Search

We report on Very Long Baseline Array polarimetric observations of NRAO 530 at 5, 8, 15, 22 and 43 GHz made during one week in 1997 February. We present the total intensity, the fractional polarization and the electric vector position angle (EVPA) distributions at all these frequencies. A model fitting has been performed to the full polarization visibility data. From this, the fitted southernmost component A is confirmed as the core of the radio structure with relatively high brightness temperature and hard spectrum between 15 and 43 GHz in comparison with the central component B of dominant flux. The relatively high degree of polarization for the component A may arise from its complex radio structure, which can be resolved at 86 GHz. In contrast, the component B shows a well-fitted power-law spectrum with a spectral index of about −0.5 ( ), and a linear correlation between EVPAs and wavelength square with an observed rotation measure of about −1062 rad m−2, indicating its structural singleness. Assuming that the component B has a comparable degree of polarization without depolarization at these frequencies, the decrease in fractional polarization with wavelength mainly results from opacity and Faraday rotation, in which the opacity plays quite a large role. A spine-sheath-like structure in fractional polarization ( m) is detected, covering almost the whole emission region at 5 and 8 GHz, with a degree of polarization relatively low along the jet spine, becoming higher towards two sides of the jet. The linear polarization at 5 GHz shows three separate polarized emission regions with alternately aligned and orthogonal polarization vectors down the jet. The polarization goes to zero between the top two regions, with the highest polarization level occurring at the top and bottom. The 5- and 8-GHz images show EVPA changes across the width of the jet as well as along the jet. These complex polarimetric properties can be explained in terms of either the presence of a large helical magnetic field or tangled magnetic fields compressed and sheared down the jet. These can be further determined by multifrequency polarimetric very long baseline interferometry observations with sufficient high resolution and sensitivity spanning an appropriate frequency range. [ABSTRACT FROM AUTHOR]

We present results on the compact steep-spectrum quasar 3C 48 from observations with the Very Long Baseline Array (VLBA), the Multi-Element Radio Linked Interferometer Network (MERLIN) and the European Very long baseline interferometry (VLBI) Network (EVN) at multiple radio frequencies. In the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright knots. The active nucleus is embedded in the southernmost VLBI component A, which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz, respectively. A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly polarized VLBI components are located at component C ∼ 0.25 arcsec north of the core, where the jet starts to bend to the north-east. The polarization angles at C show gradual changes across the jet width at all observed frequencies, indicative of a gradient in the emission-weighted intrinsic polarization angle across the jet and possibly a systematic gradient in the rotation measure; moreover, the percentage of polarization increases near the curvature at C, likely consistent with the presence of a local jet–interstellar-medium interaction and/or changing magnetic-field directions. The hot spot B shows a higher rotation measure, and has no detected proper motion. These facts provide some evidence for a stationary shock in the vicinity of B. Comparison of the present VLBI observations with those made 8.43 yr ago suggests a significant northward motion for A2 with an apparent transverse velocity . The apparent superluminal motion suggests that the relativistic jet plasma moves at a velocity of if the jet is viewed at an inclination angle less than . A simple precessing jet model and a hydrodynamical isothermal jet model with helical-mode Kelvin–Helmholtz instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined by the bright knots. [ABSTRACT FROM AUTHOR]

We present a new interpretation of the optical knot, HST-1, in the jet of M 87. High-sensitivity 22 GHz Very Large Array images have located HST-1 to within 6 mas of the jet axis immediately upstream. Based on 1.7 GHz Very Long Baseline Array images of a bright flare in 2005, we see that preponderance of emission in the early stages originates from an elongated region that is tilted 12.5° from the jet axis. The superluminal motion, shape, location, and the large jet-aligned optical/UV polarization suggest an identification with the putative relativistic spine of the jet. As such, energy flux estimates for HST-1, ∼870 mas from the nucleus, published in 2006, indicate that the central engine injected, Qspine ≈ 2.5 × 1041 ergs s−1, into the base of the spine about 200 yr earlier. Furthermore, previous studies have revealed a tubular protonic jet on sub-mas scales that envelopes a low luminosity core, presumably the faint spine base. It was estimated that the central engine injected, Qtubular jet ≈ 6.1 × 1041 ergs s−1, about 1.5 yr earlier. If one component of the jet is inherently more powerful, a firm constraint on total jet power in the recent past would then exist. If the emitted jet is inherently dominated by the spine (tubular jet), then the total bilaterally symmetric jet power emitted from the central engine was < 4Qspine ≈ 1.0 × 1042 ergs s−1 (< 4Qtubular jet ≈ 2.4 × 1042 ergs s−1) ∼200 (∼1.5) yr earlier. Assuming a nearly constant central engine injected jet power for ∼200 yr indicates a total jet power of ≲2 × 1042 ergs s−1 in epochs of modern observation or ≲3.5% jet production efficiency for an accretion rate of 0.001 M⊙ yr−1. Seemingly, the focus of Event Horizon Telescope Collaboration (EHTC) numerical models should be biased toward jet powers of ≲2 × 1042 ergs s−1, as opposed to larger estimates from ejections many centuries or millennia earlier. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter Array (ALMA) serendipitously detected H2O |$J_{K_{\rm a}, K_{\rm c}} = 10_{2,9}$| –93, 6 emission at 321 GHz in NGC 1052. This is the first submillimeter maser detection in a radio galaxy and the most luminous 321 GHz H2O maser known to-date with the isotropic luminosity of |$1090\, L_{\odot }$|⁠. The line profile consists of a broad velocity component with FWHM = 208 ± 12 km s−1 straddling the systemic velocity and a narrow component with FWHM = 44 ± 3 km s−1 blueshifted by 160 km s−1. The profile is significantly different from the known 22 GHz 61, 6–52, 3 maser which shows a broad profile redshifted by 193 km s−1. The submillimeter maser is spatially unresolved with a synthesized beam of |${0{^{\prime \prime}_{.}}68} \times {0{^{\prime \prime}_{.}}56}$| and coincides with the continuum core position within 12 pc. These results indicate amplification of the continuum emission through high-temperature (>1000 K) and dense [ n (H2O) > 104 cm−3] molecular gas in front of the core. [ABSTRACT FROM AUTHOR]

Active galactic nuclei show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures in the radio spectral energy distribution. Tracing these signatures requires resolved spectral index measurements over a broad frequency range. We present such a study for the radio galaxy 3C 293. Using the International Low Frequency Array Telescope (ILT), we probe spatial scales as fine as ∼0.2″ at 144 MHz and to constrain the spectrum, we combine these data with Multi‐Element Radio Linked Interferometer Network and Very Large Array archival data at frequencies up to 8.4 GHz. In the inner lobes (∼2 kpc), we detect the presence of a compact steep spectrum (CSS) source with a spectral turnover at ∼225 MHz, caused by free‐free absorption from the rich surrounding medium. We find evidence of interaction of the lobes with the rich interstellar medium of the host galaxy. The outer lobes (extending up to ∼100 kpc) have a remarkably uniform spatial distribution of spectral index and only mild spectral curvature (of ∼0.2). Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub‐group of restarted galaxies with short interruption time periods (≲1 Myr). This is the first time a spatially resolved study has been performed that simultaneously studies a young CSS source as well as the older outer lobes at such low frequencies. [ABSTRACT FROM AUTHOR]

The peaked spectrum (PS) radio sources are sub‐kpc in size and have a peak in the radio spectrum. The compact steep specrum (CSS) sources range from about 1 kpc to an adopted upper limit on projected linear size of 20 kpc. Because of their galactic‐scale range of size, the PS and CSS radio sources interact strongly with the ISM of this host galaxy. This means that the PS and CSS sources can be a significant source of the AGN feedback. The PS and CSS sources are a key phase in radio source evolution. Yet, there is substantial uncertainty regarding the origin of these sources, their lifetimes, and their evolution. There are many interesting open questions regarding the PS and CSS radio sources. Here, in this short introduction to the topic, I focus on the origin stories for these sources and how progress might be made. [ABSTRACT FROM AUTHOR]

Our recently reported lack of Intra-Night Optical Variability (INOV) among Broad-Absorption-Line (BAL) quasars exhibiting some blazar-like radio properties, either questions polar ejection of BAL clouds, and/or hints at a physical state of the relativistic jet modified due to interaction with the thermal BAL wind on the innermost sub-parsec scale. As a robust check on this unexpected finding for the BAL_blazar candidates, we report here the INOV study of a new and much more rigorously defined comparison sample consisting of 9 normal (non-BAL) blazars, matched in both magnitude and redshift to the aforementioned sample of BAL_blazar candidates. The present campaign spanning 27 sessions yields an INOV duty cycle of ∼23 per cent for the comparison sample of normal blazars, employing the enhanced F-test. However, even this more sensitive test does not detect INOV for the sample of BAL_blazar candidates. This stark INOV contrast found here between the BAL_blazar candidates and normal blazars can probably be traced to a physical interaction of the relativistic jet with the thermal wind, within sub-parsec range from the nucleus. The consequent enfeebling of the jet would additionally explain the striking deficiency among BAL quasars of powerful FR II radio lobes on the much larger scale of 10 – 100 kpc. [ABSTRACT FROM AUTHOR]

We report the detection of HI 21-cm absorption in a member of the rare and recently discovered class of compact radio sources: extremely inverted spectrum extragalactic radio sources (EISERS). The EISERS conceivably form a special subclass of the inverted spectrum radio galaxies since the spectral index of the optically thick part of the spectrum for these sources crosses the synchrotron self-absorption limit of α = +2.5 (S(ν) ∝ να). We searched for HI absorption in two EISERS using the recently upgraded Giant Metrewave Radio Telescope (uGMRT) and detected an absorption feature in one of them. The strong associated HI absorption detected against the source J1209−2032 (z = 0.4040) implies an optical depth of 0.178 ± 0.02, corresponding to an HI column density of 34.8 ± 2.9 × 1020 cm−2, for an assumed HI spin temperature of 100 K and covering factor of 1. This is among the highest known optical depth and HI column densities found for compact radio sources of peaked spectrum type and supports the free-free absorption model for the steeply inverted radio spectrum of this source. For the other source, J1549+5038 (z = 2.171), no HI absorption was detected in our observations. [ABSTRACT FROM AUTHOR]

Context. As the Square Kilometer Array (SKA) is expected to be operational in the next decade, investigations of the radio sky in the range of 100 MHz–10 GHz have become important for simulating SKA observations. In determining physical properties of galaxies from radio data, the radio spectral energy distribution (SED) is often assumed to be described by a simple power law, usually with a spectral index of 0.7 for all sources. Even though radio SEDs have been shown to exhibit deviations from this assumption, both in differing spectral indices and complex spectral shapes, it is often presumed that their individual differences can be canceled out in large samples. Aims. Since the average spectral index around 1 GHz (observed-frame) is important for determining physical properties of large samples of galaxies, we aim to test whether individual differences in the spectra of radio-identified active galactic nuclei align with the simple assumption of α = 0.7 and test the evolution of the parameters of the synchrotron aging model with redshift and radio luminosity. Methods. We use a sample of 744 radio-excess active galactic nuclei (RxAGN), defined as those that exhibit more than a 3σ radio luminosity excess with respect to the value expected only from the contribution from star formation, out to z ∼ 4. We constructed their average radio SED by combining Very Large Array (VLA) observations of the COSMOS field at 1.4 GHz and 3 GHz with Giant Meterwave Radio Telescope (GMRT) observations at 325 MHz and 610 MHz. To account for nondetections in the GMRT maps, we employed the survival analysis technique. We binned the RxAGN sample into luminosity- and redshift-complete subsamples. In each bin, we constrained the shape of the average radio SED by fitting a broken power-law model. Results. We find that the RxAGN sample can be described by a spectral index of α1 = 0.28 ± 0.03 below the break frequency νb = (4.1 ± 0.2) GHz and α2 = 1.16 ± 0.04 above it, while a simple power-law model, capturing fewer spectral features, yields a single spectral index of 0.64 ± 0.07. By binning in 1.4 GHz of radio luminosity and redshift, we find that the power-law spectral index is positively correlated with redshift and that the broken power-law spectral index above 4 GHz is positively correlated with both the redshift and source size. By selecting sources with sizes less than 1 kpc, we find a subsample of flat-spectrum sources, which can be described by a spectral index of α = 0.41 ± 0.07 and a broken power-law spectral index of α1 = 0.1 ± 0.1 (α2 = 0.55 ± 0.09) below (above) a break frequency of νb = (2.7 ± 0.5) GHz. Conclusions. We have constrained the radio SED for a sample of RxAGN in the COSMOS field using available VLA and GMRT data, corresponding to the rest-frame frequency range from ∼0.3 GHz to ∼10 GHz. We describe our derived average radio SED of RxAGN using power-law and broken power-law models, yielding a radio SED that steepens above ∼4 GHz. [ABSTRACT FROM AUTHOR]

Context. High-resolution very long baseline interferometry (VLBI) observations of NGC 1052 show a two sided jet with several regions of enhanced emission and a clear emission gap between the two jets. This gap shrinks with increasing frequency and vanishes around ν ∼ 43 GHz. The observed structures are due to both the macroscopic fluid dynamics interacting with the surrounding ambient medium including an obscuring torus and the radiation microphysics. In order to model the observations of NGC 1052 via state-of-the art numerical simulations both the fluid-dynamical and emission processes have to be taken into account. Aims. In this paper we investigate the possible physical conditions in relativistic jets of NGC 1052 by directly modelling the observed emission and spectra via state-of-the-art special-relativistic hydrodynamic (SRHD) simulations and radiative transfer calculations. Methods. We performed SRHD simulations of over-pressured and pressure-matched jets using the special-relativistic hydrodynamics code Ratpenat. To investigate the physical conditions in the relativistic jet we coupled our radiative transfer code to evolutionary algorithms and performed simultaneous modelling of the observed jet structure and the broadband radio spectrum. During the calculation of the radiation we consider non-thermal emission from the jet and thermal absorption in the obscuring torus. In order to compare our model to VLBI observations we take into account the sparse sampling of the u-v plane, the array properties and the imaging algorithm. Results. We present for the first time an end-to-end pipeline for fitting numerical simulations to VLBI observations of relativistic jets taking into account the macro-physics including fluid dynamics and ambient medium configurations together with thermal and non-thermal emission and the properties of the observing array. The detailed analysis of our simulations shows that the structure and properties of the observed relativistic jets in NGC 1052 can be reconstructed by a slightly over-pressured jet (dk ∼ 1.5) embedded in a decreasing pressure ambient medium [ABSTRACT FROM AUTHOR]

Context. Few active galactic nuclei (AGN) reveal double-sided jet systems. However, these systems are crucial to understand basic physical properties of extragalactic jets. Aims. We address the questions whether jets in AGN are symmetric in nature, how well they are collimated on small scales, and how they evolve with time. Methods. We monitored the sub-parsec scale morphology of NGC 1052 with the Very Long Baseline Array at 43 GHz from 2005 to 2009. Results. A detailed study of 29 epochs show a remarkable asymmetry between both jets. A kinematic analysis of the outflows reveals higher apparent velocities for the eastern (approaching) jet as compared to the western (receding) jet, i.e., βej = 0.529 ± 0.038 and βwj = 0.343 ± 0.037, respectively. Contradictory to previous studies, we find higher flux densities for the western jet as compared to the eastern. The distribution of brightness temperature and jet width features well-collimated jets up to 1 mas distance to the dynamic center and a nearly conical outflow further outward. By combining flux density ratios and velocities of the jet flows, we were unable to find a combination of intrinsic velocities and inclination angles of the jets that is consistent for all four years of observation; this contradicts findings for symmetrically evolving jets. Spectral index maps between quasi-simultaneous 22 GHz and 43 GHz observations support the existence of an optically thick absorber covering the innermost ≃1.6 mas around the 43 GHz central feature and an optically thin jet emission with a spectral index of ≤−1. Conclusions. Our results fit into a picture in which we expect larger internal energy and/or magnetic flux in the western jet and higher kinetic energy in the eastern jet. Previous observations at lower frequencies have found slower velocities of the moving jet features as compared to this work. Considering the different velocities in different areas, we suggest a spine-sheath structure with a faster inner layer and slower outer layer. [ABSTRACT FROM AUTHOR]

We report on the variation in the optical polarization of the blazar PKS 1749+096 observed in 2008-2015. The degree of polarization (PD) tends to increase in short flares, having a time-scale of a few days. The object favors a polarization angle (PA) of 40°-50° at the flare maxima, which is close to the position angle of the jet (20°-40°). Three clear polarization rotations were detected in the negative PA direction associated with flares. In addition, a rapid and large decrease in the PA was observed in the other two flares, while another two flares showed no large PA variation. The light-curve maxima of the flares possibly tended to lag behind the PD maxima and color-index minima. The PA became -50° to -20° in the decay phase of active states, which is almost perpendicular to the jet position angle. We propose a scenario to explain these observational features, where transverse shocks propagate along curved trajectories. The favored PA at the flare maxima suggests that the observed variations were governed by the variations in the Doppler factor, δ. Based on this scenario, the minimum viewing angle of the source, θmin=4°.8-6°, and the location of the source, Δr ≥ 0.1pc, from the central black hole were estimated. In addition, the acceleration of electrons by the shock and synchrotron cooling would have a time-scale similar to that of the change in δ. The combined effect of the variation in δ and acceleration/cooling of electrons is probably responsible for the observed diversity of the polarization variations in the flares. [ABSTRACT FROM AUTHOR]

The compact and, with $${\sim }4.3\pm 0.3\times 10^6$$ M $$_{\odot }$$ , very massive object located at the center of the Milky Way is currently the very best candidate for a supermassive black hole (SMBH) in our immediate vicinity. The strongest evidence for this is provided by measurements of stellar orbits, variable X-ray emission, and strongly variable polarized near-infrared emission from the location of the radio source Sagittarius A* (SgrA*) in the middle of the central stellar cluster. Simultaneous near-infrared and X-ray observations of SgrA* have revealed insights into the emission mechanisms responsible for the powerful near-infrared and X-ray flares from within a few tens to one hundred Schwarzschild radii of such a putative SMBH. If SgrA* is indeed a SMBH it will, in projection onto the sky, have the largest event horizon and will certainly be the first and most important target for very long baseline interferometry observations currently being prepared by the event horizon telescope (EHT). These observations in combination with the infrared interferometry experiment GRAVITY at the very large telescope interferometer and other experiments across the electromagnetic spectrum might yield proof for the presence of a black hole at the center of the Milky Way. The large body of evidence continues to discriminate the identification of SgrA* as a SMBH from alternative possibilities. It is, however, unclear when the ever mounting evidence for SgrA* being associated with a SMBH will suffice as a convincing proof. Additional compelling evidence may come from future gravitational wave observatories. This manuscript reviews the observational facts, theoretical grounds and conceptual aspects for the case of SgrA* being a black hole. We treat theory and observations in the framework of the philosophical discussions about '(anti)realism and underdetermination', as this line of arguments allows us to describe the situation in observational astrophysics with respect to supermassive black holes. Questions concerning the existence of supermassive black holes and in particular SgrA* are discussed using causation as an indispensable element. We show that the results of our investigation are convincingly mapped out by this combination of concepts. [ABSTRACT FROM AUTHOR]

The Seyfert galaxy NGC985 is known to show a high-frequency excess in its radio continuum spectrum at a milli-Jansky level on the basis of previous observations at 1.4°C 15 GHz; a steep spectrum at low frequencies (a spectral index, α =.1.10±0.03) changes at ~10 GHz into an inverted spectrum at higher frequencies (α = +0.86±0.09). We conduct new observations at 15-43 GHz using the Very Large Array and at 100 GHz using the Nobeyama Millimeter Array. As a result, the high-frequency excess has been confirmed as continuing at even higher radio frequencies, up to 43 GHz. The non-detection at 100 GHz was not so strong a constraint, and therefore the spectral behavior above 43 GHz remains unclear. The astrometric position of the high-frequency excess component coincideswith the optical position of the Seyfert nucleus and the low-frequency radio position to an accuracy of 0. 1, corresponding to 80 pc; the radio source size is constrained to be <0. 02, corresponding to <16 pc. We discuss the physical origin of the observed highfrequency excess component. Dust emission at the Rayleigh?CJeans regime, free-free emission from X-ray radiating high-temperature plasma, free-free emission from the ensemble of broad-line region clouds, or thermal synchrotron from hot accretion flow cannot be responsible for the observed radio flux. Compact jets under synchrotron selfabsorption may be unlikely in terms of observed time scales. Alternatively, we cannot rule out the hypotheses of synchrotron jets free?Cfree absorbed by a circumnuclear photoionized region, and self-absorbed nonthermal synchrotron from disk corona, as the origin of the high-frequency excess component. [ABSTRACT FROM AUTHOR]

The GHz-Peaked Spectrum (GPS) quasar OQ172 (J1445+0958) has an extremely high rest-frame rotation measure (RM > 20000 rad m-2) and an RM gradient in its inner nucleus. Its jet observed with Very Long Baseline Interferometry (VLBI) is strongly bent along an arc spanning > 100 pc. Near infrared (NIR) spectra reveal an unusually large [O III] line width which suggests a large mass within the NLR and/or strong interactions between the emerging jet and the dense material therein. We present our VLBI Space Observatory Programme (VSOP) and Multi-frequency Very Long Baseline Array (VLBA) observation of the GPS quasar OQ172. The observations will help us to explore the VLBI radio properties and to better understand the circumnuclear environment of OQ172. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We present K-band (22 GHz) properties of 10 central AGNs in galaxy clusters of various dynamic states. The data were obtained using the Korean VLBI Network (KVN) with a goal to study how the AGN hosted by the central cluster galaxy interacts with the cluster core while undergoing feedback. In KVN single-dish observations, 9 out of 10 targets are detected. Using our K-band flux and archival low-frequency radio data, we evaluate spectral indices of the sample. We find a rough trend that central AGNs in the cluster with shorter cooling timescale show a more flat radio spectrum. This result is suggestive of young synchrotron age of the AGNs located in the cluster with a cool core. High resolution KVN VLBI follow up observations indicate some hints of the presence of parsec-scale jets only in the cool core cluster AGNs. To-gether with our finding from KVN single-dish observations, the parsec-scale jet morphology of the sample suggests that the AGNs in cool-core clusters are more recently (re)activated compared to their counterparts in non cool-core clusters. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources represent a large fraction of the extragalactic objects in flux density-limited samples. They are compact, powerful radio sources whose synchrotron peak frequency ranges between a few hundred MHz to several GHz. CSS and GPS radio sources are currently interpreted as objects in which the radio emission is in an early evolutionary stage. In this contribution I review the radio properties and the physical characteristics of this class of radio sources, and the interplay between their radio emission and the ambient medium of the host galaxy. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Detailed and long-term VLBI (Very Long Baseline Interferometry) studies of the variable jets of supermassive black holes helps us to understand the emission processes of these fascinating phenomena. When observed and traced precisely, jet component kinematics reveals details about the potential motion of the jet base. Following this motion over decades with VLBI monitoring reveals - in some cases - the signatures of precession. While several processes can cause precession, the most likely cause seems to be a supermassive binary black hole in the central region of the AGN. We present examples of the analysis of high-resolution VLBI observations which provides us with insight into the physics of these objects and reveals evidence for the presence of double black hole cores. EHT (Event Horizon Telescope) observations will probably soon tell us more about the jet origin and launching mechanism at the very centers of nearby active galactic nuclei. An important question to be addressed by the EHT and related observations will be whether Sgr A*, the supermassive black hole in the Galactic Center, has a jet as well. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We investigated the archival ground-based VLBI images of the extragalactic radio sources included in both the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck catalogues, and selected 49 bright and compact sources as potential targets for space Very Long Baseline Interferometry (VLBI) observations at mm wavelengths. These sources have a flat radio continuum spectrum between 33 and 94 GHz. They are identified as core-dominated active galactic nuclei (AGN), located at declinations above −40, and have never been observed with ground-based VLBI at 86 GHz. The radio properties of the 49 new sources are presented. We compare this new sample with similar samples of compact AGN available from earlier studies. The new candidates, together with the existing bright compact AGN sample identified from 86-GHz ground-based VLBI imaging surveys, form a catalogue of more than 160 AGN. These could be primary targets for mm-VLBI observations on the ground, as well as for future mm-wavelength space VLBI missions such as the project with two satellites currently under study in China. [ABSTRACT FROM AUTHOR]

Light curves are usually constructed from discrete observational data by interpolation. In most cases, the observation data is temporally uneven, and therefore the light curve is usually derived by the interpolation of the binned data with the spline function, which is intended for reducing the 'high sample noise' (i.e., the variability in the timescales comparable with the bin width). Such a practice of course reduces the time resolution of the light curve. It is known that function approximation is one of the most important applications of the artificial neural networks (ANN). In this work, for the first time we tentatively use the ANN to construct light curves from unevenly sampled variability data. To demonstrate the advantages of ANN for signal reconstruction over commonly used cubic spline function scheme, two sets of simulated periodic functions are used with random noises of varying magnitudes, one single frequency based and one multiple (two) frequency based. These signal reconstruction tests show that the ANN is clearly superior to the cubic spline scheme. As a case study, we use the uneven long-term multi-band monitoring data of BL lacertae to derive the light curves with ANN. It is found that the light curves derived with ANN have higher time resolution than those with the cubic spline function adopted in previous works. We recommend using ANN for the signal reconstruction in astrophysical data analysis as well as that of in other fields. [ABSTRACT FROM AUTHOR]

Aims. The radio galaxy NGC1275, recently identified as a very high energy (VHE, >100 GeV) γ-ray emitter by MAGIC, is one of the few non-blazar active galactic nuclei detected in the VHE regime. The purpose of this work is to better understand the origin of the γ-ray emission and locate it within the galaxy. Methods. We studied contemporaneous multifrequency observations of NGC1275 and modeled the overall spectral energy distribution. We analyzed unpublished MAGIC observations carried out between October 2009 and February 2010, and the previously published observations taken between August 2010 and February 2011. We studied the multiband variability and correlations by analyzing data of Fermi-LAT in the 100 MeV-100 GeV energy band, as well as Chandra (X-ray), KVA (optical), and MOJAVE (radio) data taken during the same period. Results. Using customized Monte Carlo simulations corresponding to early MAGIC stereoscopic data, we detect NGC1275 also in the earlier MAGIC campaign. The flux level and energy spectra are similar to the results of the second campaign. The monthly light curve above 100 GeV shows a hint of variability at the 3.6σ level. In the Fermi-LAT band, both flux and spectral shape variabilities are reported. The optical light curve is also variable and shows a clear correlation with the γ-ray flux above 100 MeV. In radio, three compact components are resolved in the innermost part of the jet. One of these components shows a similar trend as the Fermi-LAT and KVA light curves. The γ-ray spectra measured simultaneously with MAGIC and Fermi-LAT from 100 MeV to 650 GeV can be well fitted either by a log-parabola or by a power-law with a subexponential cutoff for the two observation campaigns. A single-zone synchrotron-self-Compton model, with an electron spectrum following a power-law with an exponential cutoff, can explain the broadband spectral energy distribution and the multifrequency behavior of the source. However, this model suggests an untypical low bulk-Lorentz factor or a velocity alignment closer to the line of sight than the parsec-scale radio jet. [ABSTRACT FROM AUTHOR]

We carried out the investigation of the properties of features seen within superluminal sources often referred to as components. Our result indicates a fairly strong correlation of r∼0.76 between component radial distance L and component size ℜ. Assumption of simple ballistic motion and free adiabatic expansion, enabled us to use the observed jet component parameters to constrain the Doppler factor, Lorentz factor and the lower limit to the viewing angle with respect to a distant observer. The estimated average Doppler factor, Lorentz factor and viewing angle respectively are 10.3±5.0, 18.3±6.2 and 3.7±2.3 for Γ=4/3; while the values obtained for Γ=5/3 are 12.2±5.9,17.2±5.1 and 2.9±1.6, where Γ is the adiabatic index. The large scatter in our results may be due to the uncertainties introduced by the assumptions made. [ABSTRACT FROM AUTHOR]

Aims. A long-timeline kinematic study of the archetypal compact symmetric object (CSO) OQ 208 sheds light on the physical properties of the most compact radio sources. Methods. Archival data from the Very Long Baseline Array (VLBA) at 15 GHz over a time span of 13.6 yr were used to investigate the kinematics of the radio source. The flux density monitoring data obtained at the Michigan 26-m radio telescope were also used as supplementary information for analyzing the geometry of the radio structure. Results. At 2.3-GHz, the radio emission is dominated by two mini-lobes separated by ~10 pc in a northeast-southwest (NE-SW) direction. At 8.4 and 15 GHz, each lobe is further resolved into two subcomponents, which are identified as hotspots. A knotty jet is linked with the NE hotspot and traces back toward the geometric center. The core is too weak to be detected. Significant flux density variation is found in the primary hotspots with a maximum level of 62% (NE1) and 19% (SW1). The flare epoch of NE1 is earlier than that of SW1 by approximately 5.00 yr, suggesting that the northeast lobe is advancing and the southwest lobe is receding. This light travel difference indicates a radial distance difference between the two hotspots of 1.53 pc, which indicates an inclination angle of about 80.8 degrees between the radio jet and the line of sight. The angular separation rate between NE1 and SW1 is 0.027 mas yr-1, corresponding to a projected speed of 0.133 c. The inner jet knot (J1) moves at 0.047 mas yr-1 (or 0.230 c), about 3.5 times the hotspot advancing speed. Conclusions. The large viewing angle and the modest jet speed suggest a mildly relativistic jet. The jet axis is close to the plane of the sky. The separation rate and the distance between the two primary hotspots result in a kinematic age of 255 ± 17 yr, confirming that OQ 208 is indeed a young radio source. In addition to the hotspot advancing motions, sideways motions provide evidence that the lobes are obstructed by the external interstellar medium. [ABSTRACT FROM AUTHOR]

While observations in the radio band are providing essential information on the innermost structures of relativistic jets in active galactic nuclei (AGN), the recent detection by Fermi of gamma-ray emission from many hundreds of blazars shows that the maximum jet power is emitted at high energies. Multi-wavelength monitoring observations further allow variability studies of the AGN spectral energy distributions over 13 orders of magnitude in frequency. The Joint Discussion offered the possibility for a comprehensive discussion of advances in the observational domain and stimulated theoretical discussion about our current understanding of jet physics. [ABSTRACT FROM PUBLISHER]

ABSTRACT The radio galaxy 3C 84 is a representative of γ-ray-bright misaligned active galactic nuclei (AGNs) and one of the best laboratories to study the radio properties of the subparsec jet in connection with the γ-ray emission. In order to identify possible radio counterparts of γ-ray emissions in 3C 84, we study the change in structure within the central 1 pc and the light curve of subparsec-sized components C1, C2 and C3. We search for any correlation between changes in the radio components and the γ-ray flares by making use of the Very Long Baseline Interferometry (VLBI) and single-dish data. Throughout the radio monitoring spanning over two GeV γ-ray flares detected by the Fermi Large Area Telescope and the MAGIC Cherenkov Telescope during 2009 April-May and 2010 June-August, the total flux density in the radio band increases on average. This flux increase mostly originates in C3. Although γ-ray flares span the time-scale of days to weeks, no clear correlation with the radio light curve on this time-scale is found. No new prominent components and change in morphology associated with the γ-ray flares are found on VLBI images. [ABSTRACT FROM AUTHOR]

A new paradigm for active galactic jet kinematics has emerged through detailed investigations of BL Lac objects using very long baseline radio interferometry. In this new scheme, most, if not all, jet components appear to remain stationary with respect to the core but show significant non-radial motions. This paper presents results from our kinematic investigation of the jets of a statistically complete sample of radio-loud flat-spectrum active galaxies, focusing on the comparison between the jet kinematic properties of BL Lacs and flat-spectrum radio-quasars. It is shown that there is a statistically significant difference between the kinematics of the two AGN classes, with BL Lacs showing more bent jets, that are wider and show slower movement along the jet axis, compared to flat-spectrum radio-quasars. This is interpreted as evidence for helically structured jets (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We conducted very long baseline interferometry observations of five radio-loud narrow-line Seyfert 1 (NLS1) galaxies in milliarcsecond resolutions at 1.7 GHz (A 18 cm) using the Very Long Baseline Array. Significant parsec (pc) scale structures were revealed for three out of the five sources with high brightness temperature by direct imaging; this is convincing evidence for nonthermal jets. FBQS J1644+2619 with an inverted spectrum showed a prominent one-sided linear structure, indicating Doppler beaming with an intrinsic jet speed of >0.74c. FBQS J1629+4007, also with an inverted spectrum, showed rapid flux variability, indicating Doppler beaming with an intrinsic jet speed of >0.88c. Thus, we found convincing evidence that these two NLSls can generate at least mildly or highly relativistic jets, which may make them apparently radio loud even if they are intrinsically radio quiet. On the other hand, the other three NLSls had steep spectra and two of them showed significantly diffuse pc-scale structures, which were unlikely to be strongly beamed. Thus, some NLS1 s have the ability to generate jets strong enough to make them intrinsically radio loud without Doppler beaming. NLS Is as a class show a number of extreme properties and radio-loud ones are very rare. We build on these radio results to understand that the central engines of radio-loud NLSls are essentially the same as that of other radio-loud active galactic nuclei in terms of the formation of nonthermal jets. [ABSTRACT FROM AUTHOR]

We present a new prediction of GeV γ-ray emission from radio lobes of young active galactic nuclei jets. In the previous work of Kino, Kawakatu & Ito, 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 ∼GeV, and they cool down to ∼MeV by the adiabatic loss. Correspondingly, the lobes initially yield bright bremsstrahlung luminosity in ∼GeV range and they fade out. We estimate these γ-ray emissions and show that nearby young radio lobes could be detected with Fermi Gamma-ray Space Telescope. [ABSTRACT FROM AUTHOR]

The flux and the spectral index in the X-ray energy band from the knots of the M87 jet as observed by Chandra indicate a possible synchrotron origin but cannot be explained by simple one-zone models with continuous injection of non-thermal electrons. In this Letter we propose a two-zone model to explain the observed spectra of the knots of the M87 jet. We consider the synchrotron emission from a region with a tangled magnetic field where relativistic non-thermal electrons are continuously injected in from an associated acceleration region. The acceleration region is assumed to be a compact zone possibly around a shock front. A power-law distribution of electrons is injected into the acceleration region and accelerated to a maximum energy determined by the acceleration time-scale and the loss processes. With the present model we are able to explain the overall broad-band features of the knots of the M87 jet. Also the present model predicts a change in spectral index at ultraviolet energies, and future observations at these energies, can be used to constrain the parameters involved in the model. [ABSTRACT FROM AUTHOR]

We present a kinematic study of the sub-parsec-scale radio jet of the radio galaxy 3C 84/NGC 1275 with the VLBI Exploration of Radio Astrometry (VERA) array at 22 GHz for 80 epochs from 2007 October to 2013 December. The averaged radial velocity of the bright component "C3" with reference to the radio core is found to be 0.27 ± 0.02  c between 2007 October and 2013 December. This constant velocity of C3 is naturally explained by the advancing motion of the head of the mini-radio lobe. We also find a non-linear component in the motion of C3 with respect to the radio core. We briefly discuss possible origins of this non-linear motion. [ABSTRACT FROM AUTHOR]

We present VLBA observation towards the nucleus of a nearby radio galaxy NGC 1052. In NGC 1052, two-sided jet structure and a dense plasma circumnuclear torus with a radius of 0.7 pc have been found around the central mass. It emits a H2O megamaser, which is redshifted with respect to the systemic velocity of the galaxy (1491 kms−1) with a large velocity width of 100 kms−1 (FWHM). The maser gas is found at the inner jet components of both the approaching and receding jets. The maser gas is positionally coincident with a plasma torus. The maser gas in NGC 1052 could be explained as a circumnuclear torus or disk, as found for the nucleus of NGC 4258. [ABSTRACT FROM PUBLISHER]