Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron IV. The quasar 3C 345 at 18 cm: Magnetic field structure and brightness temperature
F. M. Pötzl, A. P. Lobanov, E. Ros, J. L. Gómez, G. Bruni, U. Bach, A. Fuentes, L. I. Gurvits, D. L. Jauncey, Y. Y. Kovalev, E. V. Kravchenko, M. M. Lisakov, T.… 展开作者
F. M. Pötzl, A. P. Lobanov, E. Ros, J. L. Gómez, G. Bruni, U. Bach, A. Fuentes, L. I. Gurvits, D. L. Jauncey, Y. Y. Kovalev, E. V. Kravchenko, M. M. Lisakov, T. Savolainen, K. V. Sokolovsky, J. A. Zensus
备注：
10 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics
Context. Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated. Aims. We study the innermost jet morphology and magnetic field strength in the AGN 3C 345 with an unprecedented resolution using images obtained within the framework of the key science programme on AGN polarisation of the Space VLBI mission RadioAstron. Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016 March 30 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode. Results. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300 $μ$as angular resolution, corresponding to a projected linear scale of about 2 pc or a few thousand gravitational radii. We identify the synchrotron self-absorbed core at the jet base and find the brightest feature in the jet 1.5 mas downstream of the core. Several polarised components appear in the Space VLBI images that cannot be seen from ground array-only images. Except for the core, the electric vector position angles follow the local jet direction, suggesting a magnetic field perpendicular to the jet. This indicates the presence of plane perpendicular shocks in these regions. Additionally, we infer a minimum brightness temperature at the largest $(u,v)$-distances of $1.1\times 10^{12}$ K in the source frame, which is above the inverse Compton limit and an order of magnitude larger than the equipartition value. This indicates locally efficient injection or re-acceleration of particles in the jet to counter the inverse Compton cooling or the geometry of the jet creates significant changes in the Doppler factor, which has to be $>11$ to explain the high brightness temperatures.
We present the first polarimetric space VLBI imaging observations at 22 GHz. BL Lacertae was observed in 2013 November 10 with the RadioAstron space VLBI mission, including a ground array of 15 radio telescopes. The instrumental polarization of the space radio telescope is found to be within 9%, demonstrating the polarimetric imaging capabilities of RadioAstron at 22 GHz. Ground-space fringes were obtained up to a projected baseline distance of 7.9 Earth's diameters in length, allowing us to image the jet in BL Lacertae with a maximum angular resolution of 21 $μ$as, the highest achieved to dat…
We present the first polarimetric space VLBI observations of OJ 287, observed with RadioAstron at 22 GHz during a perigee session on 2014 April 4 and five near-in-time snapshots, together with contemporaneous ground VLBI observations at 15, 43, and 86 GHz. Ground-space fringes were obtained up to a projected baseline of 3.9 Earth diameters during the perigee session, and at a record 15.1 Earth diameters during the snapshot sessions, allowing us to image the innermost jet at an angular resolution of $\sim50μ$as, the highest ever achieved at 22 GHz for OJ 287. Comparison with ground-based VLBI o…
Earth-space interferometry with RadioAstron provides the highest direct angular resolution ever achieved in astronomy at any wavelength. RadioAstron detections of the classic quasar 3C273 on interferometric baselines up to 171,000 km suggest brightness temperatures exceeding expected limits from the "inverse-Compton catastrophe" by two orders of magnitude. We show that at 18 cm, these estimates most probably arise from refractive substructure introduced by scattering in the interstellar medium. We use the scattering properties to estimate an intrinsic brightness temperature of 7*10^12 K, which…
The high brightness temperatures, $T_\mathrm{b}\gtrsim 10^{13}$ K, detected in several active galactic nuclei by RadioAstron space VLBI observations challenge theoretical limits. Refractive scattering by the interstellar medium may affect such measurements. We quantify the scattering properties and the sub-mas scale source parameters for the quasar B0529+483. Using RadioAstron correlated flux density measurements at 1.7, 4.8, and 22 GHz on projected baselines up to 240,000 km we find two characteristic angular scales in the quasar core, about 100 $μ$as and 10 $μ$as. Some indications of scatter…
Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of $10^{11.5}$ K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of $10^{13}$ K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26 $μ$as (2.7 light months) and brightness temperature in excess of $10^{13}$ K.…
We present RadioAstron Space VLBI imaging observations of the BL Lac object S5 0716+71 made on January 3-4 2015 at a frequency of 22 GHz (wavelength $λ=1.3$ cm). The observations were made in the framework of the AGN Polarization Key Science Program. The source was detected on projected space-ground baselines up to 70 833 km (5.6 Earth diameters) for both, parallel hand and cross-hand interferometric visibilities. We have used these detections to obtain a full-polarimetric image of the blazar at an unprecedented angular resolution of 24 $μ$as, the highest for this source to date. This enabled …
RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R satellite, launched in 2011, performing Space Very Long Baseline Interferometry (SVLBI) observations supported by a global ground array of radio telescopes. With an apogee of about 350 000 km, it is offering for the first time the possibility to perform μas-resolution imaging in the cm-band. We present observations at 22 GHz of 3C 273, performed in 2014, designed to reach a maximum baseline of approximately nine Earth diameters. Reaching an angular resolution of 0.3 mas, we study a particularly low-activity state of the sou…
The RadioAstron space radio telescope provides a unique opportunity to study the extreme brightness temperatures ($\mathrm{T_B }$) in AGNs with unprecedented long baselines of up to 28 Earth diameters. Since interstellar scintillation (ISS) may affect the visibilities observed with space VLBI (sVLBI), a complementary ground based flux density monitoring of the RadioAstron targets, which is performed near in time to the VLBI observation, could be beneficial. The combination/comparison with the sVLBI data can help to unravel the relative influence of source intrinsic and ISS induced effects, whi…
RadioAstron is a project to use the 10m antenna on board the dedicated SPEKTR-R spacecraft, launched on 2011 July 18, to perform Very Long Baseline Interferometry from space - Space-VLBI. We describe the strategy and highlight the first results of a 92/18/6/1.35cm fringe survey of some of the brighter radio-loud Active Galactic Nuclei (AGN) at baselines up to 25 Earth diameters (D_E). The survey goals include a search for extreme brightness temperatures to resolve the Doppler factor crisis and to constrain possible mechanisms of AGN radio emission, studying the observed size distribution of th…
Ground-space interferometer RadioAstron provides unique opportunity to probe detail structure of the distant active galactic nuclei at $μ$as scales. Here we report on RadioAstron observations of the BL Lac object S5 0716$+$71, performed in a framework of the AGN Polarization and Survey Key Science Programs at 22 GHz during 2012-2018. We obtained the highest angular resolution image of the source to date, at $57\times24 μ$as. It reveals complex structure of the blazar jet in the inner 100 $μ$as, with emission regions that can be responsible for the blazar variability at timescales of a few days…
The largest Key Science Program of the RadioAstron space VLBI mission is a survey of active galactic nuclei (AGN). The main goal of the survey is to measure and study the brightness of AGN cores in order to better understand the physics of their emission while taking interstellar scattering into consideration. In this paper we present detection statistics for observations on ground-space baselines of a complete sample of radio-strong AGN at the wavelengths of 18, 6, and 1.3 cm. Two-thirds of them are indeed detected by RadioAstron and are found to contain extremely compact, tens to hundreds of…
We present results from a multiwavelength study of the blazar PKS 1954-388 at radio, UV, X-ray, and gamma-ray energies. A RadioAstron observation at 1.66 GHz in June 2012 resulted in the detection of interferometric fringes on baselines of 6.2 Earth-diameters. This suggests a source frame brightness temperature of greater than 2x10^12 K, well in excess of both equipartition and inverse Compton limits and implying the existence of Doppler boosting in the core. An 8.4 GHz TANAMI VLBI image, made less than a month after the RadioAstron observations, is consistent with a previously reported superl…
Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure an…
The limit on the intrinsic brightness temperature, attributed to `Compton catastrophe', has been established being $10^{12}$~K. Somewhat lower limit of the order of $10^{11.5}$~K is implied if we assume that the radiating plasma is in equipartition with the magnetic field --- the idea that explained why the observed cores of active galactic nuclei sustained the limit lower than the `Compton catastrophe'. Recent observations with unprecedented high resolution by the RADIOASTRON revealed systematic exceed in the observed brightness temperature. We propose means of estimating the degree of non-eq…
Upcoming Event Horizon Telescope (EHT) observations will provide a unique opportunity to reveal the innermost region of the radiative inefficient accretion flow (RIAF) around the Galactic black hole, Sgr A*. Depending on the flow dynamics and accumulated magnetic flux, the innermost region of an RIAF could have a quasi-spherical or disk-like geometry. Here we present a phenomenological model to investigate the characteristics of the black hole shadow images with different flow geometries, together with the effect of black hole spin and flow dynamics. The resulting image consists in general of …
Revisiting 3C 279 jet morphology with space VLBI at 26 microarcsecond resolution
Instituto de Astrofísica de Andalucía-CSIC（安达卢西亚天体物理研究所-CSIC）；Max-Planck-Institut für RadioAstronomie（马克斯·普朗克射电天文研究所）；INAF Istituto di RadioAstronomia（意大利国家天体物理研究所射电天文台）；Aalto University（阿尔托大学）
We present results on the intrinsic brightness temperature of a sample of compact radio sources observed at 86 GHz using the Global Millimeter VLBI Array. We use the observed brightness temperatures at 86 GHz and the observed superluminal motions at 15 GHz for the sample in order to constrain the characteristic intrinsic brightness temperature of the sample. With a statistical method for studying the intrinsic brightness temperatures of innermost jet cores of compact radio sources, assuming that all sources have the same intrinsic brightness temperature and the viewing angles of their jets are…
On the kiloparsec scale, extragalactic radio jets show two distinct morphologies related to their power: collimated high-power jets ending in a bright termination shock and low-power jets opening up close to the core and showing a more diffuse surface brightness distribution. The emergence of this morphological dichotomy on the parsec scale at the innermost jet regions can be studied with very-long-baseline interferometry (VLBI) radio observations of blazars in which the jet emission is strongly Doppler boosted due to relativistic bulk motion at small angles between the jet direction and the l…
Context. Monitoring of BL Lacertae at 15 GHz with the Very Long Baseline Array (VLBA) has revealed a quasi-stationary radio feature in the innermost part of the jet, at 0.26 mas from the radio core. Stationary features are found in many blazars, but they have rarely been explored in detail. Aims. We aim to study the kinematics, dynamics, and brightness of the quasi-stationary feature of the jet in BL Lacertae based on VLBA monitoring with submilliarcsecond resolution (subparsec-scales) over 17 years. Methods. We analysed position uncertainties and flux leakage effects of the innermost quasi-st…
In quasars which are lensed by galaxies, the point-like images sometimes show sharp and uncorrelated brightness variations (microlensing). These brightness changes are associated with the innermost region of the quasar passing through a complicated pattern of caustics produced by the stars in the lensing galaxy. In this paper, we study whether the universal properties of optical caustics could enable extraction of shape information about the central engine of quasars. We present a toy model with a crescent-shaped source crossing a fold caustic. The silhouette of a black hole over an accretion …
We present the first very-long-baseline interferometric (VLBI) observations of the blazar OJ287 carried out jointly with the Global Millimeter VLBI Array (GMVA) and the phased Atacama Large Millimeter/submillimeter Array (ALMA) at 3.5 mm on April 2, 2017. Participation of phased-ALMA not only has improved the GMVA north-south resolution by a factor of ~3, but also has enabled fringe detection with signal-to-noise ratios up to 300 at baselines longer than 2 Gλ. The high sensitivity has motivated us to image the data with the newly developed regularized maximum likelihood imaging methods, reveal…
We study the properties of the innermost jet of the flat spectrum radio quasar 1633+382 (4C~38.41) based on VLBI data from the radio monitoring observations of the Boston University VLBI program at 43~GHz. Analysis of the components suggests a semi-parabolic jet geometry with jet radius $R$ following the relation $R\propto r^{0.7}$ with distance $r$, with indications of a jet geometry break towards a conical geometry. Brightness temperature falls with distance following $T_B\propto r^{-2.1}$. Combining this information, magnetic field and electron densities are found to fall along the jet as $…
Extragalactic jets originating from the central supermassive black holes of active galaxies are powerful, highly relativistic plasma outflows, emitting light from the radio up to the gamma-ray regime. The details of their formation, composition and emission mechanisms are still not completely clear. The combination of high-resolution observations using very long baseline interferometry (VLBI) and multiwavelength monitoring provides the best insight into these objects. Here, such a combined study of sources of the TANAMI sample is presented, investigating the parsec-scale and high-energy proper…
Accreting black holes tend to display a characteristic dark central region called the black-hole shadow, which depends only on spacetime/observer geometry and which conveys information about the black hole's mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black-hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the …
We present first results from a very deep (~650 ksec) Chandra X-ray observation of Abell 2052, as well as archival VLA radio observations. The data reveal detailed structure in the inner parts of the cluster, including bubbles evacuated by the AGN's radio lobes, compressed bubble rims, filaments, and loops. Two concentric shocks are seen, and a temperature rise is measured for the innermost one. On larger scales, we report the first detection of an excess surface brightness spiral feature. The spiral has cooler temperatures, lower entropies, and higher abundances than its surroundings, and is …
M87 is one of the nearest radio galaxies with a central SMBH and a prominent relativistic jet. Due to its close distance to the observer and the large SMBH mass, the source is one of the best laboratories to obtain strong observational constraints on the theoretical models for the formation and evolution of the AGN jets. In this article, we present preliminary results from our ongoing observational study about the innermost jet of M87 at an ultra-high resolution of $\sim$50$μ$as achieved by the Global Millimeter-VLBI Array (GMVA). The data obtained between 2004 and 2015 clearly show limb-brigh…
The X-ray binary SS 433, embedded in the W50 nebula (or supernova remnant W50), shows bipolar jets that are ejected with mildly relativistic velocities, and extend toward the east and west out to scales of tens of parsecs. Previous X-ray observations revealed twin lobes along the jet precession axis that contain compact bright knots dominated by synchrotron radiation, which provide evidence of electron acceleration in this system. Particle acceleration in this system is substantiated by the recently detected gamma rays with energies up to at least 25 TeV. To further elucidate the origin of the…
The quasar 3C~286 is one of two compact steep spectrum sources detected by the {\it Fermi}/LAT. Here, we investigate the radio properties of the parsec(pc)-scale jet and its (possible) association with the $γ$-ray emission in 3C~286. The Very Long Baseline Interferometry (VLBI) images at various frequencies reveal a one-sided core--jet structure extending to the southwest at a projected distance of $\sim$1 kpc. The component at the jet base showing an inverted spectrum is identified as the core, with a mean brightness temperature of $2.8\times 10^{9}$~K. The jet bends at about 600 pc (in proje…
Context. Radio-loud active galactic nuclei in the early Universe are rare. The quasars J0906+6930 at redshift z=5.47 and J2102+6015 at z=4.57 stand out from the known sample with their compact emission on milliarcsecond (mas) angular scale with high (0.1-Jy level) flux densities measured at GHz radio frequencies. This makes them ideal targets for very long baseline interferometry (VLBI) observations. Aims. By means of VLBI imaging we can reveal the inner radio structure of quasars and model their brightness distribution to better understand the geometry of the jet and the physics of the source…
The twin-jet system in the FRII radio galaxy 3C 452: A sub-parsec scale VLBI study
Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）；Julius-Maximilians-Universität Würzburg（维尔茨堡朱利叶斯-马克西米利安大学）；INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所博洛尼亚空间科学与天体物理观测站）；INAF–Istituto di Radioastronomia（意大利国家天体物理研究所射电天文研究所）；Università degli Studi di Bologna（博洛尼亚大学）
We present LOFAR observations of one of the most spectacular objects in the radio sky: Abell 2255. This is a nearby ($z = 0.0806$) merging galaxy cluster hosting one of the first radio halos ever detected in the intra-cluster medium (ICM). The deep LOFAR images at 144 MHz of the central $\sim10$ Mpc$^2$ region show a plethora of emission on different scales, from tens of kpc to above Mpc sizes. In this work, we focus on the innermost region of the cluster. Among the numerous interesting features observed, we discover remarkable bright and filamentary structures embedded in the radio halo. We i…
Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), the eponymous blazar of BL Lac objects class, complicate the scenario in this already puzzling class of objects. Understanding the jet geometry, in connection with the jet kinematics and the physical conditions in the surrounding medium, is fundamental to better constrain the formation, acceleration and collimation mechanisms in extragalactic jets. With the aim of investigating the jet geometry in the innermost regions of the jet of BL Lac, and solving the controversy, we explore the radio jet in this source, using h…
Context: Few Active Galactic Nuclei (AGN) reveal a double sided jet system. However, these 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 do they evolve with time. Methods: We monitored the sub-parsec scale morphology of NGC1052 with the VLBA at 43GHz from 2005 to 2009. Results: A detailed study of 29 epochs show an asymmetry between both jets. A kinematic analysis of the outflows reveals higher apparent velocities for the eastern jet as …
Radio-loud active galactic nuclei (AGNs), hosting powerful relativistic jet outflows, provide an excellent laboratory for studying jet physics. Very long baseline interferometry (VLBI) enables high-resolution imaging on milli-arcsecond (mas) and sub-mas scales, making it a powerful tool to explore the inner jet structure, shedding light on the formation, acceleration and collimation of AGN jets. In this paper, we present Very Long Baseline Array (VLBA) observations of ten radio-loud AGNs at 43 and 86~GHz, which were selected from the {\it Planck} catalogue of compact sources and are among the …
