arXiv subjects

A. P. Lobanov

Publications and source records attributed to A. P. Lobanov.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic field and plasma number density from radio and millimeter core measurements in AGN jets

Understanding the mechanism for launching relativistic jets in active galactic nuclei relies upon measuring the magnetic field strength and emitting plasma number density, tracing their evolution along the jet, and determining the relation between their rest frame energy densities. This can be achieved using measurements of the size and brightness temperature of the compact region at the jet base (the ``core'') obtained with very long baseline interferometry (VLBI) across frequencies from 2 to 230~GHz. We develop a framework for independently estimating the magnetic field B* and the emitting plasma number density N* as functions of the jet width $d$, using multifrequency VLBI observations of the core size and brightness temperature. We apply the standard model of self-absorbed synchrotron emission, assuming power-law dependencies of the jet Doppler factor, Lorentz factor, magnetic field strength, and plasma density on the jet width. For an arbitrary jet boundary shape, we derive the dependencies B*(d) and N*(d), and explore a possible relation between the rest frame energy densities of the magnetic field and the emitting plasma. Analysis of core widths and brightness temperatures measured at multiple frequencies points to the possible presence of a magnetic flux decay and effective plasma acceleration within the observed cores at least in some sources of the sample.

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arXiv2025

Revealing a ribbon-like jet in OJ 287 with RadioAstron

We present space-based very long baseline interferometry observations of the BL Lac type object OJ 287 taken with RadioAstron at 22 GHz on April 25, 2016, in conjunction with a ground array comprising 27 radio telescopes. We detect ground-space fringes at projected baselines extending up to 4.6 Earth diameters, which allowed us to image the jet in OJ 287 with an angular resolution of ~47 μas. Applying an advanced regularized maximum likelihood imaging method, we resolved the innermost jet structure with a complex morphology at a resolution of ~15 μas (~0.1 pc projected distance). For the first time, due to a favorable geometrical position of the jet in tandem with high data quality, we detect multiple sharp bends that form a "ribbon-like" jet structure that extends down to 1 mas. Two-dimensional Gaussian model-fitting reveals regions of the jet with brightness temperatures of more than 10^13 K, indicative of strong Doppler boosting. Polarimetric imaging reveals that the electric vector position angles are predominantly perpendicular to the innermost jet direction, implying a dominant poloidal magnetic field component near the central engine. Complementary multi-epoch Very Long Baseline Array observations at 43 GHz provide a multifrequency view of the jet evolution. Ridgeline analysis of the 43 GHz data shows significant variations in the jet position angle from 2014 to 2017, behavior consistent with a rotating helical jet structure. Finally, we confirm the emergence of a new jet component (B15 or K), which may be associated with the source's first TeV flare, and offer new observational constraints relevant to models involving a supermassive black hole binary.

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arXiv2025

Shared-optical-path VLBI frequency phase transfer from 86 to 258 GHz on an 8600 km baseline: Demonstrated with the APEX and IRAM 30 m telescopes

The receiver N3AR operating at a frequency range between 67 and 116 GHz has been commissioned at the APEX telescope in October 2024. This adds a new low-frequency band for APEX, with the capability of simultaneous dual-frequency observations using a dichroic beamsplitter. The 3 mm receiver also allows APEX to join the existing 3 mm global VLBI network. One of our commissioning goals was to perform simultaneous dual-band VLBI observations at 86 and 258 GHz using receivers with shared-optical-paths (SOP) to correct the atmospheric phase fluctuations using the frequency phase transfer (FPT) technique. This was possible together with the IRAM 30 m telescope, which has already developed such a capability. We aimed to verify the expected phase coherence and sensitivity improvement at the higher frequency achievable by applying FPT. With the dual-band, single baseline data, we applied the FPT method, which uses the lower frequency data to correct the simultaneously observed higher-frequency data. We evaluate the improvement compared to the conventional single-band observing mode by analyzing the coherence factor in the higher frequency data. Our results show that the phase fluctuations at the two bands are well correlated. After applying FPT, the interferometric phases at the higher frequency vary much slower and the coherence factor is significantly improved. Our analysis confirms the feasibility of applying FPT to frequencies above 250 GHz with SOP receivers. Future observations in this mode could dramatically improve the sensitivity and imaging fidelity of high-frequency VLBI.

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arXiv2024

A Collection of German Science Interests in the Next Generation Very Large Array

The Next Generation Very Large Array (ngVLA) is a planned radio interferometer providing unprecedented sensitivity at wavelengths between 21 cm and 3 mm. Its 263 antenna element array will be spatially distributed across North America to enable both superb low surface brightness recovery and sub-milliarcsecond angular resolution imaging. The project was developed by the international astronomy community under the lead of the National Radio Astronomy Observatory (NRAO), and is anticipated to be built between 2027 and 2037. Two workshops have been held in 2022 and 2023 with the goal to discuss and consolidate the scientific interests in the ngVLA within the German astronomical community. This community paper constitutes a collection of 48 science ideas which the German community aims to pursue with the ngVLA in the 2030s. This is not a complete list and the ideas are not developed at the level of a "Science Book", such that the present document is mainly meant provide a basis for further discussion within the community. As such, additional contributions are welcome, and will be considered for inclusion in future revisions.

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arXiv2023

Properties of the jet in M87 revealed by its helical structure imaged with the VLBA at 8 and 15 GHz

We present full-track high-resolution radio observations of the jet of the galaxy M87 at 8 and 15 GHz. These observations were taken over three consecutive days in May 2009 using the Very Long Baseline Array (VLBA), one antenna of the Very Large Array (VLA), and the Effelsberg 100 m telescope. Our produced images have dynamic ranges exceeding 20,000:1 and resolve linear scales down to approximately 100 Schwarzschild radii, revealing a limb-brightened jet and a faint, steep spectrum counter-jet. We performed jet-to-counter-jet analysis, which helped estimate the physical parameters of the flow. The rich internal structure of the jet is dominated by three helical threads, likely produced by the Kelvin-Helmholtz (KH) instability developing in a supersonic flow with a Mach number of approximately 20 and an enthalpy ratio of around 0.3. We produce a CLEAN imaging bias-corrected 8-15GHz spectral index image, which shows spectrum flattening in regions of helical thread intersections. This further supports the KH origin of the observed internal structure of the jet. We detect polarised emission in the jet at distances of approximately 20 milliarcseconds from the core and find Faraday rotation which follows a transverse gradient across the jet. We apply Faraday rotation correction to the polarisation position angle and find that the position angle changes as a function of distance from the jet axis, which suggests the presence of a helical magnetic field.

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arXiv2023

RadioAstron discovery of a mini-cocoon around the restarted parsec-scale jet in 3C 84

We present RadioAstron space-based very long baseline interferometry (VLBI) observations of the nearby radio galaxy 3C84 (NGC1275) at the centre of the Perseus cluster. The observations were carried out on September 21-22, 2013 and involved a global array of 24 ground radio telescopes observing at 5 GHz and 22 GHz, together with the Space Radio Telescope (SRT). Furthermore, the Very Long Baseline Array (VLBA) and the phased Very Large Array (VLA) observed the source quasi-simultaneously at 15 GHz and 43 GHz. Fringes between the ground array and the SRT were detected on baseline lengths up to 8.1 times the Earth's diameter, providing unprecedented resolution for 3C 84 at these wavelengths. We note that the corresponding fringe spacing is 125 microarcsec at 5 GHz and 27 microarcsec at 22 GHz. Our space-VLBI images reveal a previously unseen sub-structure inside the compact 1 pc long jet that was ejected about ten years earlier. In the 5 GHz image, we detected, for the first time, low-intensity emission from a cocoon-like structure around the restarted jet. Our results suggest that the increased power of the young jet is inflating a bubble of hot plasma as it carves its way through the ambient medium of the central region of the galaxy. Here, we estimate the minimum energy stored in the mini-cocoon, along with its pressure, volume, expansion speed, and the ratio of heavy particles to relativistic electrons, as well as the density of the ambient medium. About half of the energy delivered by the jet is dumped into the mini-cocoon and the quasi-spherical shape of the bubble suggests that this energy may be transferred to a significantly larger volume of the interstellar medium than what would be accomplished by the well-collimated jet on its own. The pressure of the hot mini-cocoon also provides a natural explanation for the almost cylindrical jet profile seen in the 22 GHz RadioAstron image.

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arXiv2021

RadioAstron reveals a spine-sheath jet structure in 3C 273

We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine/sheath structure observed by Space-VLBI with RadioAstron in 3C 273

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arXiv2021

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

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.

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arXiv2019

Multiband RadioAstron space VLBI imaging of the jet in quasar S5 0836+710

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 and physics of the relativistic jet in this object. The RadioAstron observations at wavelengths of 18cm, 6cm, and 1.3cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02mas to 200mas. Brightness temperatures in excess of $10^{13}$\,K are measured in the jet, requiring Doppler factors of $\ge 100$ for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin-Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be $M_\mathrm{j}\approx 12$ and $η\approx 0.33$. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations.

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arXiv2019

Derivation of the physical parameters of the jet in S5 0836+710 from stability analysis

A number of extragalactic jets show periodic structures at different scales that can be associated with growing instabilities. The wavelengths of the developing instability modes and their ratios depend on the flow parameters, so the study of those structures can shed light on jet physics at the scales involved. In this work, we use the fits to the jet ridgeline obtained from different observations of S5 B0836$+$710 and apply stability analysis of relativistic, sheared flows to derive an estimate of the physical parameters of the jet. Based on the assumption that the observed structures are generated by growing Kelvin-Helmholtz (KH) instability modes, we have run numerical calculations of stability of a relativistic, sheared jet over a range of different jet parameters. We have spanned several orders of magnitude in jet-to-ambient medium density ratio, and jet internal energy, and checked different values of the Lorentz factor and shear layer width. This represents an independent method to obtain estimates of the physical parameters of a jet. By comparing the fastest growing wavelengths of each relevant mode given by the calculations with the observed wavelengths reported in the literature, we have derived independent estimates of the jet Lorentz factor, specific internal energy, jet-to-ambient medium density ratio and Mach number. We obtain a jet Lorentz factor $γ\simeq 12$, specific internal energy of $\varepsilon \simeq 10^{-2}\,c^2$, jet-to-ambient medium density ratio of $η\approx 10^{-3}$, and an internal (classical) jet Mach number of $M_\mathrm{j}\approx 12$. We also find that the wavelength ratios are better recovered by a transversal structure with a width of $\simeq 10\,\%$ of the jet radius. This method represents a powerful tool to derive the jet parameters in all jets showing helical patterns with different wavelengths.

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arXiv2019

Significant core shift variability in parsec-scale jets of active galactic nuclei

The apparent position of jet base (core) in radio-loud active galactic nuclei changes with frequency because of synchrotron self-absorption. Studying this `core shift` effect enables us to reconstruct properties of the jet regions close to the central engine. We report here results from core shift measurements in AGNs observed with global VLBI at 2 and 8 GHz at epochs from 1994 to 2016. Our sample contains 40 objects observed at least 10 times during that period. The core shift is determined using a new automatic procedure introduced to minimize possible biases. The resulting multiple epoch measurements of the core position are employed for examining temporal variability of the core shift. We argue that the core shift variability is a common phenomenon, as established for 33 of 40 AGNs we study. Our analysis shows that the typical offsets between the core positions at 2 and 8 GHz are about 0.5 mas and they vary in time. Typical variability of the individual core positions is about 0.3 mas. The measurements show a strong dependence between the core position and its flux density, suggesting that changes in both are likely related to the nuclear flares injecting denser plasma into the flow. We determine that density of emitting relativistic particles significantly increases during these flares, while relative magnetic field changes less and in the opposite direction.

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arXiv2018

Frequency-Dependent Core Shifts in Ultracompact Quasars

Results of a pilot project with the participation of the "Kvazar-KVO" radio interferometry array in observations carried out with the European VLBI Network are presented. The aim of the project was to conduct and analyze multi-frequency (1.7, 2.3, 5.0, 8.4 GHz) observations of the parsec-scale jets of 24 active galactic nuclei. Three observing sessions were successfully carried out in October 2008. Maps of the radio intensity distributions have been constructed in all four frequencies using phase referencing. A method for measuring the frequency-dependent shift of the position of the VLBI core by applying relative astrometry to observations of close triplets of radio sources has been developed. The fundamental possibility of detecting core shifts in ultra-compact sources for which traditional methods based on the achromatic positions of optically thin regions of the jet are not suitable is demonstrated. The conditions for successful measurement of this shift are discussed; these are determined by the closeness of the calibrator used, the effective resolution of the system, the quality of the filling of the $uv$ plane, the relative orientations of the jets in the triplets, and the brightnesses of the sources.

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arXiv2018

A wide and collimated radio jet in 3C 84 on the scale of a few hundred gravitational radii

Understanding the launching, acceleration, and collimation of jets powered by active galactic nuclei remains an outstanding problem in relativistic astrophysics. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very long baseline interferometry that has thus far been able to probe the transverse jet structure in the acceleration and collimation zone of only two sources. Here we report radio interferometric observations of 3C 84 (NGC 1275), the central galaxy of the Perseus cluster, made with an array including the orbiting radio telescope of the RadioAstron mission. The obtained image transversely resolves the edge-brightened jet in 3C 84 only 30 microarcseconds from the core, which is ten times closer to the central engine than what has been possible in previous ground-based observations, and it allows us to measure the jet collimation profile from ~ 100 to ~10000 gravitational radii from the black hole. The previously found, almost cylindrical jet profile on scales larger than a few thousand r_g is now seen to continue at least down to a few hundred r_g from the black hole and we find a broad jet with a transverse radius larger than about 250 r_g at only 350 r_g from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally on scales smaller than 100 r_g. If this is not the case, then this jet sheath is likely launched from the accretion disk.

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arXiv2018

The extreme blazar AO 0235+164 as seen by extensive ground and space radio observations

Clues to the physical conditions in radio cores of blazars come from measurements of brightness temperatures as well as effects produced by intrinsic opacity. We study the properties of the ultra compact blazar AO 0235+164 with RadioAstron ground-space radio interferometer, multi-frequency VLBA, EVN and single-dish radio observations. We employ visibility modeling and image stacking for deriving structure and kinematics of the source, and use Gaussian process regression to find the relative multi-band time delays of the flares. The multi-frequency core size and time lags support prevailing synchrotron self absorption. The intrinsic brightness temperature of the core derived from ground-based VLBI is close to the equipartition regime value. In the same time, there is evidence for ultra-compact features of the size of less than 10 $μ$as in the source, which might be responsible for the extreme apparent brightness temperatures of up to $10^{14}$ K as measured by RadioAstron. In 2007--2016 the VLBI components in the source at 43 GHz are found predominantly in two directions, suggesting a bend of the outflow from southern to northern direction. The apparent opening angle of the jet seen in the stacked image at 43 GHz is two times wider than that at 15 GHz, indicating a collimation of the flow within the central 1.5 mas. We estimate the Lorentz factor $Γ= 14$, the Doppler factor $δ=21$, and the viewing angle $θ= 1.7^\circ$ of the apparent jet base, derive the gradients of magnetic field strength and electron density in the outflow, and the distance between jet apex and the core at each frequency.

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arXiv2016

Kinematics of the jet in M87 on scales of 100 -- 1000 Schwarzschild radii

Very long baseline interferometry (VLBI) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching, acceleration, and collimation of relativistic outflows. The two-dimensional structure and kinematics of the jet in M\,87 (NGC\,4486) have been studied by applying the Wavelet-based Image Segmentation and Evaluation (WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array (VLBA) observations made in January-August 2007 at 43 GHz ($λ= 7$ mm). The WISE analysis recovers a detailed two-dimensional velocity field in the jet in M\,87 at sub-parsec scales. The observed evolution of the flow velocity with distance from the jet base can be explained in the framework of MHD jet acceleration and Poynting flux conversion. A linear acceleration regime is observed up to $z_{obs} \sim 2$\,mas. The acceleration is reduced at larger scales, which is consistent with saturation of Poynting flux conversion. Stacked cross correlation analysis of the images reveals a pronounced stratification of the flow. The flow consists of a slow, mildly relativistic layer (moving at $β\sim 0.5\,c$), associated either with instability pattern speed or an outer wind, and a fast, accelerating stream line (with $β\sim 0.92$, corresponding to a bulk Lorentz factor $γ\sim 2.5$). A systematic difference of the apparent speeds in the northern and southern limbs of the jet is detected, providing evidence for jet rotation. The angular velocity of the magnetic field line associated with this rotation suggests that the jet in M87 is launched in the inner part of the disk, at a distance $r_0 \sim 5\, R_\mathrm{s}$ from the central engine. The combined results of the analysis imply that MHD acceleration and conversion of Poynting flux to kinetic energy play the dominant roles in collimation and acceleration of the flow in M\,87.

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arXiv2016

A Highly Magnetized Twin-Jet Base Pinpoints a Supermassive Black Hole

Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical models based on Blandford & Znajek extract the rotational energy from a Kerr black hole, which could be the case for NGC1052, to launch these jets. This requires magnetic fields of the order of $10^3\,$G to $10^4\,$G. We imaged the vicinity of the SMBH of the AGN NGC1052 with the Global Millimetre VLBI Array and found a bright and compact central feature, smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the magnetic field at 1 Schwarzschild radius to lie between 200 G and ~80000 G consistent with Blandford & Znajek models.

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arXiv2016

RadioAstron Observations of the Quasar 3C273: a Challenge to the Brightness Temperature Limit

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. These measurements challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive black holes and require a much higher Doppler factor than what is determined from jet apparent kinematics.

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arXiv2016

Detection of multiple velocity components in partially overlapping emitting regions

Velocity measurements made from multiple-epoch astronomical images of evolving objects with optically thin continuum emission (e.g. as relativistic jets or expanding supernova shells) may be confused as a result of the overlap of semi-transparent features moving at different speeds. Multi-scale wavelet decomposition can be effectively applied to identify and track such overlapping features, provided that their respective structural responses can be separated over the spatial scales used for the decomposition. We developed a new method that combines the stacked cross-correlation with the wavelet-based image segmentation and evaluation (WISE) technique of decomposition of two-dimensional structures, to separate and track dominant spatial responses of overlapping evolving features. The method is tested on a set of simulated images of a stratified relativistic jet, demonstrating the robust detection of both the faster spine and the slower sheath speeds. The method is applied to mutliple-epoch images from the MOJAVE survey, revealing two different superluminal streams inside the jet in 3C 273 and the acceleration of the flow in 3C 120. The method can be applied to densely monitored objects with composite structural evolution such as the parsec-scale jet in M87 or heavily resolved expanding supernova shells.

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