Engines Of Active Galactic Nuclei Research Articles

Evidence of jet-caused 12-year optical periodicity of blazar OJ 287

Blazar OJ 287 is one of the first candidates with the assumed compact system of two supermassive black holes in the center. Orbital interaction in this system has been used to explain the for over a century light curve, in which optical flares are repeated with a quasi-period of 12 years and have almost yearly duration. The absence of the predicted flare in 2022 casts doubt on the dominant model of a close system of binary black holes. The detection of optical flares outside the 12-year period and their interpretation by processes in the jet complicates the construction of a complete picture of the blazar OJ 287 optical variability. Here, we analyze the 50-year evolution of the optical spectrum and prove that the changing coefficient of relativistic boosting of two regions in a helical jet subject to an age-long orientation change forms all flares. Our findings indicate the absence of the compact binary black hole system and the impossibility of drawing reliable conclusions about the central engine of active galactic nuclei based only on the quasi-periodic brightness variability of blazars.

From the Editors

Welcome to the third volume of JoAE. Similar to the previous two editorials, here we offer an update regarding the journal, as well as ponder one of the stupefying dilemmas we have encountered: the “So What?” question.Last year we found ourselves in some manuscript submission conundrums. The primary issue involved the processing of manuscripts. As of this writing (September 2021), we have 90+ submissions for this year, and more than 200 manuscripts in some form of revision or review from the previous two years. Using Google’s G Suite for manuscript submission and correspondence was not sustainable. We needed to evolve.We migrated to the web-based journal management system Scholastica. We made some beginners’ mistakes, but the system makes organizing submissions quicker and easier. Moreover, we felt we were overwhelming our editorial board and ad hoc reviewers. After consultation with experienced journal editors, we began issuing more “desk rejects.” If manuscripts do not follow submission guidelines1 and/or disregards our previous editors’ introductions2 or the JoAE blog,3 we no longer send them out for peer review. We appreciate our reviewers and want to protect their time and effort; they offer their passion and unpaid labor to make this journal go.Another issue was the overwhelming number of COVID-19 submissions. We made the decision to reject a majority of these. Why? Most were chronologies: “This happened, and this happened, and this happened.” They did not satisfy the showing-telling characteristic of solid autoethnographic writing. Moreover, while providing personal experience, a majority of manuscripts did not answer the all-important “So What?” question: Why does (or should) your experience matter to others? Why should readers care about your issues and experiences? For COVID-19 manuscripts, we believe the So What? question was (is) difficult to answer because we are all still living with the pandemic.4The “So What?” dilemma was not just a challenge for COVID-19 submissions. Many submissions we desk rejected also did not answer the “So What?” question. In some manuscripts it was not clear if the author even considered the “So What?” question. Therefore, in this editorial, we thought it would be meaningful to consider the “So What?” question not only for JoAE but for autoethnographic research in general.5We reflected on why some autoethnographers appear to have difficulty answering the “So What?” question. We suggest that to answer the question, autoethnographers need to seriously consider four “How” questions:This list is not all-encompassing, nor do all the “how” questions need to be answered to fully answer the “So What?” question.Answering the “So What?” question is clearer when pursuing research in the sciences. “This analysis is the first in the United States to assess COVID-19 vaccination coverage among pregnant women.”8 This statement (and article) has obvious important health care implications. The Event Horizon Telescope Collaboration answered the “So What?” question when they showed “Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei.”9 They are moving physics forward. Even in traditional quantitative and qualitative social science research, the “So What?” question is more easily answered. “Group identification and stream familiarity were found to be positively related to perceived credibility. These findings hold implications for using video game streams as a marketing tool…”10 But, for autoethnography, answering the question requires much more skill, deliberation, and nuance.We want to publish articles that use autoethnography to add to understandings of the complexities of personal and social experience. Or manuscripts that add to or complicate our understandings of the philosophical underpinnings of autoethnography. Or manuscripts that examine new subjects through an autoethnographic lens. The “So What?” question is critical, and authors must show and tell why their research is important and necessary, as well as why we, readers, should care about their work. Insightful and nuanced answers and analyses will help steer manuscripts away from a dreary desk reject

First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

Abstract When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42 ± 3 μas, which is circular and encompasses a central depression in brightness with a flux ratio ≳10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 ± 0.7) × 109 M ⊙. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

RADIATION-DRIVEN OUTFLOWS FROM AND RADIATIVE SUPPORT IN DUSTY TORI OF ACTIVE GALACTIC NUCLEI

ABSTRACT Substantial evidence points to dusty, geometrically thick tori obscuring the central engines of active galactic nuclei (AGNs), but so far no mechanism satisfactorily explains why cool dust in the torus remains in a puffy geometry. Near-Eddington infrared (IR) and ultraviolet (UV) luminosities coupled with high dust opacities at these frequencies suggest that radiation pressure on dust can play a significant role in shaping the torus. To explore the possible effects of radiation pressure, we perform three-dimensional radiative hydrodynamics simulations of an initially smooth torus. Our code solves the hydrodynamics equations, the time-dependent multi–angle group IR radiative transfer (RT) equation, and the time-independent UV RT equation. We find a highly dynamic situation. IR radiation is anisotropic, leaving primarily through the central hole. The torus inner surface exhibits a break in axisymmetry under the influence of radiation and differential rotation; clumping follows. In addition, UV radiation pressure on dust launches a strong wind along the inner surface; when scaled to realistic AGN parameters, this outflow travels at and carries M ⊙ yr−1, where M, , and are the mass, UV luminosity, and Eddington luminosity of the central object respectively.

General relativistic magnetohydrodynamical simulations of the jet in M 87

(abridged) The connection between black hole, accretion disk, and radio jet can be best constrained by fitting models to observations of nearby low luminosity galactic nuclei, in particular the well studied sources Sgr~A* and M87. There has been considerable progress in modeling the central engine of active galactic nuclei by an accreting supermassive black hole coupled to a relativistic plasma jet. However, can a single model be applied to a range of black hole masses and accretion rates? Here we want to compare the latest three-dimensional numerical model, originally developed for Sgr A* in the center of the Milky Way, to radio observations of the much more powerful and more massive black hole in M87. We postprocess three-dimensional GRMHD models of a jet-producing radiatively inefficient accretion flow around a spinning black hole using relativistic radiative transfer and ray-tracing to produce model spectra and images. As a key new ingredient to these models, we allow the proton-electron coupling in these simulations depend on the magnetic properties of the plasma. We find that the radio emission in M87 is well described by a combination of a two-temperature accretion flow and a hot single-temperature jet. The model fits the basic observed characteristics of the M87 radio core. The best fit model has a mass-accretion rate of Mdot approx 9x10^{-3} MSUN/YR and a total jet power of P_j \sim 10^{43} erg/s. Emission at 1.3mm is produced by the counter jet close to the event horizon. Its characteristic crescent shape surrounding the black hole shadow could be resolved by future millimeter-wave VLBI experiments. The model was successfully derived from one for the supermassive black hole in center of the Milky Way by appropriately scaling mass and accretion rate. This suggests the possibility that this model could also apply to a larger range of low-luminosity black holes.

Astrophysical ZeV acceleration in the relativistic jet from an accreting supermassive blackhole

An accreting supermassive blackhole, the central engine of active galactic nucleus (AGN), is capable of exciting extreme amplitude Alfven waves whose wavelength (wave packet) size is characterized by its clumpiness. The pondermotive force and wakefield are driven by these Alfven waves propagating in the AGN (blazar) jet, and accelerate protons/nuclei to extreme energies beyond Zetta-electron volt (ZeV=1021eV). Such acceleration is prompt, localized, and does not suffer from the multiple scattering/bending enveloped in the Fermi acceleration that causes excessive synchrotron radiation loss beyond 1019eV. The production rate of ZeV cosmic rays is found to be consistent with the observed gamma-ray luminosity function of blazars and their time variabilities.

Estimations of the magnetic field strength in the torus of IC 5063 using near-infrared polarimetry

An optically and geometrically thick torus obscures the central engine of Active Galactic Nuclei (AGN) from some lines of sight. From a magnetohydrodynamical framework, the torus can be considered to be a particular region of clouds surrounding the central engine where the clouds are dusty and optically thick. In this framework, the magnetic field plays an important role in the creation, morphology and evolution of the torus. If the dust grains within the clouds are assumed to be aligned by paramagnetic alignment, then the ratio of the intrinsic polarisation and visual extinction, P(%)/Av, is a function of the magnetic field strength. To estimate the visual extinction through the torus and constrain the polarisation mechanisms in the nucleus of AGN, we developed a polarisation model to fit both the total and polarised flux in a 1.2" (~263pc) aperture of the type 2 AGN, IC5063. The polarisation model is consistent with the nuclear polarisation observed at K being produced by dichroic absorption from aligned dust grains with a visual extinction through the torus of 48$\pm$2 mag. We estimated the intrinsic polarisation arising from dichroic absorption to be P$_{K}^{dic}$=12.5$\pm$2.7%. We consider the physical conditions and environment of the gas and dust for the torus of IC5063. Then, through paramagnetic alignment, we estimate a magnetic field strength in the range of 12-128mG in the NIR emitting regions of the torus of IC5063. Alternatively, we estimate the magnetic field strength in the plane of the sky using the Chandrasekhar-Fermi method. The minimum magnetic field strength in the plane of the sky is estimated to be 13 and 41 mG depending of the conditions within the torus of IC5063. These techniques afford the chance to make a survey of AGN, to investigate the effects of magnetic field strength on the torus, accretion, and interaction to the host galaxy.

Constraining clumpy dusty torus models using optimized filter sets

Recent success in explaining several properties of the dusty torus around the central engine of active galactic nuclei has been gathered with the assumption of clumpiness. The properties of such clumpy dusty tori can be inferred by analyzing spectral energy distributions (SEDs), sometimes with scarce sampling given that large aperture telescopes and long integration times are needed to get good spatial resolution and signal. We aim at using the information already present in the data and the assumption of clumpy dusty torus, in particular, the CLUMPY models of Nenkova et al., to evaluate the optimum next observation such that we maximize the constraining power of the new observed photometric point. To this end, we use the existing and barely applied idea of Bayesian adaptive exploration, a mixture of Bayesian inference, prediction and decision theories. The result is that the new photometric filter to use is the one that maximizes the expected utility, which we approximate with the entropy of the predictive distribution. In other words, we have to sample where there is larger variability in the SEDs compatible with the data with what we know of the model parameters. We show that Bayesian adaptive exploration can be used to suggest new observations, and ultimately optimal filter sets, to better constrain the parameters of the clumpy dusty torus models. In general, we find that the region between 10 and 200 um produces the largest increase in the expected utility, although sub-mm data from ALMA also prove to be useful. It is important to note that here we are not considering the angular resolution of the data, which is key when constraining torus parameters. Therefore, the expected utilities derived from this methodology must be weighted with the spatial resolution of the data.

GRAVITATIONAL SHOCKS AS A KEY INGREDIENT OF GAMMA-RAY BURSTS

We identify a novel physical mechanism that may be responsible for energy release in γ-ray bursts. Radial perturbations in the neutron core, induced by its collision with collapsing outer layers during the early stages of supernova explosions, can trigger a gravitational shock, which can readily eject a small but significant fraction of the collapsing material at ultra-relativistic speeds. The development of such shocks is a strong-field effect arising in near-critical collapse in general relativity and has been observed in numerical simulations in various contexts, including, in particular, radially perturbed neutron star collapse, albeit for a tiny range of initial conditions. Therefore, this effect can be easily missed in numerical simulations if the relevant parameter space is not exhaustively investigated. In the proposed picture, the observed rarity of γ-ray bursts would be explained if the relevant conditions for this mechanism appear in only about one in every 104–105 core collapse supernovae. We also mention the possibility that near-critical collapse could play a role in powering the central engines of active galactic nuclei.

Radiation pressure, absorption and AGN feedback in the Chandra Deep Fields

The presence of absorbing gas around the central engine of Active Galactic Nuclei (AGN) is a common feature of these objects. Recent work has looked at the effect of the dust component of the gas, and how it enhances radiation pressure such that dusty gas can have a lower effective Eddington limit than ionised gas. In this work, we use multi-wavelength data and X-ray spectra from the 2 Ms exposures of the Chandra Deep Field North and Chandra Deep Field South surveys, to characterise the AGN in terms of their Eddington ratio and hydrogen column density. Their distributions are then compared with what is predicted when considering the coupling between dust and gas. Our final sample consists of 234 objects from both fields, the largest and deepest sample of AGN for which this comparison has been made up to date. We find that most of the AGN in our sample tend to be found at low Eddington ratios (typically between 1e-4 and 1e-1) and high column density (>1e22 cm^-2), with black hole masses between ~1e8 and 1e9 solar masses. Their distribution is in agreement with that expected from the enhanced radiation pressure model, avoiding the area where we would predict the presence of outflows. We also investigate how the balance between AGN radiation pressure and gravitational potential influences the behaviour of clouds in the galactic bulge, and describe a scenario where an enhanced radiation pressure can lead to the fundamental plane of black hole/galaxy scaling relations.

BLACK HOLE AURORA POWERED BY A ROTATING BLACK HOLE

We present a model for high-energy emission sources generated by a standing magnetohydrodynamical (MHD) shock in a black hole magnetosphere. The black hole magnetosphere would be constructed around a black hole with an accretion disk, where a global magnetic field could be originated by currents in the accretion disk and its corona. Such a black hole magnetosphere may be considered as a model for the central engine of active galactic nuclei, some compact X-ray sources and gamma-ray bursts. The energy sources of the emission from the magnetosphere are the gravitational and electromagnetic energies of magnetized accreting matters and the rotational energy of a rotating black hole. When the MHD shock generates in MHD accretion flows onto the black hole, the plasma's kinetic energy and hole's rotational energy can convert to radiative energy. In this letter, we demonstrate the huge energy output at the shock front by showing negative energy postshock accreting MHD flows for a rapidly rotating black hole. This means that the extracted energy from the black hole can convert to the radiative energy at the MHD shock front. When axisymmetric shock front is formed, we expect a ring-shaped region with very hot plasma near the black hole; the look would be like an "aurora". The high energy radiation generated from there would carry to us the information for the curved spacetime due to the strong gravity.

The evolution of radio-loud active galactic nuclei as a function of black hole spin

Recent work on the engines of active galactic nuclei jets suggests that their power depends strongly and perhaps counter-intuitively on black hole spin. We explore the consequences of this on the radio-loud population of active galactic nuclei and find that the time evolution of the most powerful radio galaxies and radio-loud quasars fits into a picture in which black hole spin varies from retrograde to prograde with respect to the accreting material. Unlike the current view, according to which jet powers decrease in tandem with a global downsizing effect, we argue for a drop in jet power resulting directly from the paucity of retrograde accretion systems at lower redshift z caused by a continuous history of accretion dating back to higher z. In addition, the model provides simple interpretations for the basic spectral features differentiating radio-loud and radio-quiet objects, such as the presence or absence of disc reflection, broadened iron lines and signatures of disc winds. We also briefly describe our models' interpretation of microquasar state transitions. We highlight our result that the most radio-loud and most radio-quiet objects both harbour highly spinning black holes but in retrograde and prograde configurations, respectively.

Dynamics and Energetics of Turbulent, Magnetized Disk Accretion around Black Holes: A First‐Principles Approach to Disk‐Corona‐Outflow Coupling

We present an analytic description of turbulent, magnetohydrodynamic (MHD) disk accretion around black holes that specifically addresses the relationship between radial and vertical mean field transport of mass, momentum, and energy, thereby complementing and extending numerical simulations. The azimuthal-vertical component of the magnetic stress is fundamental to an understanding of disk-corona-outflow coupling: when it is important for driving the angular momentum transport and mass accretion in the disk, it also has an important influence on the disk-corona-outflow energy budget. The Poynting flux derived from the product of this term with the Keplerian velocity also dominates the Poynting flux into the corona. The ratio of the coronal Alfven velocity to the Keplerian velocity is an important parameter in disk-corona-outflow physics. If this parameter is greater than unity, then energetically significant winds and Poynting flux into the corona occur. However, significant effects could also occur when this parameter is much less than unity. A limiting solution describing the case of angular momentum transport solely by the vertical-azimuthal stress has the property that all of the accretion power is channeled into a wind, some of which would be dissipated in the corona. More realistic solutions in which there is both radial and vertical transport of angular momentum would have different fractions of the accretion power emitted by the disk and corona, respectively. These results have important implications for existing accretion disk theory and for our interpretation of high-energy emission and nuclear outflows from the central engines of active galactic nuclei and galactic black hole candidates.

Colour changes in quasar light curves

The study of quasar variability has long been seen as a way to understanding the structure of the central engine of active galactic nuclei, and as a means of verifying the morphology of the standard model. Much work has already been done on the statistical properties of light curves monitored in one colour, and it is now possible to use these observations to test predictions of theoretical models. The addition of a second colour can add enormously to the power of such tests, and put tight constraints on the nature of the variations. In this paper a yearly monitoring programme of several hundred quasars in blue and red passbands covering 21 years is presented. The statistics of colour changes are examined for a 15 year period of homogeneous data with Fourier power spectrum analysis, in a form suitable for testing against theoretical predictions. The results of the Fourier analysis show that there is more power in blue light on all timescales than in the red. Examination of the light curves shows several different modes of colour change. However, if allowance is made for the effects of the underlying host galaxy, the variations become close to achromatie. There are however structural differences between red and blue light curves which cannot be accounted for in this way, and various modes of variability including disc instability and microlensing are examined to provide explanations for these features.

Light element nucleosynthesis from jet-cloud interactions in active galactic nuclei

The production of light nuclei via the interactions of jets from the central engines of active galactic nuclei (AGNs) and the surrounding medium are studied. Several environments ranging from hot, dense knots of gas near the central engine to the cold broad line region clouds are simulated by a nuclear reaction network that couples the thermonuclear processes in the cloud to the reactions between the jet particles and the cloud. Reaction products from the jet-cloud interactions are followed until they react or are thermalized, which may involve several subsequent reactions. Enhanced production of light nuclei well above their primordial abundances is possible even, under some conditions, of CNO nuclei. In these scenarios, the jets can enhance abundances of CNO nuclei by first producing excess amounts of nuclei with A<8, then by increasing the cloud density to the point at which the thermonuclear reaction rates become important. The comparison to observed abundances in quasars (QSOs) leads to the conclusion that the interactions of ejected matter from AGNs may be responsible for large observed abundances of light nuclei in addition to significant abundances of nuclei in the CNO region.

Light‐Element Nucleosynthesis From Jet‐Cloud Interactions in Active Galactic Nuclei

The production of light nuclei via the interactions of jets from the central engines of active galactic nuclei and the surrounding medium is studied. Several environments ranging from hot, dense knots of gas near the central engine to the cold broad-line region clouds are simulated by a nuclear reaction network that couples the thermonuclear processes in the cloud to the reactions between the jet particles and the cloud. Reaction products from the jet-cloud interactions are followed until they react or are thermalized, which may involve several subsequent reactions. The effects of nucleosynthesis from the introduction of a jet into the cloud can result in enhanced production of light nuclei well above their primordial abundances, and enhancement of CNO nuclei is also predicted above thermally produced abundances. In these scenarios, the jets can enhance abundances of CNO nuclei by first producing excess amounts of nuclei with A < 8, then by increasing the cloud density to the point at which the thermonuclear reaction rates become important. The comparison to observed abundances in quasars leads to the conclusion that the interactions of ejected matter from AGNs may be responsible for large observed abundances of light nuclei in addition to significant abundances of nuclei in the CNO region.

Reprocessed emission line profiles from dense clouds in geometrically thick accretion engines

The central engines of active galactic nuclei (AGN) contain cold, dense material as well as hot X-ray emitting gas. The standard paradigm for the engine geometry is a cold thin disc sandwiched between hot X-ray coronae. Strong support for this geometry in Seyferts comes from the study of fluorescent iron line profiles, although the evidence is not ubiquitously air tight. The thin disc model of line profiles in AGN and in X-ray binaries should be bench marked against other plausible possibilities. One proposed alternative is an engine consisting of dense clouds embedded in an optically thin, geometrically thick X-ray emitting engine. This model is further motivated by studies of geometrically thick engines such as advection dominated accretion flows (ADAFs). Here we compute the reprocessed iron line profiles from dense clouds embedded in geometrically thick, optically thin X-ray emitting discs near a Schwarzchild black hole. We consider a range of cloud distributions and disc solutions, including ADAFs, pure radial infall, and bipolar outflows. We find that such models can reproduce line profiles similar to those from geometrically thin, optically thick discs and might help alleviate some of the problems encountered from the latter.

The Formation and Merger of Compact Objects in the Central Engine of Active Galactic Nuclei and Quasars: Gamma‐Ray Burst and Gravitational Radiation

The production rate of compact objects, i.e., neutron stars (NSs) and black holes (BHs), in active galactic nuclei (AGNs) and quasars (QSOs), where frequent supernova explosions are used to explain the high metallicity, is very high because of the interaction between the accretion disk and main-sequence stars in the nucleus of the quasar. The compact object red giant (RG) star binaries can be easily formed because of the large captured cross section of the RG stars. The (NS/BH, NS/BH) binary can be formed after the supernova explosion of the (NS/BH, RG) binary. Intense transient gamma-ray emission (gamma-ray burst) and gravitational radiation can result from the merger of these two compact objects. Collision between the helium core (Hc) of the RG and the BH may also take place and may also result in long-duration gamma-ray bursts but no gravitational waves. We estimate that the merger rate of (NS/BH, NS/BH) binaries and (Hc, BH) is proportional to the metal abundance N V/C IV and can be as high as 10-3 [(N V/C IV)/0.01] yr-1 per AGN/QSO.

Central Engines of Active Galactic Nuclei: Properties of Collimated Outflows and Applications for Cosmology

Powerful extended radio galaxies in the 3CR survey are observed out to redshifts of about 2. For redshifts greater than about 0.3, the average lobe-lobe size of the sources decreases monotonically with redshift for all reasonable cosmological parameter choices. This suggests that the characteristic time for which an AGN produces a highly collimated outflow that powers the radio emission is shorter for the high-redshift sources than it is for low-redshift sources. The fact that the characteristic time for which an AGN produces a collimated outflow decreases with redshift implies that the physical mechanism that gives rise to the large scale jets is not Eddington limited. Higher redshift sources have higher radio powers due the flux limit of the sample, and sources with higher radio powers have higher beam powers relative to lower redshift sources. Both the beam power and active lifetime of the AGN are controlled by processes occurring at the heart of the AGN, and may be related for non-Eddington limited processes. The relation between the active lifetime, $t_*$, and the beam power, $L_j$, of powerful extended radio galaxies is investigated here and found to be $t_*\proptoL_j^{-2/3}$. A comparison of the redshift evolution of the characteristic source sizes with that of the average lobe-lobe size for the 3CR sample of radio galaxies could be used to constrain cosmology.

Effects of Shocks on Emission from the Central Engines of Active Galactic Nuclei. I.

In this paper we show that perturbations of the accretion flow within the central engines of some active galactic nuclei (AGNS) are likely to form shock waves in the accreting plasma. Such shocks, which may be either collisional or collisionless, can contribute to the observed high-energy temporal and spectral variability. Our rationale is the following: Observations show that the continuum emission probably originates in an optically thin, hot plasma in the AGN central engine. The flux and spectrum from this hot plasma varies significantly over light crossing timescales. Several authors have suggested that macroscopic perturbations contained within this plasma are the sources of this variability. In order to produce the observed emission the perturbations must be radiatively coupled with the optically thin hot matter and must also move with high velocities. We suggest that shocks, which can be very effective in randomizing the bulk motion of the perturbations, are responsible for this coupling. Shocks should form in the central engine, because the temperatures and magnetic fields are probably reduced below their virial values by radiative dissipation. Perturbations moving at Keplerian speeds, or strong non-linear excitations, result in supersonic and super-Alfvenic velocities leading to shock waves within the hot plasma. We show that even a perturbation smaller than the emitting region can form a shock that significantly modifies the continuum emission in an AGN, and that the spectral and temporal variability from such a shock generally resembles those of radio-quiet AGNS. As an example, the shock inducing perturbation in our model is a small main-sequence star, the capturing and eventual accretion of which are known to be a plausible process. We argue that shocks in the central engine may also provide a natural triggering mechanism for the cold component of Guilbert & Rees two-phase medium and an efficient mecha- nism for angular momentum transfer. Current and future missions, such as ASCA, XTE, XMM, AXAF, and ASTRO-E may determine the importance of shock-related emission from the central engines of AGNS.