X-ray emission from AGN is believed to be produced via Comptonization of optical/UV seed photons emitted by the accretion disk, up-scattered by hot electrons in a corona surrounding the black hole. A critical compactness vs. temperature threshold is predicted above which any increase in the source luminosity would generate positron-electron pairs rather than continue heating the coronal...
Outflows from active galactic nuclei are found over a broad range of distances from the supermassive black hole and with a large range of velocities. The so-called "warm absorbers" cover a broad range of ionization parameters but at modest velocities of a few hundred to thousand km/s and relatively modest column densities of typically less than 1% of the Thomson depth. On the other extreme,...
The observed overlap between soft X-ray emission and the NLR in obscured AGN is commonly interpreted as evidence for a constant gas pressure multiphase medium. Radiation pressure compression (RPC) also leads to a density distribution, since a gas pressure (hence density) gradient must arise within each cloud to counteract the ionizing radiation pressure. RPC leads to a well-defined ionization...
Water megamaser emission is powerful in tracing the inner region of active nuclei, mapping accretion disks and providing important clues on their absorption properties. From the broad band X-ray spectra of AGN it is possible to estimate the intrinsic power of the central engine and the obscuring column density. The synergy between X-ray and water maser studies allows to tackle the AGN inner...
In the past decade, spectral-timing measurements of accreting black holes have revealed reverberation light travel echoes between the X-ray emitting corona, and the accretion disc. Together, XMM-Newton, NuSTAR, Swift and NICER have measured light echoes from size scales ranging from the broad line region down to the ISCO. In this talk, I will give an overview of the progress over the past...
Recent observing campaigns revealed the occurrence of “obscured states” in a few Seyfert galaxies, produced by streams of outflowing, lowly ionized gas, partially eclipsing the X-ray source. These events appear to be transient, temporarily modifying the X-ray properties of the source. The increasing number of detections suggests this might be a common phenomenon. It is therefore important to...
Several AGN have shown UV/optical variability lagging behind the X-ray emission by a few days. The simplest and most straightforward interpretation is that the variable X-ray flux from the corona illuminates the accretion disc below where it is partially reflected and observed as fast X-ray reverberation signal, and partially absorbed and thermalised in the disc, which produces a slow UV and...
We present current X-ray observations and simulations showing how gravitational lensing is used to infer the structure near the event horizons of black holes, constrain the spin of the supermassive black hole and its evolution over cosmic time and test general relativity in the strong-gravity regime.
We also show how these observations can be expanded to a statistically large sample of z =...
AGN radiative feedback, driven by radiation pressure on dust, is a key physical mechanism connecting the accreting black hole to its surrounding environment. The actual importance of radiative feedback can be observationally tested by analysing how X-ray-selected AGN samples populate the so-called "NH-lambda plane", defined by the column density versus the Eddington ratio. A "forbidden" region...
We report results from a series of X-ray observations of a local galactic core whose long-term properties over the course of the past ten years are consistent with a long-lived tidal disruption event. A new spectacular phenomenon is occurring in this system 2018 December onwards, when XMM-Newton has discovered massive, repeating X-ray flares. During these "quasi-periodic eruptions" (or QPEs)...
Accretion in active galactic nuclei (AGN) is accompanied by two modes of outflow: winds and jets. However, the connection between the winds, jets, and the accretion flows is not fully understood. I present the results of a recently-published paper (Mehdipour & Costantini 2019), where we have investigated the relation between the parameters of the ionised wind and the jet in a sample of...
The growing evidence for energy-conserving outflows in powerful and luminous AGN supports the idea that high-velocity X-ray winds launched from the accretion disc evolve after undergoing a shock with the ambient medium, with the ultimate effect to expel enough mass and energy so as to produce the so-called AGN feedback, often invoked in galaxy formation and evolution. This talk will present...
Almost twenty years of X-ray grating observations allowed us to characterize the multi-component ionized gas, outflowing from active galactic nuclei, with unprecedented accuracy. XMM-Newton combined with HST have been key in unveiling the secrets of outflowing winds.
Here I will show the results of our most recent multi-wavelength campaign on the narrow line Seyfert 1 IZw1 (230 ks with XMM...
I will review our understanding of the AGN as detected by the current suite of X-ray observatories. I will discuss the complementary parameter space from the broad swathe of extragalactic X-ray surveys undertaken to date. I will describe the types of AGN detected in these surveys, their overall properties and cosmic evolution. A key objective of this talk is to provide the context for many of...
From observations of active galactic nuclei (AGN) we may improve our understanding of the growth of black holes across cosmic time and their impact on their host galaxies, but observations alone may be limited by complex biases. Here I will present the results from modeling the whole AGN population while accounting for observational biases.
I will begin by showing that the Eddington ratio...
It has been claimed for decades that almost all galaxies in the local Universe host at their centre a super-massive black hole (SMBH) the mass of which appears to be tightly correlated with the stellar mass and the random motion ("velocity dispersion", sigma) of the stars of the host galaxy. In this talk I will first highlight that significant biases affect local black hole-galaxy...
The cosmological concordance model (ΛCDM) well accounts for a wealth of observations, from the existence of Cosmic Microwave background (CMB) to the discovery of the accelerated expansion of the universe from Type Ia supernovae. Yet, it assumes a still unknown form of dark energy and matter and some tensions arose recently as, for instance, the discovery of a >3σ discrepancy between the local...
The formation channels of Supermassive Black holes (SMBHs) seeds are currently debated, as are their accretion limits, growth and co-evolution with galaxies. Independent of these questions, we investigate the occurance of SMBH seeds with a simple generic framework. We consider that when halos outgrow a mass threshold, a fraction of them have formed black holes. Following the well-understood...
We are performing a systematic study of the the X-ray properties of the most luminous (L$_{bol}>10^{47}$ erg/s) quasars in the Universe spanning from optical- NIR- MIR-selected sources at z=2-3. These AGN exhibit widespread outflow signatures at all scales and they are the sources where we expect quasar feedback to manifests in full force. Hence they are in a transit phase, predicted in quasar...
Some AGN are known to be efficient producers of strong, relativistic jets which power the extended radio sources. Most spectacular in respect of powers and sizes are the radio sources associated with AGN hosted by giant elliptical galaxies. However even among them, the production of powerful jets is a very rare phenomenon and the unanswered question remains why it is so. Since relativistic...
Despite the importance of dual active galactic nuclei to wide-ranging astrophysical fields such as galaxy formation and gravitational waves, the rate of dual AGNs has yet to be accurately measured. However, the rate of dual AGNs can inform us of the role galaxy mergers play in triggering AGN, timescales for post-merger SMBHs to sink to the center of the potential well (or, the effectiveness of...
X-ray emission is an ubiquitous property of Active Galactic Nuclei and, being produced within a few gravitational radii from the supermassive black hole, can provide fundamental information about the structure and geometry of the circumnuclear material, as well as on the characteristics of the accretion flow. In my talk I will present recent studies of the obscuration and accretion properties...
An accurate assessment of the fraction of heavily obscured, "Compton-thick" AGN in the local Universe provides important insights into the composition and structure of AGN X-ray obscuration, as well as its connection with the evolution of supermassive black holes and their surrounding host galaxies. However, current estimates of the Compton-thick fraction vary dramatically between ~20-70%,...
The AGN effect on host galaxies is an interesting topic that has been often discussed so far. The AGN is usually X-ray luminous, and thus X-ray irradiation by the AGN is unavoidable for its host galaxy. We report our recent study on X-ray-irradiated gas in the central ~100 pc of the Circinus galaxy (TK+19), a Compton-thick AGN host, at 10-pc resolution using Chandra and ALMA. Based on ~200...
The ability of the X-rays to penetrate and measure large columns of obscuring material is crucial to characterise the emission from the Active Galactic Nuclei (AGN), especially those with heavy obscuration and/or weak emission.
In this work we exploit the largest energy band available in the X-rays ($\sim$0.5-30 keV) to accurately assess the intrinsic power and obscuration for a sample of 30...
Through the years, AGN accretion theory has lagged behind with respect to the plethora of observational signatures driven by accretion onto supermassive black holes. For instance, the smoking gun of the disk-corona interplay in radiatively-efficient AGN is given by the correlation observed between monochromatic $\log L_{X}-\log L_{UV}$. Despite being used for decades (since the introduction of...
Studies of recent decades have led to the conclusion that there is an inextricable link between galaxies and the Supermassive Black Holes (SMBHs) located in their centre. However, it is still unclear how the activity of the SMBH affects the properties of the host galaxy. Furthermore, there is a scientific debate whether the absorption we observe in some AGN is a geometric effect or an...
Scaling relations are the most powerful astrophysical tools to set constraints to the physical mechanisms of astronomical sources and to infer properties for objects where they cannot be accessed directly. We have re-investigated one of these scaling relations using powerful type 1 Seyferts; the so-called X-ray variability plane (or mass-luminosity-timescale relation, McHardy et al. 2006)....
Determining the processes responsible for the initial formation and subsequent growth of the very first supermassive black holes, primarily taking place at very high redshifts (z>6), will be a major challenge for the next decade of X-ray astronomy and beyond. I will review what is currently known about black hole growth during this early period of cosmic time (from both an X-ray and...
Chandra and XMM-Newton showed that AGN existed in the early Universe (at z~7.5) and this challenges our understanding of SMBH formation. Chandra detected serendipitously only an handful of z>4-5 sources and the majority of the constrains on the nature of early Black holes come from Cosmic background fluctuations studies or stacking/follow up of Infrared selected sources. I will present recent...
More than 200 quasars (QSOs) with spectroscopic redshift z > 6 have been discovered so far.
Multi-wavelength observations showed that these objects are evolved systems with large black hole masses ($10^8−10^{10} M_{\odot}$), and that their broad-band spectral energy distributions (SEDs) and rest-frame UV spectra have not significantly evolved over cosmic time.
The formation of their Super...
