium to divide Seyfert galaxies (or simply Seyferts) into two groups (Khachikian and Weedman1974). Type 1 Seyfert galaxies (or Seyferts 1) exhibit spectra with the Balmer lines being significantly broader than the forbidden lines, while the Balmer and forbidden lines feature similar widths in the case of type 2 Seyfert galaxies (or Seyferts 2).

Independently of studies on Seyfert galaxies, the technological advancement in radio astronomy in the 40s and 50s led to the discovery and study of quasars, which were identified as point-like extragalactic sources of extreme luminosity (Schmidt 1963). It was early considered that quasars and Seyfert galaxies might be of similar nature (e.g. Weedman 1977, and references therein). Nowadays, it is broadly accepted that quasars and Seyfert galaxies are powered by the same physical processes and are both classified as subclasses of AGN, with quasars being on average more luminous than Seyferts.

Since their discovery, AGN have been intensively studied in all wavebands of the electromagnetic spectrum. This led to the identification of several observational

1Or “extragalactic nebulae", as they are called in the original paper.

1

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characteristics, unique to AGN. The detection of these characteristics may serve as a criterium to classify celestial sources as AGN, whereas their investigation can shed light into the mysteries of AGN. Below, I briefly describe the main observational features, which will set the stage for the subsequently presented work.

High Luminosity

AGN comprise the most luminous non-transient²objects in the sky. Their bolometric luminosity ranges from about 10⁴²to 10⁴⁸erg/s, which corresponds roughly to 10⁸− 10¹⁵ times the solar luminosity. AGN may, thus, frequently outshine their host galaxy, which has a typical bolometric luminosity of nearly 10⁴⁴ erg/s. It may be worth noticing that the constant (in a non-transient sense) emission of large luminosities hints a large reservoir of energy for these objects. The vast amount of radiated energy was one of the first indications that the AGN emission can not be explained by stellar processes. Assuming, for example, the production of the observed energy through stellar fusion, hydrogen of a mass equal to or larger than the solar mass needs to be burnt to helium per year to produce a typical AGN luminosity. Instead, it is currently accepted that AGN are powered by the accretion of matter onto a supermassive black hole, which is a very efficient way to extract energy from matter (see Section 1.3).

Spectral lines

The high ionisation emission lines of abundant elements were the first characteristic to separate AGN from other celestial sources. As a result, their line spectrum has been excessively studied. A large variety of emission and absorption lines have been observed in the spectrum of AGN, adding to the complexity of these sources.

In the visible spectrum, the emission lines are commonly divided into two groups, broad and narrow, based on their width. The broad lines correspond to permitted atomic transitions and, assuming Doppler broadening, their widths indicate velocities from about 10³ km/s up to even higher than 10⁴ km/s. On the contrary, AGN feature narrow emission lines of both permitted and forbidden transitions with typical velocities between 200 and 800 km/s. Both the narrow and broad emission lines are believed to be the result of photoionisation of the surrounding matter by the ultraviolet (UV) emission of the central source. However, broad lines are present only in the spectra of a subclass of AGN (Section1.2), while narrow lines are detected for almost every AGN. A typical spectrum of AGN with both broad and narrow lines is plotted in Fig. 1.1.

The specific characteristics of the various lines have been studied to derive conclusions on the line emitting regions. The broad lines are produced in the so-called broad line region (BLR), which has a typical number density of 10⁹−10¹⁰ cm⁻³ and a relatively large covering factor, of the order of 10%. Moreover, the variability of broad lines with respect to the continuum UV variability suggests that BLR is typically located at a distance of several light days to less than a pc from the central source (Kaspi et al. 2005; Peterson1993).

The narrow lines originate from the narrow line region (NLR), which is thought to be less dense than the BLR with typical densities below 10⁵ cm⁻³. NLR is also

2I call AGN non-transient, as they are not regularly observed to transit from an inactive to an active state and faint away afterwards. Nevertheless, AGN have been suggested to have a duty cycle of the order of hundred million years (Woltjer1959).

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Figure 1.1: Composite UV/optical spectrum of quasars, as estimated by Vanden Berk et al. (2001) using data from the sloan digital sky survey (SDSS). Several emission lines are labelled on the plot. The dotted and dashed lines denote the power-law fits to the underlying continuum emission.

located at a larger distance from the centre, typically at 100 pc to 200 kpc, which makes it possible to spatially resolve it for the closest AGN. This has revealed a biconical shape for NLR (e.g. Tadhunter and Tsvetanov 1989). It should be mentioned that the exact geometry, nature, and production of both BLR and NLR is still rather unclear, despite the decades of investigations. The recent high-quality data sets, though, seem promising in providing new insights into these regions (e.g.

Horne et al. 2020; Xiao et al.2018).

In addition to the optical lines, many more absorption and emission features are detected outside the visible spectrum, most notably in the infrared and soft X- rays. One of the most important and widely studied lines is the Fe Kα emission line detected at 6.4 keV in the X-ray spectrum of AGN, which will be discussed in detail in Section 1.4.1.

Continuum emission

A typical broadband spectrum of AGN is shown in Fig. 1.2. The dashed and solid line correspond to radio-loud and radio-quiet AGN, respectively, which will be introduced in the next section. Deferring also a detailed discussion of the X-ray spectrum until Section 1.4.1, a few intriguing remarks can be deduced from Fig. 1.2.

First, AGN are luminous in all wavelengths, from radio waves to X-rays, or even γ-rays in some cases; suggesting that a rather extraordinary source powers these objects and that several emission mechanisms take place in them. Second, the AGN spectral energy distribution (SED) is nearly flat within an order of magnitude over several decades of frequency; that is, AGN emit at a rather constant energy rate from around 100 µmin infrared to tens of keV in X-rays.

Furthermore, several features are present in the SED of AGN. Undoubtedly, one of the most prominent is the so-called big blue bump, found in the UV part of the spectrum. It corresponds to the increase of radiated energy at around 3000 Å

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Elvis et al., 1994, ApJS, 95, 1

IR Opt-UV

Big Blue Bump  (0.1~1μm) 

So3 Excess  (~0.1 keV)  1μm 

inﬂec<on  mm break 

(~100μm)  Radio

Compton  Hump      (~10‐30keV) 

Lν = ν^‐3 (dust) 

EUV

Radio‐loud 

Radio‐quiet 

X-ray

Figure 1.2: Average spectral energy distribution of radio-quiet (solid line) and radio-loud (dashed line) quasars as estimated by Elvis et al. (1994). Several spectral features are denoted on the plot.

compared to the emission in longer wavelengths (Richstone and Schmidt1980). The shape of this spectral feature seems to resemble roughly the spectrum of blackbody emission (Malkan and Sargent 1982). However, its peak indicates a body with a typical temperature T&2·10⁴ K. These values of temperature are extreme for the majority of stars and as a result, this spectral feature cannot be explained even by a population of diverse stars. As will be shown in Section1.3.2, the big blue bump can be naturally explained as thermal emission from an accretion disc around a massive black hole.

Jets and outflows

A fraction of AGN has been observed to host large scale collimated outflows of relativistic velocities, which are referred to as jets. Jets arise from the core of AGN and extend from few parsecs to tens of kpc. They are luminous mainly in radio waves and their detected synchrotron radiation indicates the existence of a relatively strong magnetic field. Due to their high velocities, jets are capable to transfer large amounts of energy from the AGN to the host galaxy or the circumgalactic medium.

The exact mechanism of jet production is still a matter of intense research.

Nonetheless, it is generally agreed upon that the presence of a magnetic field near the core of AGN is most likely needed for a jet formation (Contopoulos et al.2015).

Moreover, it is still unclear whether jets are present in all AGN, but potentially too faint to be detected, and thus, whether they are a universal characteristic of accretion process; or they are present in only a portion of AGN with specific characteristics. Interestingly, a few sources without a strong radio emission were found to host a jet after being observed for sufficiently large exposures (e.g. Blundell et al.

2003; Blundell and Rawlings 2001).

In addition to jets, AGN host outflows of mass, which are associated with winds originating from the accretion disc. Their existence is commonly inferred through the detection of absorption or emission lines, usually at energies shifted compared to the

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expected rest frame value. Contrarily to jets, these outflows are less extended and not spatially resolved. AGN feature a variety of outflows with different velocities, ionisation levels, and densities, even within the same source (e.g. Tombesi et al.

2013). The different outflows are expected to originate from different regions of the disc. The launching mechanism of these outflows is rather debated, although it is expected that several mechanisms might operate simultaneously in different parts of the disc (Elvis 2012). The disc outflows are of scientific importance, since they provide a mechanism for the AGN to interact directly with its host galaxy, while they have also been proposed as a source of the BLR (e.g. Czerny et al. 2017).

Variability

The AGN emission has been found to vary significantly in all wavebands and in various time scales. Variability is such a characteristic feature of AGN that it may be used as a classification criterium for these objects (De Cicco et al.2020; Pouliasis et al. 2019). The variability amplitude, at least on short time scales, seems to increase with energy, namely the emission in shorter wavelengths is more variable than the one in longer wavelengths. In addition, the characteristic time scale of variability increases for decreasing frequency. The X-ray emission is usually found to be variable in time scales of minutes to hours, the UV/optical flux varies typically in one to several days and the infrared radiation has a characteristic variability time of the order of months. The different variability amplitudes and time scales indicate that the observed emission in different wavebands originate from different regions within the source.

The observed variability can be used to derive a rough estimation of the source’s size using the simple expressionR≤ct, whereRis the typical radius of the (assumed spherical) source, tis the characteristic time scale, andc denotes the speed of light.

The above relation expresses the fact that a perturbation can not propagate within a source faster than the speed of light³. Hence, the X-ray source is constrained within a region of several light-minutes, where one light-minute is approximately 10¹² cm, while the UV and optical source may extend to a region around 50 to 100 times larger. Such a physical picture is further supported by the results of recent microlensing studies (Chartas et al. 2016).

Finally, the detection of variability in all wavebands has enabled the conduction of correlation studies between the different emissions, which have provided important insights into the inner as well as the outer geometry of AGN. For instance, reverberation mapping studies of the BLR have provided information about this region’s structure, while they have also concluded that a mass of the order of million solar masses or more lies in the innermost regions of AGN (e.g. Bentz and Katz 2015, and references therein).

To recapitulate, AGN are extremely bright sources, typically emitting as much light as a whole galaxy, restrained on a small central region of a galaxy, with a typical size of less than a few light-days, featuring a broadband non thermal spectrum, which extends from radio waves up to γ-rays, and harbour a significantly large amount of mass. Clearly, AGN are unique celestial objects, the physical origin of which is not straightforwardly deduced. This thesis is an effort to improve our understanding of AGN by studying their hard X-ray spectrum.

3This line of argument does not hold in the case of highly anisotropic and non spherical sources.

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In this introductory chapter, I will discuss the phenomenology of AGN, introducing the different AGN classes (Section1.2), and I will describe the currently accepted model for their source of energy (Section 1.3). After a detailed presentation of the X-ray features of AGN (Section 1.4) and a description of the efforts to include all AGN under a common scheme (Section 1.5), the contemporary view of AGN will be summarised in Section 1.6. The cosmic X-ray background and its connection to AGN population will be briefly introduced in Section1.7. This chapter is concluded with a description of the main objectives of the present thesis.

1.2 AGN Taxonomy

Studying the AGN radiation in different parts of the electromagnetic spectrum, as- tronomers have defined several subclasses of AGN over the years. Since we still lack a complete understanding of the physical processes taking place in an AGN, the derived taxonomy has mostly been based on observational characteristics. This might even be misleading in some cases, as these characteristics do not necessarily correspond to physical differences between the various classes. In this section, I briefly outline the main AGN classes, which are frequently encountered in the literature, focusing mainly in their observational features. A more detailed presentation of all the AGN classes and how these are detected in the different wavebands can be found in Padovani et al. (2017).

It is common to divide AGN into two broad categories, the radio-loud and radio- quiet sources⁴. Such a classification is driven by the early studies of quasars (e.g.

Sandage 1965). As is evident from the used names, the defining criterium for an AGN to be classified as radio-loud or radio-quiet is its observed radio flux density with respect to the optical one.

The strong radio emission in radio-loud sources⁵ is attributed to the presence of a jet. Relativistic particles are launched by the central source in a preferential direction forming the observed jet, which emits a synchrotron radiation due to the interaction of the particles with the magnetic field. It has been found that about 15% of AGN are radio-loud (e.g. Kellermann et al. 1989; Lal and Ho 2010).

The main focus of this thesis is the radio-quiet sources, which represent the vast majority of observed AGN. It should be noted that radio quiet does not mean radio silent. Instead, radio-quiet AGN have been found to emit in the radio waveband (e.g. Kimball et al. 2011). This radiation, though, is significantly fainter than the radio emission of radio-loud AGN (see Fig. 1.2). The exact source of this emission is still controversial (e.g. Padovani 2016), although a significant fraction originates, probably, from the host galaxy.

The radio-quiet AGN can be further divided into (radio-quiet) quasars and Seyferts. Observationally, quasars differ from Seyferts as they outshine their host galaxy and are usually (at least in the earlier surveys) observed as point-like sources.

It is well accepted that quasars are similar to Seyfert galaxies, but exhibit larger bolometric luminosities. Typically, one may define as quasars all radio-quiet AGN

4Note that Padovani (2017) propose the use of “jetted" and “non-jetted" terms instead of radio- loud and radio-quiet, respectively, since the latter (older) terms might be significantly misleading when used for AGN of moderate luminosity.

5The radio-loud group comprise the AGN classified as blazars, radio-loud quasars, and radio galaxies. The study of these sources is outside the scope of this work and they are only mentioned here for consistency.

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with an X-ray luminosity from 2 to 10 keV larger than 10⁴⁴ erg/s, while the corresponding luminosity range for Seyferts is L_{2−10 keV} = 10⁴²−10⁴⁴ erg/s (Padovani et al. 2017).

Moreover, several subcategories of Seyfert galaxies are usually defined based on their optical spectrum. The main classification criterium in this case is the detection of broad wings in the permitted emission lines. The two major subclasses are Seyfert 1 and Seyfert 2. As noted by Khachikian and Weedman (1974), Seyfert 1 galaxies feature broader Balmer than forbidden lines. On the contrary, Seyferts 2 feature a similar width (typically below 1000 km/s) for both Balmer and forbidden lines. Based on this classification, Osterbrock (1977) defined the additional categories Seyfert 1.2, Seyfert 1.5, and Seyfert 1.8. Seyfert 1.5 galaxies correspond to an intermediate state between Seyfert 1 and Seyfert 2 sources, with a narrow Hβ profile being superimposed on an observed broad line. Seyfert 1.2 and 1.8 sources correspond to intermediate states between Seyfert 1 and Seyfert 1.5 or Seyfert 1.5 and Seyfert 2, respectively. The former sources show a weaker narrow Hβ line in comparison to Seyferts 1.5, while the opposite is true for the latter. Finally, Oster- brock (1981) defined the supplementary class of Seyfert 1.9 galaxies, which feature a broad Hα emission line, but, in contrast to Seyfert 1.8, no broad Hβ line is evident in their spectra.

An additional Seyfert subclass of scientific interest consists of the so-called Nar-

4400 4600 4800 5000 5200 5400 5600 0

.5

1 Sy2 (Mrk 1066)

NLS1 (Mrk 42)

Sy1 (NGC 3516)

Hβ He II

He II [O III]

Fe II

Wavelength

Relative Flux

Figure 1.3: Comparison between the spectrum of a Seyfert 1 (NGC 3516, lower line), a Seyfert 2 (Mrk 1066, top), and a NLS1 (Mrk 42, center) in the waveband around the Hβ line. The Hβ line is clearly broader in the case of the Seyfert 1 NGC 3516, while it features a similar width in the Seyfert 2 and NLS1 galaxies. On the other hand, the [OIII] line is remarkably more prominent in the spectrum of the Seyfert 2 galaxy. Figure retrieved from Pogge (2000).

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row Line Seyfert 1 galaxies (NLS1, Goodrich 1989; Osterbrock and Pogge 1985).

The optical spectrum of these objects exhibits mostly narrow Balmer lines, but, in contrast to Seyferts 2, the forbidden [O III] line is relatively weak with respect to the detected Hβ one. Hence, NLS1 feature observational characteristics of both Seyfert 1 and Seyfert 2 class. The typical spectrum of NLS1s is compared to that of Seyferts 1 and Seyferts 2 in Fig. 1.3.

The efforts to combine all the different classes in one big picture led to the development of the unification model (Section 1.5). It is now generally accepted that all AGN are powered by the same central engine, that is the liberation of gravitational energy through accretion onto a supermassive black hole. The main observational differences between the various classes are then considered to originate from observing the source at different inclination angles. Although recent results seem to challenge the simplified unification model, it has been able to account for many of the observational properties of AGN.

1.3 Black Hole paradigm

1.3.1 Energy reservoir

The continuum emission of AGN in the optical and UV part of the electromagnetic spectrum can not be explained by stellar emission. As already mentioned, the big blue bump in AGN spectra is typically found below 3000 Å, which, translated to temperature using Wien’s law, corresponds to a blackbody emission from a source with T >10⁴ K. This is significantly larger than the temperatures of typical stars.

In addition, the large amount of energy radiated by AGN in combination with their small size, as inferred from their optical variability, indicates that AGN are not powered by stellar thermonuclear processes.

It was early realised that matter accretion onto massive objects is an efficient mechanism to release significant amount of energy and may thus explain the observations of quasars and Seyfert galaxies (Lynden-Bell1969; Salpeter1964; Zel’dovich 1964). According to the currently accepted model, a supermassive black hole (with mass M_BH > 10⁵M, where M is the solar mass) resides in the centre of every AGN^6,7. Surrounding matter accretes onto the black hole in the form of a disc and part of the energy released during this process may then be radiated away. A crude estimation of this mechanism’s efficiency can be retrieved as follows.

Assuming a supermassive black hole at rest, matter approaching the black hole with a slight angular momentum⁸, so that it does not free-fall into it, will be set in circular orbit around the black hole. If a sufficient mechanism to reduce the matter’s angular momentum is further assumed, the orbiting matter will start to fall inwards.

As matter moves closer to the black hole, its gravitational energy is reduced and it

6In this sense, active galaxies are no more special than normal galaxies since the contemporary belief is that most massive galaxies host a supermassive black hole in their centre.

7There have been several indications supporting the existence of a massive black hole in an AGN.

For example, observations of the water maser in the spectrum of the AGN NGC 4258 suggest the existence of a very massive object in its core (e.g. Moran et al.1995). Reverberation mapping studies of the BLR have, also, been successful in evaluating the total mass within light-days from the AGN centre to be more than a million solar masses (e.g. Bentz and Katz2015).

8The initial angular momentum of the approaching matter with respect to the black hole cannot be arbitrarily high to allow for the accretion to occur in the required timescales (King2008).

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will eventually enter the black hole. The total amount of this energy difference for a mass m falling from infinity is:

∆U = GM_BHm

R_ISCO (1.1)

where G is the gravitational constant and R_ISCO is the radius of the innermost stable circular orbit. Inwards of this radius, the matter spirals onto the black hole in timescales faster than needed for the available energy to be radiated. For a Schwarzschild, meaning non-rotating, black hole this radius is equal to R_ISCO = 6Rg = 6^GM_c2^BH, where R_g is the gravitational radius and c the speed of light. The above equation (1.1) is then reduced to:

∆U '0.17mc² (1.2)

Assuming for simplicity that half of this energy is radiated, the total luminosity expected from accretion is:

L= dU

dt '0.085 ˙mc² (1.3)

where ˙m denotes the rate of matter being accreted.

This simple consideration has revealed the high efficiency of the accretion process in converting mass into energy, which is of the order of 10% (eq. 1.3). This efficiency is far greater than the nearly 0.7% efficiency of the nuclear reactions in solar-like stellar cores. In general, the efficiency of accretion is around 5-42% (King 2008), depending on the black hole’s spin. Assuming a prograde accretion, i.e. the angular momentum of the accreting matter and the spin of the black hole have the same direction, R_ISCO is decreasing as the spin is increasing, leading thus to an increase of the total energy available to be emitted.

For typical values of AGN luminosity one may also estimate the rate of accretion required to power these objects. An accretion rate of ˙m'10²⁹ kg/yr is found to be sufficient to produce luminosities of L= 10⁴⁴ erg/s, assuming a 10% efficiency. This value is not unreasonably high and definitely smaller than the Eddington limit⁹.

A major question that might rise form the above description is the origin of accreting material. Matter has to be located near the black hole and with a small angular momentum around it for the accretion to take place. This led to the sug- gestion that AGN activity is triggered by merging events of galaxies. The merging of two galaxies can lead to the tidal disruption of their core matter providing the necessary initial conditions of the accretion model. However, recent studies indicate

9The Eddington limit of mass accretion is the accretion rate that corresponds to the Eddington luminosity, which is the maximum luminosity a body can emit. For luminosities above this limit, the outward force due to radiation pressure outweights the binding gravitational force and the accreting matter is expelled away. Assuming spherical symmetry and matter of ionised hydrogen, the Eddington luminosity is given by:

LEdd=4πGMBHmpc σT

'1.26·10³⁸ M M

erg·s⁻¹ (1.4)

wheremp is the proton mass andσTdenotes the Thomson cross section. Although retrieved under specific assumptions that do not hold in the case of AGN disks, the above expression provides a rough estimation for the limiting luminosity of AGN, as well. The corresponding Eddington accretion rate depends on the efficiency of the accretion process, η, and is equal to ˙mEdd = ^L_ηc^Edd2 . It is also customary to define the Eddington ratio asλEdd= _L^L

Edd.

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that the majority of AGN do not show any signatures of merging history (Zhao et al.

2019). The exact trigerring mechanism that activates the cores of galaxies is still a subject of scientific research.

1.3.2 The disc spectrum

It is commonly assumed that matter accretes onto the black hole in the form of a disc.

It is reasonably expected that the dynamic evolution of an initially less structured accretion will eventually lead to the more steady configuration of accretion disc.

However, a detailed analytical description of the disc structure has not been possible so far. The main reason for that is the lack of a proper understanding of the viscous processes that lead to the transport of angular momentum. While it is broadly considered that magnetorotational instabilities might provide a base for the needed viscosity (Balbus and Hawley 1991), this is still a matter of active research and our understanding remains far from complete.

Nevertheless, as noted by Shakura and Sunyaev (1973), one may reach rough conclusions even in the absence of a robust theory for the viscous processes in action.

The nature of the viscosity associated with turbulent motions¹⁰ make it possible to describe it asv=αc_sH, wherec_s is the speed of sound,H denotes the disc thickness and α is a parameter smaller than 1. Although the above parametrisation is not enlightening in elustrating the physics involved, it has been useful in algebraically expressing the disc properties. A detailed derivation of the various equations may be found in Frank et al. (2002). Below, a short description on the disc temperature profile and the disc emission is given.

Assuming a steady, geometrically thin, and optically thick disc, the conservation of mass and angular momentum can be combined to yield the following equation for the energy dissipated per unit surface per time in the disc (Pringle 1981):

D(R) = 3GMBHm˙ 8πR³

"

1−

R_ISCO R

1/2#

(1.5) This is the total energy rate available to be radiated, where it was also assumed that the disc extends all the way down to the innermost stable circular orbit and half of the liberated gravitational energy is transformed to kinetic energy.

An important result of eq. (1.5) is that the dissipated energy, which is an ob- servable quantity, does not depend explicitely on viscosity. This allows us to study the emission of AGN and derive results without the need to understand in detail the exact energy dissipation mechanism. Furthermore, it can be shown that the maximum energy per unit area corresponds to a distance R = 1.36R_ISCO from the black hole. The energy dissipated closer to the black hole corresponds to only about 5% of the total disc luminosity.

Assuming, in addition, that the liberated energy can be dissipated locally, meaning that the disc is optically thick at every radius, and as a result, each disc annulus at radiusR emits a black body radiation, the disc temperature can be estimated using the Stefan-Boltzmann law and eq. (1.5). The disc temperature profile is, hence, given by:

10A similar description can be derived in the presence of magnetic stresses (Shakura and Sunyaev 1973).

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T(R) =

(3GMBHm˙ 8πR³σ

"

1−

R_ISCO R

1/2#)1/4

(1.6) where σ is the Stefan-Boltzmann constant. For large radii, R R_ISCO, the above equation is simplified to:

T(R) =3GM_BHm˙ 8πR³σ

1/4

(1.7) which shows that the disc temperature increases closer to the black hole, having the radial dependence of T ∝R⁻^3/4. This is an important prediction which can be tested by observations. Expressing the various parameters in terms of typical AGN values, eq. (1.7) can be further reduced to:

T(R)'2.8·10⁵ K M_BH 10⁸M

−1/4 m˙

˙ m_Edd

1/4 R 6R_g

!−3/4

(1.8) Thus, for a a given accretion rate, the disc temperature is larger for less massive black holes, while for a given black hole mass, the disc temperature increases with the accretion rate. For typical values of the accretion rate and the black hole mass, the disc temperature reaches values of the order of 10⁵ K. Such large temperatures can explain the big blue bump in the spectra of AGN.

Finally, it is important to derive the predicted spectrum of the accretion disc emission. Following the assumptions made above, one may conceptualise the disc radiation as the sum of black body emissions¹¹, each originating from a disc annulus with radius R and width ∆R. Subsequently, the total disc emission is given by:

Fν ∝

Z RISCO

Rout

Bν(T(R))2πRdR (1.9)

where Bν is the Planck function describing the black body emission and T(R) is the temperature at each radius as given by eq. (1.6). For frequencieshν kTmax, where kis the Boltzmann constant, h denotes the Planck constant, andT_max is the maximum temperature of the disc, the disc emission follows a Wien-like exponential drop, which corresponds to the high-energy spectral emission of the hottest annuli.

For hν kT_out, where T_out is the temperature at the outer disc, the disc emission follows the Rayleigh-Jeans tail of the coolest annuli. Finally, in the intermediate range kTout hν kTmax one, starting from eq. (1.9), may show that the disc emission follows a power-law distribution of F_ν ∝ν^1/3.

A typical disc spectrum is shown in Fig. 1.4. The spectrum increases with energy, exhibiting a bump feature in the ultraviolet before dropping exponentially.

The accretion disc emission has been found to reproduce well the observed spectrum of quasars (Malkan1983).

To sum up, the black hole/accretion disc assumption has been quite succesful in explaining several observational characteristics of AGN, among which the emission of large luminosities from a small region and the big blue bump. However, our understanding of accretion process remains largely incomplete. Open questions, like the nature of viscosity and the triggering of accretion, are still a subject of intense research.

11Such an approach is useful to deduce an approximate estimation of the disc’s spectrum. It should be noted though that it ignores the electron scatter opacity, which becomes important in the inner parts of the disc for an AGN (Novikov and Thorne1973; Shakura and Sunyaev1973). In this case, the emission of the inner annuli does not follow a black body distribution.

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1 10 100 E (eV)

10

FE (arbitrary units)

ν²

ν^1/3

e^-hν/kT

Figure 1.4: Expected disc spectrum for a black hole with mass M_BH = 10⁷M, accreting at a 10% mass rate of the Eddington limit. The plotted spectrum was estimated using the disk model in XSPEC software. The frequency dependence at the different energy bands, as deduced from eq. (1.9) is labelled on the figure.

1.3.3 Time scales

As was already mentioned, variability is a ubiquitous feature of AGN. The investigation of spectral or flux variability is a powerful tool to obtain information about the geometry of AGN and the physical mechanisms shaping their emission. Therefore, I find it useful to briefly describe here the characteristic time scales associated with the system of accretion disc.

Assuming a Keplerian, geometrically thin disc, the shortest time scale of the system is the dynamical time scale:

t_dyn' R vφ '

s R³

GM_BH (1.10)

where vφdenotes the rotational velocity. This time scale corresponds to the orbital period of matter at distanceR. It may also be shown to be equal to the characteristic time scale within which hydrostatic equilibrium in the vertical direction of the disc is achieved.

The thermal time scale is defined as the ratio between the internal energy and the dissipation rate. It is given by:

t_th' Σc²_s

D(R) (1.11)

where Σ is the surface density andc_s is the speed of sound. The thermal time scale denotes the characteristic time needed to reach thermal equilibrium. Within the α parametrisation of the disc accretion, the thermal time scale is connected to the dynamical one as:

t_th' t_dyn

α (1.12)

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Finally, one may define the viscous time scale, which measures the characteristic time on which mass moves inwards, given by:

t_visc ' R²

v (1.13)

where v is again the viscosity. The three time scales are associated as:

t_dyn'αt_th'α H

R 2

t_visc (1.14)

where H is the disc’s thickness, assumed HR for a thin disc.

1.4 AGN in X-rays

1.4.1 The X-ray spectrum

AGN are luminous X-ray sources (Elvis et al. 1978). In fact, the X-ray luminosity of an object serves as a defining criterium for AGN (Hickox and Alexander 2018).

All the celestial sources with a constant (i.e. non-transient) luminosity above 10⁴² erg/s in the range from 0.5 to 10 keV are selected in cosmic surveys as AGN.

However, the X-ray radiation has a different origin than the UV/optical one.

For typical values of black hole mass and accretion rate, the inner disc reaches temperatures of the order of 10⁵ K or, equivalently, kT ∼ 10 eV (see eq. 1.8).

Therefore, it cannot account for the observed X-ray emission. Instead, the X-rays are believed to be produced by Comptonisation of soft photons in an optically thin region of high temperature.

Figure 1.5: Average Seyfert 1 X-ray spectrum, as estimated by Ginga/OSSE (up- per) and Ginga/EXOSAT data (lower pannel). The dashed lines in both panels denote a power-law spectrum with a high-energy cutoff and photo-electric absorption in lower energies; the dotted lines indicate the reflected (including the iron line) emission, and the solid lines correspond to the sum of the two. This figure was retrieved from Gondek et al. (1996).

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A typical X-ray spectrum of Seyfert 1 galaxies, as was estimated by Gondek et al.

(1996) combiningGinga,EXOSAT, and OSSE data, is shown in Fig. 1.5. In general, the AGN spectrum above a few keV consists of mainly three components, namely the primary intrinsic continuum (dashed lines in Fig. 1.5), the Compton reflection hump, and a Fe Kαemission line. The latter two emission components (dotted lines in Fig. 1.5) are often called jointly the reflected emission¹². The three emission components as well as their modification due to interaction with matter along our line of sight are detailed below.

X-ray continuum

The primary X-ray continuum has been found to be well described by a power-law model with an exponential cutoff at high energies (e.g. Haardt and Maraschi1993).

The cutoff is usually between several tens to few hundreds keV (e.g. Baloković et al. 2020; Kamraj et al. 2018; Malizia et al. 2014), while the photon index ranges typically from around 1.5 up to 2.2 (e.g. Ricci et al. 2017b).

A power-law emission is indicative of a Comptonisation of soft photons by hot plasma (Sunyaev and Titarchuk 1980). The UV/optical photons emitted by the disc are thought to interact with a compact region of high energy plasma and to, consequently, be Compton upscattered to X-rays. In this case, the high-energy cutoff is associated with the finite value of the Comptonising plasma’s temperature.

However, the otherwise featureless nature of a power-law distribution complicates the effort to infer the physics of the X-ray source, since different combinations of the seed emission and the nature and geometry of the source result in similar power-law distributions. This has fuelled studies correlating the observed X-ray spectrum with other physical parameters of the system in an effort to improve our understanding of the X-ray source. For instance, the photon index, Γ, that is the power-law slope, has been shown to correlate with the Eddington ratio, λ_Edd, which is the ratio between the bolometric luminosity and the Eddington limit (Brightman et al. 2013;

Shemmer et al. 2006, see Fig. 1.6). Defined in this way,λ_Edd provides a measure of the accretion rate. Therefore, the X-ray spectrum seems to depend on the accretion rate, with sources of higher accretion state featuring softer X-ray spectra. This might indicate that the energy cooling of the X-ray source due to an increased UV/optical emission is higher than its heating due to the larger amount of energy available.

In any case, the exact shape of this correlation can be used to constrain accretion models. It should be mentioned, though, that an opposite correlation, meaning a decrease of Γ when λ_Edd increases, is found in studies of local AGN, although using significantly smaller samples (Kammoun et al.2020; Winter et al.2009b), suggesting that the sign of the correlation might evolve with redshift or luminosity.

The situation is even less clear when the dependence of Γ on the X-ray luminosity is studied, with several contradictory results being published in the literature. For example, a positive correlation was reported by Saez et al. (2008) when studying bright AGN from the Chandra deep field north and south surveys. On the contrary, the existence of an anticorrelation was concluded by Scott et al. (2011), when analysing the X-ray spectrum of a number of AGN in the XMM-Newton senendipi- tous source catalogue. Finally, Winter et al. (2009a) considered the X-ray spectra of

12In this study, the term “reflected emission" will be mainly used to refer to the emission related to the Compton hump, which is indeed the result of reflection (or scattering, to be precise). The iron line will be treated as independent of the reflection hump to allow for comparison between the two emissions.

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Figure 1.6: Correlation between the X-ray photon index and the Eddington ratio for a sample of AGN in the Extended Chandra Deep Field South and Cosmic Evolution Survey, calculated by Brightman et al. (2013). The top panel plots the measurements for the individual sources. The green and red squares denote the sources for which the Eddington ratio was estimated using the Mg II and Hα line, respectively. The Spearman rank correlation coefficient, rs, and the corresponding null hypothesis probability are also listed in the upper plot. The lower panel plots the binned average values using the data of the upper panel. The red and black dotted lines denote the correlation found by the previous studies of Risaliti et al.

(2009b) and Shemmer et al. (2008), respectively. Figure adapted from Brandt and Alexander (2015).

a rather unbiased sample of nearby AGN and found no evidence for the existence of any correlation between Γ and the X-ray luminosity. The apparent contradiction of results might either point to an evolution of any assumed correlation with redshift or luminosity, or suggest that the detected correlation is driven by an underlying parameter of the system.

Reflected emission

Assuming naturally an omnidirectional emission of X-rays, part of this emission is then expected to interact with the surrounding matter. There are two main ways that X-rays interact with relatively cold matter. The high energy photon is either absorbed, which is followed by fluoresence or Auger emission¹³, or is Compton scattered by free or bound electrons, which would alter its energy. Consequently, the

13When a photon is absorbed, an electron from an inner atomic shell receives its energy and leaves the atom. The produced vacancy is then filled with an electron from a higher energy state. This transition of the electron to a state of lower energy may either result in the emission of a photon, in which case we observe a fluoresent line with an energy equal to the difference between the two atomic transitions; or in the ejection of another outer electron from the atom, which received the energy surplus. The latter case corresponds to the Auger effect (Meitner1922).

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0.1 1

F E (arbitrary units)

i=10°

i=30° i=60°

i=80°

10

E (keV)

0.1 1 10

ξ=10ξ=10² ξ=10³ ξ=10⁴

Figure 1.7: Variation of the reflected emission, from 3 to 78 keV, as the system’s inclination (upper) or the reflector’s ionisation level (lower panel) changes. The different lines correspond to different values of ionisation or inclination, as indicated in the figure’s inset. An ionisation level ofξ = 10 was assumed in producing the upper plot, whereas an inclination i = 10^◦ was adapted for the lower plot. The spectra were estimated using the relxill model and a power-law shape was assumed for the primary X-ray continuum (see texts for details).

interaction of X-rays with the nearby matter will be imprinted in the X-ray spectrum of AGN. The resulting spectral component is usally called the reflected emission and its investigation has been the focus of many studies.

The competition between two effects, the increase of photo-absorption cross section in softer X-rays and the photon energy loss per Compton scattering in high energies, in combination with the spectral shape of the scattered continuum, results in a characteristic “hump" shape for the reflected spectrum (Lightman and White 1988, and references therein; see also George and Fabian1991, and Fig. 1.7above).

It should be mentioned that the production of a hump requires reflection from a Compton thick medium, that is a medium with column densityN_H&1.5·10²⁴cm⁻². The actual shape of the so-called Compton hump depends, among others, on the geometry, the chemical composition and the ionisation level of the reflecting surface.

On average, this spectral feature becomes important above 10 keV and peaks at around 20-30 keV, although it might dominate the whole X-ray spectrum of AGN in some cases (e.g. Baloković et al. 2014).

Figure1.7 shows the dependence of the reflected emission on the reflector’s ionisation state and on the system’s inclination, assuming a lamp post geometry for the X-ray source (see Fig. 1.13) and reflection from the accretion disc. The plotted spectra have been produced using the relxill model (García et al. 2014). The

NuSTAR view of Active Galactic Nuclei: Probing the reflected emission

Thesis

Reference

NuSTAR view of Active Galactic Nuclei: Probing the reflected emission

NuSTAR view of Active Galactic Nuclei: Probing the reflected emission

THÈSE

Christos Panagiotou

Preveza (Grèce)

DE GENEVE

DOCTORAT ES SCIENCES, MENTION ASTRONOMIE ET ASTROPHYSIQUE

Thdse de Monsieur Ghristos PANAGIOTOU

<<NUSTAR View of Active Galactic Nuclei: Probing the Reflected Emission>>

thdse

Madame E. KARA,

of

of

S.

ordinaire

et

Thdse

-

Résumé

Abstract

Acknowledgments

List of my publications

Contents

Introduction

1.1 Observational characteristics

1.2 AGN Taxonomy

1.3 Black Hole paradigm

1.4 AGN in X-rays