In next five years, dramatic progress is anticipated for the AGN studies, as we have two important missions to observe celestial sources in the high energy regime: GLAST and Suzaku. In this talk, I will summarize recent highlights in studies of AGN jets, focusing on the high-sensitivity X-ray observations that may shed new light on the forthcoming GLAST era. I will especially present some examples from most recent Suzaku observations of blazars, which provides important hints for the shock acceleration in sub-pc scale jets, as well as particle content in jets. Then I will focus on the neutral iron-line feature observed in some broad line radio galaxies, as a probe of jet launching and/or the disk-jet connection. Finally, I will discuss new results of large scale (kpc to Mpc) jets recently resolved with Chandra X-ray observatory. Simultaneous monitoring observations in various wavelengths will be particularly valuable for variable blazar sources, allowing the cross correlations of time series as well as detailed modeling of the spectral evolution between the X-ray and gamma-ray energy bands. Possible impacts of these new observations across the electromagnetic spectrum on various spatial scales are discussed to challenge the long-standing mystery of AGN jet sources.
Comparing the properties AGN and Herbig-Haro jets can be a useful exercise for understanding the physical mechanisms at work in collimated outflows that propagate in such different environments. In the case of Herbig-Haro jets, the presence of emission lines in the spectra and the continuous evolution of the observation techniques greatly favor our knowledge of the physical parameters of the jets instead, for AGN jets, the process of constraining the jet parameters is hampered by the nature of the emission from these objects that is non-thermal. I will discuss how one cannot directly constrain…
We argue the behavior of Poynting flux-dominated outflows from AGN in the galactic cluster systems by performing three-dimensional MHD simulations within the framework of the "magnetic tower" model. Of particular interests are the structure of MHD waves, the cylindrical radial force balance, the (de)collimation, and the stability properties of magnetic tower jets. Transition between the jet/lobe and the formation of wiggling jet by growing current-driven instability are discussed.
We discuss a new VLBA program to investigate structural and polarization changes in the brightest AGN jets in the northern sky. Our study represents a significant improvement over previous surveys in terms of image fidelity, size, and completeness, and will serve to characterize the kinematics of AGN jets and determine how these are related to other source properties. We discuss several preliminary results from our program, including the detection of parsec-scale circularly polarized jet emission, enhanced magnetic field ordering at the sites of apparent bends, and intrinsic differences in the…
I review Chandra's achievements in the last two years in AGN research. I concentrate on some topics of my interest; some others are reviewed in an accompanying article by K. Nandra. I comment briefly on the need to allow for the discovery space to ensure rapid progress in the field.
Compact relativistic jets in active galactic nuclei offer an effective tool for investigating the physics of nuclear regions in galaxies. The emission properties, dynamics, and evolution of jets in AGN are closely connected to the characteristics of the central supermassive black hole, accretion disk and broad-line region in active galaxies. Recent results from studies of the nuclear regions in several active galaxies with prominent outflows are reviewed in this contribution.
Most of energy carried by relativistic AGN jets remains undetected until hundreds of kiloparsecs where interaction with intergalactic medium produces hot spots. The jet's hidden energy is only partially dissipated at smaller scales, from parsecs to kiloparsecs. Several media may play the role of the ``prime mover'': ions, pairs or large scale magnetic fields. Analyzing VLBI polarization structures of relativistic parsec scale jets we conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. This implies that large-scale magnetic fields …
Results of high-resolution simultaneous multi-frequency 8.1-15.4 GHz VLBA polarimetric observations of relativistic jets in active galactic nuclei (the MOJAVE-2 project) are analyzed. We compare characteristics of VLBI features with jet model predictions and test if adiabatic expansion is a dominating mechanism for the evolution of relativistic shocks in parsec-scale AGN jets. We also discuss magnetic field configuration, both predicted by the model and deduced from electric vector position angle measurements.
The highest energy cosmic rays could be produced by drifts in magnetized, cylindrically collimated, sheared jets of powerful active galaxies (i.e. FR II radiogalaxies; radio loud quasars and high power BL Lacs). We show that in such scenarios proton synchrotron radiation can give rise to detectable photon fluxes at energies ranging from hundreds of keV to tens of MeV.
Recent advances in estimating black hole masses for AGN show that radio luminosity is dependent on black hole mass and accretion rate. In this paper we outline a possible scheme for unifying radio-quiet and radio-loud AGN. We take the ``optimistic'' view that the mass and spin of the central black hole, the accretion rate onto it, plus orientation and a weak environmental dependence, fully determine the observed properties of AGN.
The sensitivity, stability, and uniformity of calibration of the VLBA has revolutionized parsec-scale polarization studies of AGN jets. Not only does polarization probe the magnetic field structures of jets, serving as a hydrodynamic tracer of shocks, bends, and shear, but polarization also probes the medium through which it propagates by encoding the signature of Faraday effects along the line of sight. I review advances made by the VLBA in studying the polarization of jets to probe their magnetic field structures, properties of the jet plasma, and properties of the external environment. This…
Consequences of anisotropic cascade radiation in AGN are investigated. The anisotropic cascades are produced by protons accelerated in a nuclear jet. The protons are assumed to cool in the UV photon field of an accretion disk by photo-production of secondary particles. Most cascade photons are produced in the direction of the accretion disk leading to strong high-energy irradiation and weak emission of observable MeV photons. Nuclear, turbulent jets rapidly disrupt and remain radio-faint, so that the scenario could hold the clue to the understanding of the X-ray spectra of radio-quiet AGN.
Until very recently, few constraints existed on the physics of jets, even though they represent the first known evidence of mass outflow in AGN. This has begun to change with HST and Chandra observations, which allow us to observe short-lived, dynamic features, and compare their spectra and morphology to those of longer-lived particles seen in the radio. We examine HST and Chandra observations of M87 and 3C273 which reveal that these two prototype objects seem radically different.
(Abridged) We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron--positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical, and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factor $Γ$. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. S…
We study the properties of ultracompact jets in several prominent radio sources (Cygnus A, 3C 309.1, 3C 345, 3C 395, 4C 39.25, and 1038+528 A), using the frequency dependence of observed position of the optically thick jet core. Frequency dependent offsets of the core positions are used for calculating the luminosities, magnetic fields, and geometrical properties of the jets. Opacity effects are studied, for both synchrotron self-absorption and external free-free absorption. Pressure and density gradients in the jets and in surrounding ambient medium are shown to be a primary factor determinin…
The phenomenology of accretion disc, coronnae and jets in X-ray binaries is rather well established. However the structure of the accretion flow in the various spectral states is still debated and the connection between the hot flow and compact jet is far from being understood. Simbol-X should address these two important questions in several ways. First, it will provide us with the capability of producing high sensivity, broad band spectra and therefore constrain simultaneously the shape and luminosity of all spectral components (iron line, reflection bump, thermal disc and comptonised emissio…
This paper summarizes some of our recent projects which try to illuminate the nature and importance of jets associated with active nuclei and compact objects. After a short introduction on jets in radio galaxies and radio loud quasars the paper focuses on jets in other source types, such as radio-quiet quasars, Seyfert and LINER galaxies, and stellar mass black holes. Radio observations of quasars, for example, have brought new evidence for the existence of relativistic jets in radio quiet quasars, while HST and VLA observations of Seyfert galaxies have now clearly established not only the pre…
We study the effect of non-relativistic, thermal matter in the jets of active galaxies (AGN) on the low-frequency non-thermal emission and the variability thereof. In matter-dominated jets, sizable quantities of gas should exist, in particular in the compression zones near the collision fronts that are an implicit ingredient of Fermi-type particle acceleration scenarios. Non-relativistic thermal gas in AGN jets noticably contributes to the optical depth at radio to infrared frequencies, and much less to the emission, with an efficiency that is strongly temperature-dependent. The observable flu…
I explore a possibility to measure the gravitational redshift (GR) effect in the gravitational field of massive central nuclei residing in active galaxies (AGN). The activity of central nuclei is associated with the bipolar jet ejection of relativistic plasma which produces strong radio emission. I consider the behavior of the flux density variations of the jet plasma as a result of GR effect from a Schwarzschild black hole, and I discuss possibilities to detect the GR effect from the relativistic jets of compact AGN with present and future radio facilities.
Distributions of Faraday rotation measure (FRM) and the projected magnetic field derived by a 3-dimensional simulation of MHD jets are investigated based on our "sweeping magnetic twist model". FRM and Stokes parameters were calculated to be compared with radio observations of large scale wiggled AGN jets on kpc scales. We propose that the FRM distribution can be used to discuss the 3-dimensional structure of magnetic field around jets and the validity of existing theoretical models, together with the projected magnetic field derived from Stokes parameters. In the previous paper, we investigat…
Models of relativistic jets filled with ultrarelativistic pair plasma are very successful in explaining the broadband radiation of $γ$-ray blazars. Assuming that the initial injection and cooling of ultrarelativistic pair plasma in an AGN jet has occurred, producing the observed high-energy $γ$-ray radiation, we investigate the further evolution of the pair plasma as it continues to move out from the central engine. The effects of thermalization and reacceleration, the emission of pair bremsstrahlung and annihilation radiation and the bulk Compton process, and the possible application to MeV b…
We discuss the fluxes of high energy neutrinos and gamma-rays expected from AGNs if hadrons can be effectively accelerated to ultra-high energies by their relativistic jets, as currently believed. Fluxes of multi-TeV neutrinos detectable by $km$-scale detectors like IceCube could be expected from powerful blazars where strong accretion-disk radiation is present in the AGN cores. Gamma-ray fluxes of hadronic origin can be important for flares in the compact jets of these sources up to GeV energies, but they will be insignificant for BL Lac objects. Production of UHE neutral beams composed of ne…
These lecture notes provide an introduction to mm/submm extragalactic astronomy, focused on AGN studies, with the final goal of preparing students to their future exploitation of the ALMA capabilities. I first provide an overview of the current results obtained through mm/submm observations of galaxies and AGNs, both local and at high redshift. Then I summarize the main mm/submm facilities that are currently available. ALMA is then presented with a general description and by providing some details on its observing capabilities. Finally, I discuss some of the scientific goals that will be achie…
Evidence is mounting that many of the transverse jet B fields observed in BL Lac objects on parsec scales represent the dominant toroidal component of the intrinsic jet B fields. If this is the case, this may give rise to rotation-measure (RM) gradients across the jets, due to the systematic change in the line-of-sight component of the jet B field. We have found evidence for such RM gradients in several BL Lac objects. We discuss these new results, together with some of their implications for our understanding of the pc-scale jets of AGN.
We discuss the possible role of reconnection in electro-magnetically dominated cores of relativistic AGN jets. We suggest that reconnection may proceed in a two-fold fashion: initial explosive collapse on the Alfven time-scale of a current-carrying jet (which is of the order of the light crossing time) and subsequent slow quasi-steady reconnection. Sites of explosive collapse are associated with bright knots, while steady-state reconnection re-energizes particles in the ``bridges'' between the knots. Ohmic dissipation in reconnection layers leads to particle acceleration either by inductive el…
We examine the hypothesis that there exists a simple scaling between the observed velocities of jets found in Young Stellar Objects (YSOs) and jets found in Active Galactic Nuclei (AGN). We employ a very simplified physical model of the jet acceleration process. We use time-dependent, spherically symmetric wind models in Newtonian and relativistic gravitational fields to ask whether the energy input rates required to produce the jet velocities observed in YSOs (of about 2 $\times$ the escape velocity from the central object) can also produce AGN jet velocities (Lorentz factors of $γ\sim$ 10). …
We have developed a model for gamma ray emission in jets of active galactic nuclei in which particle acceleration takes place at a shock in the relativistic jet plasma due to a massive star in the central region of the host galaxy moving through the jet. The gamma rays are produced in a pair-Compton cascade in the radiation field of the star initiated by accelerated electrons. Our model may account for the observed GeV to TeV gamma ray spectrum and variability of Markarian 421 and other blazars detected by the EGRET instrument on the Compton Gamma Ray Observatory.
According to the latest versions of synthesis modeling of the Cosmic X-ray Background, Compton Thick AGN should represent ~50% of the total absorbed AGN population. However, despite their importance in the cosmological context, only a few dozens of Compton Thick AGN have been found and studied so far. We will briefly review this topic and discuss the improvement in this field offered by the Simbol-X mission with its leap in sensitivity (E>10 keV) of more than a factor 500 with respect to previous X-ray missions.
Millimeter-VLBI images probe as deep as never before the nuclei of AGN. Global 3mm-VLBI where the VLBA is combined with sensitive European stations, gives CLEAN maps of a quality, which never before was so good. VLBI at 147 GHz yields transatlantic fringes for the first time. Now it will become possible to study the relation between jet kinematics and spectral activity with a few ten micro-arcsecond resolution at the shortest wavelengths.
We consider different scenarios of collisions of compact objects (clouds, massive stars, supernova shock waves, or young pulsars) with jets in active galactic nuclei. The purpose is to find out if such collisions can become plausible explanations for the gamma-ray production in blazars. We conclude that the relativistic proton beam - cloud collision scenario has problems with explanation of the $γ$-ray spectrum in the observed energy range from blazars. As a result of collisions of massive stars, supernova shock waves, or young pulsars with the jet plasma, a highly oblique shocks should be for…
The fine-scale structure and the kinematics of relativistic active galactic nuclei (AGN) jets have been studied by very-long-baseline interferometry at very high resolutions since 1998 at 2 cm wavelength for a sample of over a hundred radio sources (VLBA 2cm Survey and MOJAVE programs). Since 2007, this is being complemented by the TANAMI project, based on southern observations with the Australian LBA at 3.6 cm and 1.1 cm wavelengths. From our observation campaign, we find that most of the radio jets show linear morphologies at parsec-scales, but some of show curvature and non-radial motions. …
X-Ray observations of groups and clusters of galaxies show that the Intra-Cluster Medium (ICM) in their cores is hotter than expected from cosmological numerical simulations of cluster formation which include star formation, radiative cooling and SN feedback. We investigate the effect of the injection of supersonic AGN jets into the ICM using axisymmetric hydrodynamical numerical simulations. A simple model for the ICM, describing the radial properties of gas and the gravitational potential in cosmological N-Body+SPH simulations of one cluster and three groups of galaxies at redshift z=0, is o…
We report on an ongoing effort to image active galactic nuclei simultaneously observed at 2.3 and 8.6 GHz in the framework of a long-term VLBI project RDV (Research and Development - VLBA) started in 1994 aiming to observe compact extragalactic radio sources in the astrometric/geodetic mode. Observations of bright extragalactic sources are carried out bi-monthly making up to six sessions per year with participation of all ten VLBA antennas and up to nine additional (geodetic and EVN) radio telescopes. Analysis of single-epoch results for 370 quasars, BL Lacs and radio galaxies is presented. We…
We show that a quasi-spherical (QS) hot accretion flow is expected to operate in all SMBHs, with its rate being capped at dot m_QS,max~0.001 (M/1E8Msun) in units of the Eddington rate. It is then proposed that AGN jet power is proportional to the product of the hot accretion rate and a SMBH spin-dependent efficiency for energy extraction in the form of jets. Predictions from this model include (1) while radio jets should emerge from all SMBHs, the maximum jet power goes with SMBH mass approximately as 1E43.6(M/1E8Msun)^{2}erg/s, (2) even although there are two separate underlying populations o…
It is suggested that the outflowing plasma in the jets of active galactic nuclei (AGNs) is inhomogeneous and consists of separate clouds. These clouds are strongly magnetized and move away from the central engine at relativistic speeds. The clouds interact with an ambient medium which is assumed to be at rest. In the process of this interaction, particles of the ambient medium are accelerated to high energies at the cloud front and flow ahead of the front. It is shown that the radiation of the accelerated particles may be responsible for the X-ray and $γ$-ray emission from AGN jets. TeV $γ$-ra…
X-ray emission from large scale extragalactic jets is likely to be due to inverse Compton scattering of relativistic particles off seed photons of both the cosmic microwave background field and the blazar nucleus. The first process dominates the observed high energy emission of large scale jets if the plasma is moving at highly relativistic speeds and if the jet is aligned with the line of sight, i.e. in powerful flat radio spectrum quasars. The second process is relevant when the plasma is moving at mildly bulk relativistic speeds, and can dominate the high energy emission in misaligned sourc…
We present a new prediction of GeV $γ$-ray emission from radio lobes of young AGN jets. In the previous work of Kino et al. (2007), MeV $γ$-ray bremsstrahlung emission was predicted from young cocoons/radio-lobes in the regime of no coolings. In this study, we include cooling effects of bremsstrahlung emission and adiabatic loss. With the initial conditions determined by observed young radio lobes, we solve a set of equations describing the expanding lobe evolution. Then we find that the lobes initially have electron temperature of $\sim$GeV, and they cool down to $\sim$MeV by the adiabatic lo…
We consider the evolution of a spherically expanding plasma cloud, where there is continuous injection of non-thermal electrons. We compute the time dependent electron distribution and resultant photon spectra taking into account synchrotron, adiabatic and inverse Compton cooling. This model is different from previous works where, instead of a continuous injection of particles, a short injection period was assumed. We apply this model to the radio/optical knots in the large scale jets of AGN, detected in X-rays by {\it Chandra} and find that the overall broadband spectral features can be repro…
The MOJAVE blazar sample consists of the 133 brightest, most compact radio-loud AGN in the northern sky, and is selected on the basis of VLBA 2 cm correlated flux density exceeding 1.5 Jy (2 Jy for declinations south of 0 degrees) at any epoch between 1994 and 2003. Since 1994 we have been gathering VLBA data on the sample to measure superluminal jet speeds and to better understand the parsec-scale kinematics of AGN jets. We have obtained 1.4 GHz VLA-A configuration data on 57 of these sources to investigate whether the extended luminosity of blazars is correlated with parsec-scale jet speed, …
The energy budget of all known optical jets is discussed. It is found that to power the extended radio lobes of radio galaxies, the jet bulk motion on kpc scales must be relativistic, on average. Based on various constraints, a ``most probable'' region centered around bulk Lorentz factor ~7.5 and jet inclination \~20 degrees is found. Because of the consequent relativistic beaming, the rest frame magnetic field is lower and electron lifetimes longer. Combining the effects of time dilation and lower emission rate, the electron diffusion length becomes fully consistent with the deprojected jet l…
There is good evidence for X-ray emission associated with AGN jets which are relativistically boosted towards the observer. But to what jet radius does such X-ray emission persist? To attempt to answer this question one can look at radio galaxies; their cores are sufficiently X-ray faint that any unbeamed X-ray emission in the vicinity of the central engine must be obscured. The jets of such sources are at unfavourable angles for relativistic boosting, and so their relatively weak X-ray emission must be carefully separated from the plateau of resolved X-ray emission from a hot interstellar, in…
We revisit a method to obtain upper limits on the jet matter content combining synchrotron self-absorption constraints and the large scale bubble energy. We use both X-ray observations, which give limits on the jet power from the energies and timescales of bubbles found in clusters of galaxies, and radio observations, which give limits on the magnetic field in the jets. Combining the two imposes constraints on the particle number density, and hence the jet content. Out of a sample of clusters which have clear radio bubbles, there are only two which have sufficient resolution in the radio image…
We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelen…
Seyfert galaxies and quasars were first discovered through optical and radio techniques, but in recent years high-energy emission, that can penetrate central gas and dust, has become essentially the defining characteristic of an AGN. AGNs with extended radio jets are of particular interest, since the jets signal source orientation. However, the jets extend into the cores, where they are faster and more compact. Special-relativistic effects then cause jet brightness and variability time-scales across the electromagnetic spectrum to be strong functions of jet orientation. Jet X-ray emission is c…
The Global mm-VLBI Array is at present the most sensitive 3 mm-VLBI interferometer and provides images of up to 40 micro-arcsecond resolution. Using this array, we have monitored the rotation of the innermost jet in the quasar NRAO 150, which shows an angular speed of ~ 7 deg./yr. Future 3 mm arrays could include additional stations like ALMA, GBT, LMT, CARMA, SRT, Yebes, Nobeyama and Noto, which would allow to push VLBI at this wavelength to sensitivity and image quality levels comparable to those of present VLBI at centimeter wavelengths. This would improve our knowledge of the accretion sys…
Simultaneous observations of blazars in different spectral regimes imply that the amplitude of variability depends on the observational band. Both in the low energy spectral component and in the high energy spectral component, the amplitude increases with the photon energy. We show that such behavior can be explained in terms of a two-component model, where the spectra observed during flares are superpositions of spectra arising from at least two distinct, spatially separated sources. Using the two-component model for blazar variability we derive constraints on jet physics in quasars. Particul…
The matter content of jets in active galactic nuclei is examined in a new way. We model the dynamical expansion of its cocoon embedded in the intra-cluster medium (ICM). By comparing the observed shape of the cocoon with that expected from the theoretical model, we estimate the total pressure ($P_{\rm c}$) and electron temperature ($T_{e}$) of the cocoon. The number density of the total electrons ($n_{e^{-}}$) is constrained by using the non-thermal spectrum of the hot spot and the analysis of the momentum balance between the jet thrust and the rum pressure of ICM. Together with the obtained $…
We model the acceleration of energetic particles due to shear and centrifugal effects in rotating astrophysical jets. The appropriate equation describing the diffusive transport of energetic particles in a collisionless, rotating background flow is derived and analytical steady state solutions are discussed. In particular, by considering velocity profiles from rigid, over flat to Keplerian rotation, the effects of centrifugal and shear acceleration of particles scattered by magnetic inhomogeneities are distinguished. In the case where shear acceleration dominates, it is confirmed that power la…
Observations made with the VLBA have led to fundamental advances in our understanding of how radio jets in AGN evolve from parsec-scales out to distances exceeding several hundred kiloparsecs. In this review I discuss current models of young radio source evolution, as well as the observational evidence for a rapid change in jet properties on scales of ~1 kpc. A central topic of current debate is the relative importance of intermittent jet fueling versus jet-environment interactions in causing a drop-off in powerful radio sources at this critical evolutionary stage. Recent 3-D hydrodynamical je…
Highly collimated supersonic jets are observed to emerge from a wide variety of astrophysical objects, ranging from Active Nuclei of Galaxies (AGN's) to Young Stellar Objects (YSOs) within our own Galaxy. Despite their different physical scales (in size, velocity, and amount of energy transported), they have strong morphological similarities. Thanks to the proximity and relatively small timescales, which permit direct observations of evolutionary changes, YSO jets are, perhaps, the best laboratories for cosmic jet investigation. In this lecture, the formation, structure, and evolution of the Y…
