Centers Of Active Galactic Nuclei Research Articles

THE DEEP LOOK AT THE HARD X-RAY SKY: THESWIFT–INTEGRALX-RAY (SIX) SURVEY

The super-massive black-holes in the centers of Active Galactic Nuclei (AGNs) are surrounded by obscuring matter that can block the nuclear radiation. Depending on the amount of blocked radiation, the flux from the AGN can be too faint to be detected by currently flying hard X-ray (above 15 keV) missions. At these energies only ~1% of the intensity of the Cosmic X-ray Background (CXB) can be resolved into point-like sources that are AGNs. In this work we address the question of the undetected sources contributing to the CXB with a very sensitive and new hard X-ray survey: the SIX survey that is obtained with the new approach of combining the Swift/BAT and INTEGRAL/IBIS X-ray observations. We merge the observations of both missions. This enhances the exposure time and reduces systematic uncertainties. As a result we obtain a new survey over a wide sky area of 6200 deg^2 that is more sensitive than the surveys of Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs (Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic sources, 3 previously detected unidentified X-ray sources, and 14 unidentified sources. The scientific outcome from the study of the sample has been properly addressed to study the evolution of AGNs at redshift below 0.4. We do not find any evolution using the 1/V_max method. Our sample of faint sources are suitable targets for the new generation hard X-ray telescopes with focusing techniques.

Influence of Black Hole Spin on the Shape of the Fe Kα Spectral Line: The Case of 3C 405

AbstractHere we analyze how the angular momenta (spins) of black holes in the centers of Active Galactic Nuclei (AGN) affect the shape of the FeK line emitted from their accretion disks. For that purpose, we compared the observed line profile in the case of radio galaxy 3C 405 (Cygnus A) with its profiles, obtained by numerical simulations based on ray-tracing method in the Kerr metric. Our results show that the spins of rotating central black holes of AGN have significant influence on their FeKα line shapes. Also, we found that in the case of Cygnus A the observed line is probably emitted from the innermost region of its slightly inclined accretion disk around very slowly rotating or even stationary central black hole.

The effect of stellar feedback on the formation and evolution of gas and dust tori in AGN

Recently, the existence of geometrically thick dust structures in active galactic nuclei (AGN) has been directly proven with the help of interferometric methods in the mid-infrared. The observations are consistent with a two-component model made up of a geometrically thin and warm central disc, surrounded by a colder, fluffy torus component. Within the framework of an exploratory study, we investigate one possible physical mechanism, which could produce such a structure, namely the effect of stellar feedback from a young nuclear star cluster on the interstellar medium in centres of AGN. The model is realized by numerical simulations with the help of the hydrodynamics code tramp. We follow the evolution of the interstellar medium by taking discrete mass-loss and energy ejection due to stellar processes, as well as optically thin radiative cooling into account. In a post-processing step, we calculate observable quantities like spectral energy distributions (SEDs) and surface brightness distributions with the help of the radiative transfer code mc3d. The interplay between injection of mass, supernova explosions and radiative cooling leads to a two-component structure made up of a cold geometrically thin, but optically thick and very turbulent disc residing in the vicinity of the angular momentum barrier, surrounded by a filamentary structure. The latter consists of cold long radial filaments flowing towards the disc and a hot tenuous medium in between, which shows both inwards and outwards directed motions. With the help of this modelling, we are able to reproduce the range of observed neutral hydrogen column densities of a sample of Seyfert galaxies as well as the relation between them and the strength of the silicate 10 μm spectral feature. Despite being quite crude, our mean Seyfert galaxy model is even able to describe the SEDs of two intermediate type Seyfert galaxies observed with the Spitzer Space Telescope.

Large-scale dynamo of accretion disks around supermassive nonrotating black holes

In this paper one presents an analytical model of accretion disk magnetosphere dynamics around supermassive nonrotating black holes in the centers of active galactic nuclei. Based on general relativistic equations of magneto hydrodynamics, the nonstationary solutions for time-dependent dynamo action in the accretion disks, spatial and temporal distribution of magnetic field are found. It is shown that there are two distinct stages of dynamo process: the transient and the steady-state regimes, the induction of magnetic field at t > 6:6665 x 1011GM/c3 s becomes stationary, magnetic field is located near the innermost stable circular orbit, and its value rises up to ~ 105 G. Applications of such systems with nonrotating black holes in real active galactic nuclei are discussed.

SGRS J0515−8100: A Fat‐Double Giant Radio Galaxy

We present here the first detailed study of a giant radio galaxy of the fat-double type. The lobes of the double radio galaxy SGRS J0515-8100 have transverse widths that are 1.3 times their extent from the center, their surface brightness is the lowest among known giant radio sources, and the lobes have relatively steep radio spectra. We infer that these wide lobes were created as a result of a highly variable and intermittent jet whose axis direction also varied significantly: the fat-double lobes in this giant radio source are a result of the ejection and deposition of synchrotron plasma over a wide range of angles over time rather than the expansion of relic lobes. In addition, the optical host shows evidence for an ongoing galaxy-galaxy interaction. SGRS J0515-8100 supports the hypothesis that interactions with companions might perturb the inner accretion disk that produces and sustains the jets at the centers of active galactic nuclei. As a result, it appears unnecessary to invoke black hole coalescence to explain such morphologies, implying that the corresponding event rates predicted for gravitational wave detectors may be overestimates.

Towards a Unified Description of the Jet-Accretion Coupling in Stellar and Super Massive Black Holes

Radio and X-ray flux from accreting stellar-mass black holes in the low/hard state display a tight non-linear correlation from the `quiescent' regime up to the transition to the high/soft state. The non-linearity of the correlation naturally leads to the possibility of a `jet dominated' power output from these systems. In such a state most of the accretion power is dissipated in the form of bulk kinetic energy of the outflow, while a standard disc-corona system may still be responsible for the radiated luminosity. With the introduction of a mass correction, the same radio/X-ray scaling has been shown to apply in super massive black holes at the centre of active galactic nuclei. Expanding on this, we have found that the same sample of AGN exhibits a `quenching' of the radio emission at about 10 per cent of the Eddington rate, just as seen in high/soft state black hole binaries. This result indicates that the same spectral phenomenology, and therefore probably jet-accretion coupling, is shown by stellar and super massive black holes and strengthens the notion of `quiescence' as a very low luminosity version of the standard low/hard state.

High Resolution Observations of Sub-Kiloparsec Circumnuclear Gas in AGN

In recent years it has become possible to use high resolution radio interferometry and other techniques to study the sub-kiloparsec scale circumnuclear gas in the centres of Active Galactic Nuclei. This gas presumably provides the fuel source for the AGN activity and also plays a role in obscuring the direct view of the central engine from certain directions. The status of high resolution observations of the ionised, atomic and molecular phases of this circumnuclear gas are briefly reviewed.

Megamaser Disks in Active Galactic Nuclei

Recent spectroscopic and VLBI-imaging observations of bright extragalactic water maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity coherence length along the line of sight could account (through self-amplification) for the entire maser flux of a high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-amplification likely contributes also to the flux of the background-amplifying ``systemic'' features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2-like galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission-line region-type) clouds.

2.9. Microlens mapping of disks in active galactic nuclei

Generally, it is believed that there is a supermassive black hole and a surrounding accretion disk in a central region of active galactic nuclei (AGN). However, it is quite difficult to obtain direct information about the center of AGN, because the accretion disk size is far too small to resolve.

Dynamical and Microphysical Constraints on Accreting Magnetospheric Clouds in Active Galactic Nuclei

AbstractThe range of microphysical and dynamical timescales at the centres of Active Galactic Nuclei (AGN) is sufficiently wide to permit multiphase structure. In particular, very dense, cool clouds can coexist with a hot, magnetically-dominated plasma. The strong dynamical forces in this central magnetosphere can give rise to clouds so small that microphysical processes determine whether they can survive for long enough to produce potentially observable spectral signatures due to thermal reprocessing. Specific physical constraints on the scale sizes of such reprocessing clouds are examined. It is found that there exists a parameter subspace in the extreme high density regime for which gas can sustain cool temperatures whilst maintaining pressure equilibrium with the ambient magnetosphere.

Black hole accretion disks with coronae

Observations suggest the existence of both hot and cold dark matter in the centers of active galactic nuclei. Recent spectral models require a major fraction of power to be dissipated in the hot matter. We study the case when the hot matter forms a corona around a standard cold alpha-disk. In particular, we investigate the case when a major fraction, f, of the power released when the cold matter accretes is transported to and dissipated in the corona. This has major effects on the cold disk, making it colder, more geometrically thin, denser, and having larger optical depths. One important consequence is the disappearance of the effectively optically thin zone as well as of the radiation pressure dominated zone for values of f sufficiently closed to unity. The disappearance of the radiation pressure dominated zone will result in a cold disk with only a gas pressure dominated zone that is stable against thermal and viscous instabilities. We also show that the pressure ( and the radiation) from the corona will only affect the surface layers of the cold disk. Our results disagree with those of other recent work on accretion disks with coronae. We find those works to be based on unphysical assumptions.

Non-LTE effects on the strength of the Lyman edge in quasar accretion disks

We have calculated UV/EUV (300 A which is less than or equal to lambda which is less than or equal to 1500 A) continuous energy distributions of accretion disks in the centers of active galactic nuclei (AGNs) for disk luminosities in the range 0.1 L(sub Edd) less than or equal to L(sub acc) less than 1.0 L(sub Edd) and central masses ranging from 10(exp 8) solar mass to 10(exp 9) solar mass. The vertical gas pressure structure of the disk and the disk height are obtained analytically; the temperature stratification and the resulting continuum radiation fields are calculated numerically. We have included non-Local Thermodynamic Equilibrium (LTE) effects of both the ionization equilibrium and the level populations of hydrogen and helium. We show that these non-LTE effects reduce the strength of the Lyman edge when comapred to the LTE case. In non-LTE we find that the edge can be weakly in emission or absorption for disks seen face-on, depending on the disk parameters.

Electromagnetic fields inside thin accretion discs around rotating black holes

The equations that describe the electromagnetic field inside a thin resistive accretion disc around a rotating black hole are obtained by using the general relativistic form of Maxwell's equations and Ohm's law in a stationary and axisymmetric geometry. The equations obtained in this paper have the same form as those that have been obtained in the non-relativistic regime. These equations are so simple that we can find some analytic solutions easily under suitable boundary conditions. These solutions may be useful in the discussion of the magnetospheres of rotating black holes, which are assumed to inhabit the centres of active galactic nuclei

Comptonization of the Lyman edge in active galactic nuclei

Recent X-ray observations indicate the presence of both relativistic particles and cold gas very close to the centre of active galactic nuclei. However, the UV radiation from this cold gas does not exhibit the Lyman edge predicted by many accretion disc models. It is shown that Comptonization of the intrinsic spectrum by gas in the form of a hot corona at the intermediate temperatures (T ∼ 10 8 K) can efficiently smear out the Lyman edge without destroying the observed iron K-fluorescent line

Sweeping-Magnetic-Twist Mechanism and Molecular Bipolar Flows

Uchida and Shibata have proposed the “sweeping-magnetic-twist” mechanism for the formation of astrophysical jets in relation to the accretion disks (disks around protostars, around black hole in the center of active galactic nuclei, and so on) in which a jet is accelerated by thejxBforce in the relaxing magnetic twist created in the winding-up of the poloidal magnetic field by the rotation of the contracting disk (Uchida and Shibata 1985a, b; Shibata and Uchida 1986a, b; Uchidaet al.1985). In this mechanism, a jet is collimated also by thejxBforce due to the large scale poloidal magnetic field whose footpoints are squeezed in the contracting disk. The main mechanism involved is different from that of centrifugal wind models (Blandford and Payne 1982, Pudritz and Norman 1983) and worked out indepentently, but the centrifugal effect itself is automatically built-in.

Primeval QSOs

view Abstract Citations (27) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Primeval QSOs Cavaliere, A. ; Szalay, A. S. Abstract The physical processes causing the formation of an increased number of QSOs at redshifts higher than 2 are discussed, along with the implications for the turn-on of active galactic nuclei (AGN) at the same time as QSOs. It is argued that optical and X-ray background data indicate that all but the brightest objects will decline in number at redshifts exceeding 2. Consideration is given to the possibility that accreting black holes are at the center of AGNs and formed within a few billion years of the turn-on of QSOs. A scenario is also explored wherein QSOs are remnant echoes of the epoch of galaxy formation and are thereby a suitable object with which to probe the parent population. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164798 Bibcode: 1986ApJ...311..589C Keywords: Astronomical Models; Galactic Evolution; Galactic Nuclei; Quasars; Active Galactic Nuclei; Density Distribution; Luminosity; Universe; Astrophysics; GALAXIES: FORMATION; GALAXIES: NUCLEI; QUASARS full text sources ADS |

Gamma-Rays from Active Galactic Nuclei

Gamma-ray observations from active galactic nuclei (AGN) are important in trying to understand the nature of their central sources. A handful of mechanisms can give rise to γ-rays either from nonthermal or from thermal processes. Hot accretion disks around massive black holes in the centers of AGN could provide the required thermal electrons, pions and relativistic electrons and positrons to explain both the X-ray and γ-ray emission.