Search

We use Gemini Near-Infrared Integral Field Spectrograph (NIFS) observations of the inner 3.5$\times$3.5 kpc$^2$ of the radio galaxy Cygnus A to map the gas excitation and kinematics at a spatial resolution of 200 pc. The emission of the ionised gas shows a biconical morphology, with half-opening angle of 45$^\circ$ and oriented along the position angle of the radio jet. Coronal line emission is seen within the cone, up to 1.75 kpc from the nucleus, with higher ionisation gas observed in the easterly side. The H$_2$ and [Fe II] emission lines are consistent with excitation by the central AGN, with some contribution of shocks to the southwest of the nucleus. The gas visual extinction and electron density are larger than those from optical-based measurements, consistent with the fact that near-IR observations penetrate deeply into the gas emission structure, probing denser and more obscured regions. The gas kinematics shows two components: (i) a rotating disc with kinematic position angle of $\Psi_0=21^\circ\pm2^\circ$, seen both in ionised and molecular gas, and (ii) outflows with velocities of up to 600 kms$^{-1}$ observed within the ionisation cone in ionised gas and restricted to inner 0.5 arcsec in molecular gas. The mass outflow rate in ionised gas is in the range $\sim100-280 {\rm M_\odot yr^{-1}}$ and the kinetic power of the outflow corresponds to 0.3-3.3 per cent of the AGN bolometric luminosity, indicating that the outflows in Cygnus A may be effective in suppressing star formation., Comment: 13 pages, Published in MNRAS, DOI: 10.1093/mnras/stab1877, Version with corrections in the axes labels of some figures

Immensely bright quasars and radio-loud active galactic nuclei (AGNs) provide an enticing opportunity to construct standard candles detectable up to the very early universe. An analytic theory is proposed to measure the distance to powerful \citeauthor{FR+1974} type-II radio sources based on their integrated flux density across a broad range of radio frequencies, and the angular size and axis ratio of their synchrotron-emitting lobes. This technique can be used at low-redshift to construct absolute standard candles in conjunction with X-ray observations of the host cluster, or at high-redshift to measure the relative distances of objects and constrain the curvature of our universe. Distances calculated with this method are consistent for dissimilar objects at the same redshift; the two lobes of Cygnus A have flux densities, linear sizes and spectral break frequencies varying by between 15-35\% yet their fitted distances are the same to within 7\%. These distance estimates together yield a transverse comoving distance to Cygnus A of $261_{-55}^{+70}\rm\, Mpc$ corresponding to a Hubble constant of $H_0 = 64_{-13}^{+17}\rm\, km\, s^{-1}\, Mpc^{-1}$. Large samples of suitable FR-II sources could provide a measure of the Hubble constant independent of existing techniques such as the cosmic microwave background, baryon acoustic oscillations, and type 1a supernovae., Comment: 12 pages, 3 figures, 3 tables; accepted in MNRAS

The archetype FR-II galaxy Cygnus A lies in a moderately rich cluster about to undergo a major merger. We study the pre-merger Cygnus cluster environment using smoothed particle hydrodynamics simulations constrained by 2Ms of Chandra observations of the hot intracluster medium. The observations constrain the total gravitating mass and concentration parameter, and the simulations provide the quiescent and merger-enhanced temperature profiles of the pre- and post-merger of the cluster excluding the central active galactic nucleus (AGN). We present the first detailed model of the sub cluster north west of Cygnus A, named CygNW. We find a lower baryon fraction and higher concentration parameter for CygA than expected from known scaling relations in the literature. The model suggests the Cygnus cluster hosts a pre-merger with a progenitor mass ratio of about 1.5:1 at the virial radius. We notice that the intra cluster medium is heated as a result of the merger, but we find no evidence for a (pre-)merger shock in the interstitial region between both cluster haloes. We attribute the merger-induced heating to compression of the cluster outskirts. The smooth model obtained from our simulations is subtracted from the observed cluster state and shows residual temperature structure that is neither hydrostatic nor merger-heated cluster gas. We speculate that this residual heating may result from previous AGN activity over the last ∼100 Myr.

Radio interferometric (RI) data are noisy under-sampled spatial Fourier components of the unknown radio sky affected by direction-dependent antenna gains. Failure to model these antenna gains accurately results in a radio sky estimate with limited fidelity and resolution. The RI inverse problem has been recently addressed via a joint calibration and imaging approach which consists in solving a non-convex minimisation task, involving suitable priors for the DDEs, namely temporal and spatial smoothness, and sparsity for the unknown radio map via an $\ell_1$-norm prior, in the context of realistic RI simulations. Building on these developments, we propose to promote sparsity of the radio map via a log-sum prior, enforcing sparsity more strongly than the $\ell_1$-norm. The resulting minimisation task is addressed via a sequence of non-convex minimisation tasks composed of re-weighted $\ell_1$ image priors, which are solved approximately. We demonstrate the efficiency of the approach on RI observations of the celebrated radio galaxy Cygnus~A obtained with the Karl G. Jansky Very Large Array at X, C, and S bands. More precisely, we showcase that the approach enhances data fidelity significantly while achieving high resolution high dynamic range radio maps, confirming the suitability of the priors considered for the unknown DDEs and radio image. As a clear qualitative indication of the high fidelity achieved by the data and the proposed approach, we report the detection of three background sources in the vicinity of Cyg~A, at S band., Comment: 22 pages,13 figures. Submitted to MNRAS

The powerful FR II radio galaxy Cygnus A exhibits primary and secondary hotspots in each lobe. A 2 Msec Chandra X-ray image of Cygnus A has revealed an approximately circular hole, with a radius of 3.9 kpc, centered on the primary hotspot in the eastern radio lobe, hotspot E. We infer the distribution of X-ray emission on our line-of-sight from an X-ray surface brightness profile of the radio lobe adjacent to the hole and use it to argue that the hole is excavated from the radio lobe. The surface brightness profile of the hole implies a depth at least 1.7 $\pm$ 0.3 times greater than its projected width, requiring a minimum depth of 13.3 $\pm$ 2.3 kpc. A similar hole observed in the 5 GHz VLA radio map reinforces the argument for a cavity lying within the lobe. We argue that the jet encounters the shock compressed intracluster medium at hotspot E, passing through one or more shocks as it is deflected back into the radio lobe. The orientation of Cygnus A allows the outflow from hotspot E to travel almost directly away from us, creating an elongated cavity, as observed. These results favor models for multiple hotspots in which an FR II jet is deflected at a primary hotspot, then travels onward to deposit the bulk of its power at a secondary hotspot, rather than the dentist drill model., Accepted to ApJ, 12 pages, 7 figures, 5 tables

We present results from a deep (174 ks) Chandra observation of the FR-II radio galaxy 3C 220.1, the central brightest cluster galaxy (BCG) of a $kT \sim$ 4 keV cluster at $z=0.61$. The temperature of the hot cluster medium drops from $\sim5.9$ keV to $\sim3.9$ keV at $\sim$ 35 kpc radius, while the temperature at smaller radii may be substantially lower. The central active galactic nucleus (AGN) outshines the whole cluster in X-rays, with a bolometric luminosity of $2.0\times10^{46}$ erg s$^{-1}$ ($\sim10$% of the Eddington rate). The system shows a pair of potential X-ray cavities $\sim35$ kpc east and west of the nucleus. The cavity power is estimated within the range of $1.0\times10^{44}$ erg s$^{-1}$ and $1.7\times10^{45}$ erg s$^{-1}$, from different methods. The X-ray enhancements in the radio lobes could be due to inverse Compton emission, with a total 2-10 keV luminosity of $\sim8.0\times10^{42}$ erg s$^{-1}$. We compare 3C 220.1 with other cluster BCGs, including Cygnus A, as there are few BCGs in rich clusters hosting an FR-II galaxy. We also summarize the jet power of FR-II galaxies from different methods. The comparison suggests that the cavity power of FR-II galaxies likely under-estimates the jet power. The properties of 3C 220.1 suggest that it is at the transition stage from quasar-mode feedback to radio-mode feedback., 14 pages, 13 figures, MNRAS accepted

We have analyzed archival VLBA data for Cygnus A between 2002 and 2013, to search for radio emission from the transient discovered in 2015 by \citet{per18} approximately 0.4\arcsec~ from the nucleus of Cygnus A (Cyg A-2). \citet{per18} use VLA and VLBA archival data (between 1989 and 1997) to show that the transient rises in flux density by a factor of at least five in less than approximately 20 years. With the additional data presented here, we revise the rise time to between approximately four years and six years, based on a new detection of the source at 15.4 GHz from October 2011. Our results strengthen the interpretation of Cyg A-2 as the result of a Tidal Disruption Event (TDE), as we can identify the location of the compact object responsible for the TDE and can estimate the angular expansion speed of the resulting radio emitting structures, equivalent to an apparent expansion speed of $10^{49}$ erg. Due to the improved temporal coverage of our archival measurements, we find that it is unlikely that Cyg A-2 has previously been in a high luminosity radio state over the last 30 years., Eight pages, three figures, accepted to ApJ Letters

The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (, 7 pages, 2 figures, accepted A&A, online data on A&A website

Supermassive black hole binaries may be detectable by an upcoming suite of gravitational wave experiments. Their binary nature can also be revealed by radio jets via a short-period precession driven by the orbital motion as well as the geodetic precession at typically longer periods. We have investigated Karl G. Jansky Very Large Array (VLA) and MERLIN radio maps of powerful jet sources for morphological evidence of geodetic precession. For perhaps the best studied source, Cygnus A, we find strong evidence for geodetic precession. Projection effects can enhance precession features, for which we find indications in strongly projected sources. For a complete sample of 33 3CR radio sources we find strong evidence for jet precession in 24 cases (73 per cent). The morphology of the radio maps suggests that the precession periods are of the order of 10^6 - 10^7 yr. We consider different explanations for the morphological features and conclude that geodetic precession is the best explanation. The frequently observed gradual jet angle changes in samples of powerful blazars can be explained by orbital motion. Both observations can be explained simultaneously by postulating that a high fraction of powerful radio sources have sub-parsec supermassive black hole binaries. We consider complementary evidence and discuss if any jetted supermassive black hole with some indication of precession could be detected as individual gravitational wave source in the near future. This appears unlikely, with the possible exception of M87., Comment: 24 pages, 36 figures, accepted by MNRAS

Recent analyses of the broad spectral energy distributions (SED) of extensive lobes of local radio-galaxies have confirmed the leptonic origin of their Fermi/LAT gamma-ray emission, significantly constraining the level of hadronic contribution. SED of distant (D > 125 Mpc) radio-galaxy lobes are currently limited to the radio and X-ray bands, hence give no information on the presence of non-thermal (NT) protons but are adequate to describe the properties of NT electrons. Modeling lobe radio and X-ray emission in 3C 98, Pictor A, DA 240, Cygnus A, 3C 326, and 3C 236, we fully determine the properties of intra-lobe NT electrons and estimate the level of the related gamma-ray emission from Compton scattering of the electrons off the superposed Cosmic Microwave Background, Extragalactic Background Light, and source-specific radiation fields., 8 pages, 4 figures; a few typos corrected; several missing references added

We use new and archival Chandra observations of Cygnus A, totalling $\sim$1.9 Ms, to investigate the distribution and temperature structure of gas lying within the projected extent of the cocoon shock and exhibiting a rib-like structure. We confirm that the X-rays are dominated by thermal emission with an average temperature of around 4 keV, and have discovered an asymmetry in the temperature gradient, with the southwestern part of the gas cooler than the rest by up to 2 keV. Pressure estimates suggest that the gas is a coherent structure of single origin located inside the cocoon, with a mass of roughly $2\times10^{10} M_{\odot}$. We conclude that the gas is debris resulting from disintegration of the cool core of the Cygnus A cluster after the passage of the jet during the early stages of the current epoch of activity. The 4 keV gas now lies on the central inside surface of the hotter cocoon rim. The temperature gradient could result from an offset between the centre of the cluster core and the Cygnus A host galaxy at the switch-on of current radio activity., 15 pages, 12 figures, accepted by MNRAS

We leverage the Sparsity Averaging Reweighted Analysis (SARA) approach for interferometric imaging, that is based on convex optimisation, for the super-resolution of Cyg A from observations at the frequencies 8.422GHz and 6.678GHz with the Karl G. Jansky Very Large Array (VLA). The associated average sparsity and positivity priors enable image reconstruction beyond instrumental resolution. An adaptive Preconditioned Primal-Dual algorithmic structure is developed for imaging in the presence of unknown noise levels and calibration errors. We demonstrate the superior performance of the algorithm with respect to the conventional CLEAN-based methods, reflected in super-resolved images with high fidelity. The high resolution features of the recovered images are validated by referring to maps of Cyg A at higher frequencies, more precisely 17.324GHz and 14.252GHz. We also confirm the recent discovery of a radio transient in Cyg A, revealed in the recovered images of the investigated data sets. Our matlab code is available online on GitHub., Comment: 14 pages, 7 figures (3/7 animated figures), accepted for publication in MNRAS

Accepted: 2019-04-12, 資料番号: SA1190006000

In 2015, a radio transient named Cygnus A-2 was discovered in Cygnus A with the Very Large Array. Because of its radio brightness ($\nu F_{\nu} \approx 6 \times 10^{39}$ erg s$^{-1}$), this transient likely represents a secondary black hole in orbit around the AGN. Using {\it Chandra} ACIS observations from 2015 to 2017, we have looked for an X-ray counterpart to Cygnus A-2. The separation of 0.42 arcsec means that Cygnus A-2 can not be spatially resolved, but by comparing the data with simulated \texttt{marx} data, we put an upper limit to the 2-10 keV X-ray luminosity of Cygnus A-2 of $1 \times 10^{43}$ erg s$^{-1}$. Using the Fundamental Plane for accreting black holes, we find that our upper limit to the X-ray flux of Cygnus A-2 in 2015-2017 disfavours the interpretation of Cygnus A-2 as a steadily accreting black hole. We suggest instead that Cygnus A-2 is the radio afterglow of a tidal disruption event (TDE), and that a peak in the 2-10 keV luminosity of the nuclear region in 2013, when it was observed by {\it Swift} and {\it NuSTAR}, is X-ray emission from the TDE. A TDE could naturally explain the X-ray light curve of the nuclear region, as well as the appearance of a short-lived, fast, and ionized outflow previously detected in the 2013 {\it NuSTAR} spectrum. Both the radio and X-ray luminosities fall in between typical luminosities for 'thermal' and 'jetted' TDE types, suggesting that Cygnus A-2 would be unlike previously seen TDE's., Comment: 14 pages, 7 figures, accepted for publication in MNRAS

We present the first detailed polarimetric studies of Cygnus A at 230 GHz with the Submillimeter Array (SMA) to constrain the mass accretion rate onto its supermassive black hole. We detected the polarized emission associated with the core at a fractional polarization of . This low fractional polarization suggests that the polarized emission is highly depolarized. One of the possible explanations is due to a significant variance in the Faraday rotation measure within the synthesized beam. By assuming the Faraday depolarization caused by inhomogeneous column density of the magnetized plasma associated with the surrounding radiatively-inefficient accretion flow within the SMA beam, we derived the constraint on the mass accretion rate to be larger than 0.15 yr−1 at the Bondi radius. The derived constraint indicates that an adiabatic inflow–outflow solution or an advection-dominated accretion flow should be preferable as the accretion flow model in order to explain the jet power of Cygnus A.

CLEAN, the commonly employed imaging algorithm in radio interferometry, suffers from a number of shortcomings: In its basic version, it does not have the concept of diffuse flux, and the common practice of convolving the CLEAN components with the CLEAN beam erases the potential for super-resolution; it does not output uncertainty information; it produces images with unphysical negative flux regions; and its results are highly dependent on the so-called weighting scheme as well as on any human choice of CLEAN masks for guiding the imaging. Here, we present the Bayesian imaging algorithm resolve , which solves the above problems and naturally leads to super-resolution. We take a VLA observation of Cygnus A at four different frequencies and image it with single-scale CLEAN, multi-scale CLEAN, and resolve. Alongside the sky brightness distribution, resolve estimates a baseline-dependent correction function for the noise budget, the Bayesian equivalent of a weighting scheme. We report noise correction factors between 0.4 and 429. The enhancements achieved by resolve come at the cost of higher computational effort.

The superfine structure of the bipolar outflow from the radio galaxy Cygnus A has been investigated at a wavelength of 2 cm. The surrounding thermal plasma inflows onto the disk and is transferred in a spiral to the center, with the plasma velocity and temperature increasing to relativistic values. The rotating bipolar outflow carries away an excess angular momentum as it is accumulated. The high-velocity central flow is surrounded by parallel chains of components, the tangential directions of the low-velocity flows. Rotation collimates the flow; ring currents, a longitudinal magnetic field, are generated in it. The size of the high-velocity jet exceeds the size of the counterjet by a factor of 3.5 due to the velocity difference: the acceleration of the flow moving along the field and its decelerationwhenmoving in a direction opposite to the magnetic field of the system. The observed features are typical of objects with active nuclei.

The composition of the relativistic plasma produced in active galactic nuclei and ejected via powerful jets into the interstellar/intergalactic medium is still a major unsettled issue. It might be a positron-electron plasma in case the plasma was created by pair production in the intense photon fields near accreting super-massive black holes. Alternatively, it might be an electron-proton plasma in case magnetic fields lift and accelerate the thermal gas of accretion discs into relativistic jets as the recent detection of $\gamma$-rays from blazars indicates. Despite various attempts to unambiguously establish the composition of the relativistic jets, this remains a major unknown. Here, we propose a way to settle the question via sensitive measurements of circular polarization (CP) in the radio emission of the hot spots of bright radio galaxies like Cygnus A. The CP of synchrotron emission is determined by the circular motions of the radiating relativistic leptons. In case of charge symmetric energy spectra of a electron-positron plasma, it should be exactly zero. In case of an electron-proton plasma the electrons imprint their gyration onto the CP and we expect the hot spots of Cygnus A to exhibit a fractional CP at a level of $10^{-3}\,(\nu/\mbox{GHz})^{-{1}/{2}}$, which is challenging to measure, but not completely unfeasible., Comment: 6 pages, no figures, accepted by JCAP, erratum with respect to accepted version applied, numerical corrections leading to a stronger effect

We present a spectral analysis of the lobes and X-ray jets of Cygnus A, using more than 2 Ms of $\textit{Chandra}$ observations. The X-ray jets are misaligned with the radio jets and significantly wider. We detect non-thermal emission components in both lobes and jets. For the eastern lobe and jet, we find 1 keV flux densities of $71_{-10}^{+10}$ nJy and $24_{-4}^{+4}$ nJy, and photon indices of $1.72_{-0.03}^{+0.03}$ and $1.64_{-0.04}^{+0.04}$ respectively. For the western lobe and jet, we find flux densities of $50_{-13}^{+12}$ nJy and $13_{-5}^{+5}$ nJy, and photon indices of $1.97_{-0.10}^{+0.23}$ and $1.86_{-0.12}^{+0.18}$ respectively. Using these results, we modeled the electron energy distributions of the lobes as broken power laws with age breaks. We find that a significant population of non-radiating particles is required to account for the total pressure of the eastern lobe. In the western lobe, no such population is required and the low energy cutoff to the electron distribution there needs to be raised to obtain pressures consistent with observations. This discrepancy is a consequence of the differing X-ray photon indices, which may indicate that the turnover in the inverse-Compton spectrum of the western lobe is at lower energies than in the eastern lobe. We modeled the emission from both jets as inverse-Compton emission. There is a narrow region of parameter space for which the X-ray jet can be a relic of an earlier active phase, although lack of knowledge about the jet's electron distribution and particle content makes the modelling uncertain., Comment: 21 pages, 8 figures

We report the detection of linearly polarized emission at 53 and 89 $\mu$m, from the radio-loud active galactic nucleus (AGN) Cygnus A using HAWC+ onboard SOFIA. We measure a highly polarized core of $11\pm3$% and $9\pm2$% with a position angle (P.A.) of polarization of $43\pm8^{\circ}$ and $39\pm7^{\circ}$ at 53 and 89 $\mu$m, respectively. We find (1) a synchrotron dominated core with a flat spectrum ($+0.21\pm0.05$) and a turn-over at $543\pm120$ $\mu$m, which implies synchrotron emission is insignificant in the infrared (IR), and (2) a $2-500$ $\mu$m bump peaking at $\sim40$ $\mu$m described by a blackbody component with color temperature of $107\pm9$ K. The polarized SED has the same shape as the IR bump of the total flux SED. We observe a change in the P.A. of polarization of $\sim20^{\circ}$ from 2 to 89 $\mu$m, which suggests a change of polarization mechanisms. The ultraviolet, optical and near-IR polarization has been convincingly attributed to scattering by polar dust, consistent with the usual torus scenario, though this scattered component can only be directly observed from the core in the near-IR. By contrast, the gradual rotation by $\sim20^{\circ}$ towards the far-IR, and the near-perfect match between the total and polarized IR bumps, indicate that dust emission from aligned dust grains becomes dominant at $10-100$ $\mu$m, with a large polarization of 10% at a nearly constant P.A. This result suggests that a coherent dusty and magnetic field structure dominates the $10-100$ $\mu$m emission around the AGN., Comment: 9 pages, 4 figures. accepted for publication in ApJL

We combine new (NGC 1275, NGC 4151, and NGC 5506) and previously published (Cygnus A, Mrk 231, and NGC 1068) sub-arcsecond resolution mid-infrared (MIR; 8-13 $��$m) imaging- and spectro-polarimetric observations of six Seyfert galaxies using CanariCam on the 10.4-m Gran Telescopio CANARIAS. These observations reveal a diverse set of physical processes responsible for the nuclear polarization, and permit characterization of the origin of the MIR nuclear polarimetric signature of active galactic nuclei (AGN). For all radio quiet objects, we found that the nuclear polarization is low (, 9 pages, 4 figures. Accepted to MNRAS

We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18-1.66 around the cocoon. We estimate a total outburst energy of $\simeq 4.7\times10^{60}\rm\ erg$, with an age of $\simeq 2 \times 10^{7}\rm\ yr$. The average postshock pressure is found to be $8.6 \pm 0.3 \times 10^{-10}\rm\ erg\ cm^{-3}$, which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are found to be lower in the western lobe than in the eastern lobe by $\simeq 20\%$. Pressure estimates for hotspots A and D from synchrotron self-Compton models imply that each jet exerts a ram pressure $\gtrsim$ 3 times its static pressure, consistent with the positions of the hotspots moving about on the cocoon shock over time. A steady, one-dimensional flow model is used to estimate jet properties, finding mildly relativistic flow speeds within the allowed parameter range. Models in which the jet carries a negligible flux of rest mass are consistent with with the observed properties of the jets and hotspots. This favors the jets being light, implying that the kinetic power and momentum flux are carried primarily by the internal energy of the jet plasma rather than by its rest mass., Comment: Accepted to ApJ, 15 pages, 7 figures, 4 tables

A phenomenological model for the evolution of classical radio galaxies such as Cygnus A is presented. An activity cycle of the host galaxy in the radio begins with the birth of radio jets, which correspond to shocks on scales ∼1 pc (the radio galaxy B0108+388). In the following stage of the evolution, the radio emission comes predominantly from formations on scales of 10–100 pc, whose physical parameters are close to those of the hot spots of Cygnus A (this corresponds to GHz-peaked spectrum radio sources). Further, the hot spots create radio lobes on scales of 103–104 pc (compact steep-spectrum radio sources). The fully formed radio galaxies have radio jets, hot spots, and giant radio lobes; the direction of the jets can vary in a discrete steps with time, creating new hot spots and inflating the radio lobes (as in Cygnus A). In the final stage of the evolutionary cycle, first the radio jets disappear, then the hot spots, and finally the radio lobes (similar to the giant radio galaxies DA 240 and 3C 236). A large fraction of radio galaxies with repeating activity cycles is observed. The close connection between Cygnus A-type radio galaxies and optical quasars is noted, as well as similarity in the cosmological evolution of powerful radio galaxies and optical quasars.

We study particle acceleration and magnetic field amplification in the primary hotspot in the northwest jet of radiogalaxy Cygnus A. By using the observed flux density at 43 GHz in a well resolved region of this hotspot, we determine the minimum value of the jet density and constrain the magnitude of the magnetic field. We find that a jet with density greater than $5\times 10^{-5}$ cm$^{-3}$ and hotspot magnetic field in the range 50-400 $��$G are required to explain the synchrotron emission at 43 GHz. The upper-energy cut-off in the hotspot synchrotron spectrum is at a frequency < $5\times 10^{14}$ Hz, indicating that the maximum energy of non-thermal electrons accelerated at the jet reverse shock is $E_{e, \rm max} \sim 0.8$ TeV in a magnetic field of 100 $��$G. Based on the condition that the magnetic-turbulence scale length has to be larger than the plasma skin depth, and that the energy density in non-thermal particles cannot violate the limit imposed by the jet kinetic luminosity, we show that $E_{e,\rm max}$ cannot be constrained by synchrotron losses as traditionally assumed. In addition to that, and assuming that the shock is quasi-perpendicular, we show that non-resonant hybrid instabilities generated by the streaming of cosmic rays with energy $E_{e, \rm max}$ can grow fast enough to amplify the jet magnetic field up to 50-400 $��$G and accelerate particles up to the maximum energy $E_{e, \rm max}$ observed in the Cygnus A primary hotspot., Accepted for publication in MNRAS

We report the appearance of a new radio source at a projected offset of 460 pc from the nucleus of Cygnus A. The flux density of the source (which we designate Cygnus A-2) rose from an upper limit of, 10 pages, 4 figures, accepted to ApJ. Includes some minor corrections, added references, and higher-resolution Figure 1

Recently published results from VLBI observations at 3 and 7 millimeters of the radio galaxy Cygnus A are reviewed in this article, and discussed within the model of a prominently stratified jet outflow. At the source redshift ( z = 0.056), mm-VLBI allows a spatial resolution down to 200 Schwarzschild radii to be achieved, providing an extremely detailed view of the two-sided jet base. Through a study of the kinematic properties of the flow and of its transverse structure, it is shown that the radio emission is produced by an accelerating, mildly relativistic, parabolically expanding disk-wind. The observed transverse stratification, both of the flux density and of the bulk speed, supports the presence of an invisible faster spine close to the jet axis, powered either by the inner regions of the accretion disk or by the spinning black hole.

Many-year measurements of the radio flux of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A were continued at 290 and 151.5 MHz. The new data are used together with previously published observations carried out at decameter, meter, centimeter, and millimeter wavelengths to derive the frequency dependence of the secular variation of the radio flux density of Cas A: $d_\nu [\% year^{ - 1} ] = - (0.63 \pm 0.02) + (0.04 \pm 0.01)\ln \nu [GHz] + (1.51 \pm 0.16) \times 10^{ - 5} (\nu [GHz])^{ - 2.1} $ . The observed slowing of the secular variations with decreasing frequency at decameter wavelengths can be explained by a decrease in the optical depth of a remnant HII zone around Cas A with time due to recombination of hydrogen atoms. The new derived frequency dependence for the rate of the secular decrease, absolute and relative measurements of the radio flux density of Cas A carried out over the last 25 years, and the absolute spectrum of Cyg A are used to construct the spectrum of Cas A in the range 5–250 000 MHz predicted for epoch 2015.5.

The low-frequency radio spectra of the hotspots within powerful radio galaxies can provide valuable information about the physical processes operating at the site of the jet termination. These processes are responsible for the dissipation of jet kinetic energy, particle acceleration, and magnetic-field generation. Here we report new observations of the powerful radio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183 MHz, at an angular resolution of ~3.5 arcsec. The radio emission of the lobes is found to have a complex spectral index distribution, with a spectral steepening found towards the centre of the source. For the first time, a turnover in the radio spectrum of the two main hotspots of Cygnus A has been directly observed. By combining our LOFAR imaging with data from the Very Large Array at higher frequencies, we show that the very rapid turnover in the hotspot spectra cannot be explained by a low-energy cut-off in the electron energy distribution, as has been previously suggested. Thermal (free-free) absorption or synchrotron self absorption models are able to describe the low-frequency spectral shape of the hotspots, however, as with previous studies, we find that the implied model parameters are unlikely, and interpreting the spectra of the hotspots remains problematic., Comment: 9 pages, 3 figures, accepted for publication in MNRAS

The extended jet structures of radio galaxies (RGs) represent an ideal acceleration site for High Energy Cosmic Rays (HECRs) and a recent model showed that the HECR data can be explained by these sources, if the arrival directions of HECRs at energies $\lesssim 8\,\text{EeV}$ from a certain RG, Cygnus A, are isotropized. First, this work introduces the inverted simulation setup in order to probe the isotropy assumption. Here, different extragalactic magnetic field models are compared showing that either a magnetic field of primordial origin that yields a high field strength in the large scale structures of the Universe is needed, or a significant contribution by a multitude of isotropically distributed sources. Secondly, the HECRs contribution by the bulk of RGs of different Fanaroff-Riley (FR) type is determined. Here, the most recent FR-type dependent radio-to-CR correlations $Q_{\rm cr}\propto L_{\rm radio}^{\beta_L}$ are used, and the impact of the slope $\beta_L$ on the HECRs is analyzed in detail. Finally, it is carved out that FR-II RGs provide a promising spectral behavior at the hardening part of the CR flux, between about $3\,\text{EeV}$ and $30\,\text{EeV}$, but most likely not enough CR power. At these energies, FR-I RGs can only provide an appropriate flux in the case of a high acceleration efficiency and $\beta_L\gtrsim 0.9$, otherwise these sources rather contribute below $3\,\text{EeV}$. Further, the required acceleration efficiency for a significant HECR contribution is exposed dependent on $\beta_L$ and the CR spectrum at the acceleration site.

We present the first direct imaging of what may be the thick torus in the active galactic nucleus (AGN) of the archetype powerful radio galaxy Cygnus A, using the Jansky Very Large Array at 18-48 GHz, with a resolution down to 45 mas. Such a torus has long been a key component of AGN models, but direct imaging on the relevant physical scales in sources of extreme (quasar-like) luminosities, remains scarce. An elongated structure, perpendicular to the radio jets and centered on the core, is well resolved, with a full length of 0.″48 (528 pc), and a full width of 0.″26 (286 pc). The radio emission spectrum is consistent with optically thin free-free emission. We present a toy model of a flaring torus, with a half-opening angle for the poloidal region of 62°. The radio jets are oriented along the poles. The observations require a clumpy gas distribution, with the free-free emission dominated by clumps with densities ≥4000 cm -3 . © 2019. The American Astronomical Society. All rights reserved..

Using the Irkutsk Incoherent Scattering Radar, it is demonstrated that the high sensitivity of such radars, which are usually used for studies of the Earth’s ionosphere, also enables their use in a passive mode for observations of astronomical radio sources. Observations of solar flares accompanied by coronal mass ejections and of quasi-stationary radio sources on the Sun have been carried out. In addition, scintillations of several of the brightest discrete radio sources (Cygnus A, Cassiopeia A, and the Crab Nebula) have been studied over several months. These data can also be useful for studies of the ionosphere and interplanetary space.

We present new XMM-Newton observations of the northwest (NW) radio relic region in the cluster Abell 3667. We detect a jump in the X-ray surface brightness and X-ray temperature at the sharp outer edge of the radio relic which indicate that this is the location of a merger shock with a Mach number of about 2. Comparing the radio emission to the shock properties implies that approximately 0.2% of the dissipated shock kinetic energy goes into accelerating relativistic electrons. This is an order of magnitude smaller than the efficiency of shock acceleration in many Galactic supernova remnants, which may be due to the lower Mach numbers of cluster merger shocks. The X-ray and radio properties indicate that the magnetic field strength in the radio relic is ≳ 3 µG, which is a very large field at a projected distance of ∼ 2.2 Mpc from the center of a cluster. The radio spectrum is relatively flat at the shock, and steepens dramatically with distance behind the shock. This is consistent with radiative losses by the electrons and the post-shock speed determined from the X-ray properties. The Cygnus A radio source is located in a merging cluster of galaxies. This appears to be an early-stage merger. Our recent Suzaku observation confirm the presence of a hot region between the two subclusters which agrees with the predicted shocked region. The high spectral resolution of the CCDs on Suzaku allowed us to measure the radial component of the merger velocity, Δvr ≈ 2650 km s–1. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study the jet and counterjet of the powerful classical double FRII radio galaxy Cygnus A as seen in the 5, 8 and 15-GHz radio bands using the highest spatial resolution and signal-to-noise archival data available. We demonstrate that the trace of the radio knots that delineate the jet and counterjet deviates from a straight line and that the inner parts can be satisfactorily fitted with the precession model of Hjellming & Johnston. The parameter values of the precession model fits are all plausible although the jet speed is rather low (< 0.5 c) but, on investigation, found to be consistent with a number of other independent estimates of the jet speed in Cygnus A. We compare the masses and precession periods for sources with known precession and find that for the small number of AGN with precessing jets the precession periods are significantly longer than those for microquasars., Accepted for publication by MNRAS, 8 pages; e-mail author for full resolution

The duty-cycle of powerful radio galaxies and quasars such as the prototype Cygnus A is poorly understood. X-ray observations of inverse-Compton scattered Cosmic Microwave Background (ICCMB) photons probe lower Lorentz-factor particles than radio observations of synchrotron emission. Comparative studies of the nearer and further lobes, separated by many 10s of kpc and thus 10s of thousands of years in light-travel time, yield additional temporal resolution in studies of the lifecycles. We have co-added all archival Chandra ACIS-I data and present a deep 200 ks image of Cygnus A. This deep image reveals the presence of X-ray emission from a counterjet i.e. a jet receding from Earth and related to a previous episode of jet activity. The non-thermal X-ray emission, we interpret as ICCMB radiation. There is an absence of any discernible X-ray emission associated with a jet flowing towards Earth. We conclude that: (1) The emission from a relic jet, indicates a previous episode of jet activity, that took place earlier than the current jet activity appearing as synchrotron radio emission. (2) The presence of X-ray emission from a relic counterjet of Cygnus A and the absence of X-ray emission associated with any relic approaching jet constrains the timescale between successive episodes of jet activity to ~10^6 years. (3) Transverse expansion of the jet causes expansion losses which shifts the energy distribution to lower energies. (4) Assuming the electrons cooled due to adiabatic expansion, the required magnetic field strength is substantially smaller than the equipartition magnetic field strength. (5) A high minimum Lorentz factor for the distribution of relativistic particles in the current jet, of a few 10^3, is ejected from the central nucleus of this active galaxy. Abridged., Comment: Accepted for publication by MNRAS, 8 pages Dates in Table 1 corrected

We present for the first time Very-Long-Baseline Interferometry images of the radio galaxy Cygnus A at the frequency of $86$ $\rm GHz$. Thanks to the high spatial resolution of only ${\sim}200$ Schwarzschild radii ($R_{\bf S}$), such observations provide an extremely detailed view of the nuclear regions in this archetypal object and allow us to derive important constraints for theoretical models describing the launching of relativistic jets. A pixel-based analysis of the jet outflow, which still appears two-sided on the scales probed, was performed. By fitting Gaussian functions to the transverse intensity profiles, we could determine the jet width in the nuclear region. The base of the jets appears wide. The minimum measured transverse width of ${\sim} (227\pm98)$ $R_{\bf S}$ is significantly larger than the radius of the Innermost Stable Circular Orbit, suggesting that the outer accretion disk is contributing to the jet launching. The existence of a faster and Doppler de-boosted inner section, powered either from the rotation of the inner regions of the accretion disk or by the spinning black hole, is suggested by the kinematic properties and by the observed limb brightening of the flow., Comment: 4 pages, 4 figures. Accepted for publication as a Letter in A&A

It is shown that composite radio spectra of the hot spots of the radio galaxy Cygnus A can be fully explained by assuming a nonuniform distribution of the magnetic fields inside the hot spots, without invoking any physical mechanisms other than synchrotron radiation. The magnetic fields are strong (B ∼ 10−2−10−1 G) at the center of the hot spots, and decreases at the hot-spot edges to the level of the magnetic field of the radio lobes in which the hot spots are embedded (B ∼ 10−4−10−5 G). The difference in the magnetic field between the hot-spot center and edge decreases during the evolution, while the average magnetic-field intensity increases.

Using deep Chandra ACIS observation data for Cygnus A, we report evidence of non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster medium (ICM). The diffuse X-ray emission, which are associated with the eastern and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The lobe spectra are reproduced with not only a single-temperature Mekal model, such as that of the surrounding ICM component, but also an additional power-law (PL) model. The X-ray flux densities of PL components for the eastern and western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and 52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are 1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal component is considered to be produced via the inverse Compton (IC) process, as is often seen in the X-ray emission from radio lobes. From a re-analysis of radio observation data, the multiwavelength spectra strongly suggest that the seed photon source of the IC X-rays includes both cosmic microwave background radiation and synchrotron radiation from the lobes. The derived parameters indicate significant dominance of the electron energy density over the magnetic field energy density in the Cygnus A lobes under the rich ICM environment., 8 pages, 5 figures, accepted for publication in ApJ

We study the particle energy distribution in the cocoon surrounding Cygnus A, using radio images between 151 MHz and 15 GHz and a 200 ks Chandra ACIS-I image. We show that the excess low frequency emission in the the lobe further from Earth cannot be explained by absorption or excess adiabatic expansion of the lobe or a combination of both. We show that this excess emission is consistent with emission from a relic counterlobe and a relic counterjet that are being re-energized by compression from the current lobe. We detect hints of a relic hotspot at the end of the relic X-ray jet in the more distant lobe. We do not detect relic emission in the lobe nearer to Earth as expected from light travel-time effects assuming intrinsic symmetry. We determine that the duration of the previous jet activity phase was slightly less than that of the current jet-active phase. Further, we explain some features observed at 5 and 15 GHz as due to the presence of a relic jet., Comment: Accepted for publication with MNRAS, 10 pages with 10 figures

We investigate the total kinetic powers (L_j) and ages (t_age) of powerful jets in FR II radio galaxies by comparison of the dynamical model of expanding cocoons with observations. We select four FR II radio sources (Cygnus A, 3C 223, 3C 284, and 3C 219), for which the mass-density profiles of intracluster medium (ICM) are known in the literature. It is found that large fractions > 0.02 - 0.7 of the Eddington luminosity (L_Edd) are carried away as a kinetic power of jet. The upper limit of estimated 2 L_j / L_Edd are larger than unity (< 10) for some sources, suggesting a possibility of super-Eddington mass accretions. As a consequence of the large powers, we also find that the total energy stored in the cocoon (E_c) exceeds the energy derived from the minimum energy condition for the energy of radiating non-thermal electrons and magnetic fields (E_min): 4< E_c /E_min, 28 pages, accepted for publication in ApJ

The Naval Research Laboratory and the National Radio Astronomy Observatory completed implementation of a low frequency capability on the VLA at 73.8 MHz in 1998. This frequency band offers unprecedented sensitivity (~25 mJy/beam) and resolution (~25 arcsec) for low-frequency observations. We review the hardware, the calibration and imaging strategies, comparing them to those at higher frequencies, including aspects of interference excision and wide-field imaging. Ionospheric phase fluctuations pose the major difficulty in calibrating the array. Over restricted fields of view or at times of extremely quiescent ionospheric ``weather'', an angle-invariant calibration strategy can be used. In this approach a single phase correction is devised for each antenna, typically via self-calibration. Over larger fields of view or at times of more normal ionospheric ``weather'' when the ionospheric isoplanatic patch size is smaller than the field of view, we adopt a field-based strategy in which the phase correction depends upon location within the field of view. This second calibration strategy was implemented by modeling the ionosphere above the array using Zernike polynomials. Images of 3C sources of moderate strength are provided as examples of routine, angle-invariant calibration and imaging. Flux density measurements indicate that the 74 MHz flux scale at the VLA is stable to a few percent, and tied to the Baars et al. value of Cygnus A at the 5 percent level. We also present an example of a wide-field image, devoid of bright objects and containing hundreds of weaker sources, constructed from the field-based calibration. We close with a summary of lessons the 74 MHz system offers as a model for new and developing low-frequency telescopes. (Abridged), 73 pages, 46 jpeg figures, to appear in ApJS

We present a comparative analysis of 5 GHz VLA and 200 ks Chandra ACIS-I image. In the 5 GHz image the familiar jet and much weaker counterjet are seen, which bend as the jet propagates towards the hotspots. Furthermore, where the lobe detected in 5 GHz emission starts to interact with the jet, we see that the jet "threads". In the 0.2-10 keV X-ray image we do not detect the jet, but do detect a relic of the counterjet. © 2007 Springer Science+Business Media B.V.

Radio flux measurements of the Crab nebula have been performed over many years relative to Orion A at 927 MHz and relative to Cygnus A and Virgo A at 151.5 MHz. The inferred average secular rates of decrease in the radio flux of the Crab nebula are d927 MHz = −0.18 ± 0.10% yr−1 over 1977–2000 and d151.5 MHz = −0.3 ± 0.1% yr−1 over 1980–2003. The weighted mean flux-decrease rate averaged over several years of relative measurements at 86, 151.5, 927, and 8000 MHz is dmw = −0.17 ± 0.02% yr−1. The secular flux decrease is frequency independent, with an upper limit of |dα/dt| < 3 × 10−4 yr−1 for the absolute value of the rate of change of the spectral index, and remains constant in time when averaged over long time intervals. The results of our measurements at 151.5 and 927 MHz combined with published absolute measurements at 81.5 and 8250 MHz are used to determine the radio spectrum of the Crab nebula for epoch 2010.0.

We present first results from the Near-infrared Integral Field Spectrograph (NIFS) located at Gemini North. For the active galaxies Cygnus A and Perseus A we observe rotationally-supported accretion disks and adduce the existence of massive central black holes and estimate their masses. In Cygnus A we also see remarkable high-excitation ionization cones dominated by photoionization from the central engine. In the T-Tauri stars HV Tau C and DG Tau we see highly-collimated bipolar outflows in the [Fe II] 1.644 micron line, surrounded by a slower molecular bipolar outflow seen in the H_2 lines, in accordance with the model advocated by Pyo et al. (2002)., Comment: Invited paper presented at the 5th Stromlo Symposium. 9 pages, 7 figures. Accepted for publication in Astrophysics & Space Science

Long-term measurements of the radio flux density of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A have been carried out at 290 and 927 MHz. We have obtained for the mean rates of the secular decrease of the radio emission of Cassiopeia A d290 MHz = −0.67 ± 0.04% year−1 for 1978–2005 and d927 MHz = −0.71 ± 0.035% year−1 for 1977–2004. The evolution of the radio spectrum of Cassiopeia A is traced based on long-term observations at 38, 151.5, 290, 927, and 2924 MHz.

High-resolution Very-Long-Baseline Interferometry observations of relativistic jets are essential to constrain fundamental parameters of jet formation models. At a distance of 249 Mpc, Cygnus A is a unique target for such studies, being the only Fanaroff-Riley Class II radio galaxy for which a detailed sub-parsec scale imaging of the base of both jet and counter-jet can be obtained. Observing at millimeter wavelengths unveils those regions which appear self-absorbed at longer wavelengths and enables an extremely sharp view towards the nucleus to be obtained. We performed 7 mm Global VLBI observations, achieving ultra-high resolution imaging on scales down to 90 $\mu$as. This resolution corresponds to a linear scale of only ${\sim}$400 Schwarzschild radii (for $M_{\mathrm{BH}}=2.5 \times 10^9 M_{\odot}$). We studied the kinematic properties of the main emission features of the two-sided flow and probed its transverse structure through a pixel-based analysis. We suggest that a fast and a slow layer, with different acceleration gradients, exist in the flow. The extension of the acceleration region is large (${\sim} 10^4 R_{\mathrm{S}}$), indicating that the jet is magnetically-driven. The limb brightening of both jet and counter-jet and their large opening angles ($\phi_\mathrm{J}{\sim} 10^{\circ}$) strongly favor a spine-sheath structure. In the acceleration zone, the flow has a parabolic shape ($r\propto z^{0.55\pm 0.07}$). The acceleration gradients and the collimation profile are consistent with the expectations for a jet in "equilibrium'' (Lyubarsky 2009), achieved in the presence of a mild gradient of the external pressure ($p\propto z^{-k}, k\leq2$).}, Comment: 9 pages, 8 figures. Accepted for publication in Astronomy and Astrophysics

Cygnus A, the nearest truly powerful radio galaxy, resides at the centre of a massive galaxy cluster. Chandra X-ray observations reveal its cocoon shocks, radio lobe cavities and an X-ray jet, which are discussed here. It is argued that X-ray emission from the outer regions of the cocoon shocks is nonthermal. The X-ray jets are best interpreted as synchrotron emission, suggesting that they, rather than the radio jets, are the path of energy flow from the nucleus to the hotspots. In that case, a model shows that the jet flow is non-relativistic and carries in excess of one solar mass per year., Comment: 6 pages, 5 figure. To appear in the Proceedings of the IAU Symposium No. 313: "Extragalactic jets from every angle," Galapagos, Ecuador, 15-19 September 2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowksa (eds.), Cambridge University Press

This paper presents observations of Cygnus A at 74 and 327 MHz at angular resolutions of approximately 10" and 3", respectively. These observations are among the highest angular resolutions obtained below 1000 MHz for this object. While the angular resolution at 74 MHz is not sufficient to separate clearly the hot spots from the lobes, guided by 151 and 327 MHz images, we have estimated the 74 MHz emission from the hot spots. We confirm that the emission from both the western and eastern hot spots flattens at low frequencies and that there is a spectral asymmetry between the two. For the eastern hot spot, a low-energy cutoff in the electron energy spectrum appears to explain the flattening, which implies a cutoff Lorentz factor \gamma_min ~ 300, though we cannot exclude the possibility that there might be a moderate level of free-free absorption. For the western hot spot, the current observations are not sufficient to distinguish between a free-free absorped power-law spectrum and a synchrotron self-absorbed spectrum., Comment: 7 pages, AASTeX, 3 figures; accepted in ApJL

Long-term measurements of the radio flux density of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A have been carried out at 38 MHz (1987–2004) and 151.5 MHz (1980–2004). Using other data from the literature, we find a secular decrease of the radio flux density of Cassiopeia A at the rates d(38 MHz) = −0.79 ± 0.14% yr−1 (for 1956–2004) and d(151.5 MHz) = −0.83 ± 0.04% yr−1 (for 1966–2004). Based on measurements made in 1997 and 1998 and data from the literature, this secular decrease at 81.5 MHz is d(81.5 MHz) = −0.86 ± 0.14% yr−1 (for 1966–1998). Absolute flux densities of Cassiopeia A at 38 and 151.5 MHz for epoch 2005.5 are calculated based on the relative flux density of Cassiopeia A and the spectrum of Cyg A, which is approximated using an empirical formula at meter and decameter wavelengths.

We have spectroscopically identified 77 new members of the Cygnus A cluster, bringing the total to 118 galaxies consistent with cluster membership. We use these data combined with the results from X-rays to deduce a dynamical model for the system. The data are consistent with a cluster-cluster merger viewed at a projection angle of 30-45 degrees, 0.2-0.6 Gyr prior to core passage. We estimate the richness of the combined cluster system at Abell richness class 2 or greater, suggesting the merger of two richness class 1 clusters., Comment: 41 pages, 7 figures, 2 tables, accepted AJ