The spatial scales of relativistic radio jets, probed by relativistic magneto-hydrodynamic jet launching simulations (RMHDs) and by most very-long-baseline interferometry (VLBI) observations differ ...by an order of magnitude. Bridging the gap between these RMHD simulations and VLBI observations requires selecting nearby active galactic nuclei (AGN), the parsec-scale region of which can be resolved. 3C 84 is a nearby bright AGN fulfilling the necessary requirements: it is launching a powerful, relativistic jet powered by a central supermassive black hole, while also being very bright. Using 22 GHz global VLBI measurements of 3C 84 we aim to study its sub-parsec region in both total intensity and linear polarisation, to explore the properties of this jet, with a linear resolution of \(\sim0.1\) parsec. We test different simulation setups by altering the bulk Lorentz factor \(\Gamma\) of the jet, as well as the magnetic field configuration (toroidal, poloidal, helical). We confirm the persistence of a limb brightened structure, which reaches deep into the sub-parsec region. The corresponding electric vector position angles (EVPAs) follow the bulk jet flow inside but tend to be orthogonal to it near the edges. Our state-of-the-art RMHD simulations show that this geometry is consistent with a spine-sheath model, associated with a mildly relativistic flow and a toroidal magnetic field configuration.
A&A 634, A112 (2020) The $\gamma$-ray production mechanism and its localization in blazars are
still a matter of debate. The main goal of this paper is to constrain the
location of the high-energy ...emission in the blazar TXS 2013+370 and to study
the physical and geometrical properties of the inner jet region on sub-pc
scales. VLBI observations at 86 GHz and space-VLBI at 22 GHz allowed us to
image the jet base with an angular resolution of $\sim$0.4 pc. By employing
CLEAN imaging and Gaussian model-fitting, we performed a thorough kinematic
analysis, which provided estimates of the jet speed, orientation, and component
ejection times. Additionally, we studied the jet expansion profile and used the
information on the jet geometry to estimate the location of the jet apex. VLBI
data were combined with single-dish measurements to search for correlated
activity between the radio and $\gamma$-ray emission. The high-resolution VLBI
imaging revealed the existence of a spatially bent jet, described by moving and
stationary features. New jet features are observed to emerge from the core,
accompanied by flaring activity in radio bands and $\gamma$ rays. The analysis
of the transverse jet width profile constrains the location of the mm core to
lie $\leq$ 2 pc downstream of the jet apex, and also reveals the existence of a
transition from parabolic to conical jet expansion at a distance of $\sim$54 pc
from the core, corresponding to $\sim$1.5$\times$10$^{\rm 6}$ Schwarzschild
radii. The cross-correlation analysis reveals a strong correlation between the
radio and $\gamma$-ray data, with the 1 mm emission lagging $\sim$49 days
behind the $\gamma$ rays. Based on this, we infer that the high energy emission
is produced at a distance of $\sim$1 pc from the VLBI core, suggesting that the
seed photon fields for the external Compton mechanism originate either in the
dusty torus or in the broad-line region.
In February 2017 the blazar OJ287, one of the best super-massive binary-black-hole-system candidates, was detected for the first time at very high energies (VHEs; E>100GeV) with the ground-based ...gamma-ray observatory VERITAS. Very high energy gamma rays are thought to be produced in the near vicinity of the central engine in active galactic nuclei. For this reason, and with the main goal of providing useful information for the characterization of the physical mechanisms connected with the observed teraelectronvolt flaring event, we investigate the parsec-scale source properties by means of high-resolution very long baseline interferometry observations. We use 86 GHz Global Millimeter-VLBI Array (GMVA) observations from 2015 to 2017 and combine them with additional multiwavelength radio observations at different frequencies from other monitoring programs. We investigate the source structure by modeling the brightness distribution with two-dimensional Gaussian components in the visibility plane. In the GMVA epoch following the source VHE activity, we find a new jet feature (labeled K) at about 0.2 mas from the core region and located in between two quasi-stationary components (labeled S1 and S2). Multiple periods of enhanced activity are detected at different radio frequencies before and during the VHE flaring state. Based on the findings of this work, we identify as a possible trigger for the VHE flaring emission during the early months of 2017 the passage of a new jet feature through a recollimation shock (represented by the model-fit component S1) in a region of the jet located at a de-projected distance of about 10 pc from the radio core.
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 G{\lambda}. The high sensitivity has motivated us to image the data with the newly developed regularized maximum likelihood imaging methods, revealing the innermost jet structure with unprecedentedly high angular resolution. Our images reveal a compact and twisted jet extending along the northwest direction with two bends within the inner 200 {\mu}as that resembles a precessing jet in projection. The component at the southeastern end shows a compact morphology and high brightness temperature, and is identified as the VLBI core. An extended jet feature that lies at ~200 {\mu}as northwest of the core shows a conical shape in both total and linearly polarized intensity, and a bimodal distribution of the linear polarization electric vector position angle. We discuss the nature of this feature by comparing our observations with models and simulations of oblique and recollimation shocks with various magnetic field configurations. Our high-fidelity images also enabled us to search for possible jet features from the secondary supermassive black hole (SMBH) and test the SMBH binary hypothesis proposed for this source.
Sources at the brightest end of QSO luminosity function during the peak epoch of star formation and black hole accretion (z~2-4, i.e. Cosmic noon) are privileged sites to study the feeding & feedback ...cycle of massive galaxies. We perform the first systematic study of cold gas properties in the most luminous QSOs, by characterising their host-galaxies and environment. We analyse ALMA, NOEMA and JVLA observations of FIR continuum, CO and CII emission lines in eight QSOs (\(L_{\rm Bol}>3\times10^{47}\) erg/s) from the WISSH sample at z~2.4-4.7. We report a 100% emission line detection rate and a 80% detection rate in continuum emission, and we find CO emission to be consistent with the steepest CO ladders observed so far. Sub-mm data reveal presence of (one or more) bright companion galaxies around 80% of WISSH QSOs, at projected distances of 6-130 kpc. We observe a variety of sizes for the molecular gas reservoirs (1.7-10 kpc), associated with rotating disks with disturbed kinematics. WISSH QSOs typically show lower CO luminosity and higher star formation efficiency than FIR matched, z~0-3 main-sequence galaxies, implying that, given the observed SFR ~170-1100 \(M_\odot\)/yr, molecular gas is converted into stars on <50 Myr. Most targets show extreme dynamical to black-hole mass ratios \(M_{\rm dyn}/M_{\rm BH}\sim3-10\), two orders of magnitude smaller than local relations. The molecular gas fraction in WISSH hosts is lower by a factor of ~10-100 than in star forming galaxies with similar \(M_*\). WISSH QSOs undergo an intense growth phase of both the central SMBH and host-galaxy. They pinpoint high-density sites where giant galaxies assemble and mergers play a major role in the build-up of the final host-galaxy mass. The observed low molecular gas fraction and short depletion timescale are likely due to AGN feedback, as traced by fast AGN-driven ionised outflows in all our targets.
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 high resolution millimeter-wave VLBI data. We collect 86GHz GMVA and 43GHz VLBA data to obtain stacked images that we use to infer the jet collimation profile by means of two comparable methods. We analyze the kinematics at 86GHz, and we discuss it in the context of the jet expansion. Finally we consider a possible implication of the Bondi sphere in shaping the different expanding region observed along the jet. We found that the jet in BL Lac expands with an overall conical geometry. A higher expanding rate region is observed between ~5 and 10 pc (de-projected) from the black hole. Such a region is associated with the decrease in brightness usually observed in high-frequency VLBI images of BL Lac. The jet retrieves the original jet expansion around 17 pc, where the presence of a recollimation shock is supported by both the jet profile and the 15GHz kinematics (MOJAVE survey). The change in the jet expansion profile occurring at ~5 pc could be associated with a change in the external pressure profile in correspondence of the Bondi radius (~3.3X10$^5$$R_s$).
The quasar 3C 454.3 is a blazar known for its rapid and violent outbursts seen across the electromagnetic spectrum. Using gamma-ray, X-ray, multi-band optical, and very long baseline interferometric ...data we investigate the nature of two such events that occurred in 2013 and 2014 accompanied by strong variations in optical polarization, including a 230 degree electric vector position angle (EVPA) rotation. Our results suggest that a single disturbance was responsible for both flaring events. We interpret the disturbance as a shock propagating down the jet. Under this interpretation the 2013-flare originated most likely due to changes in the viewing angle caused by perhaps a bent or helical trajectory of the shock upstream of the radio core. The 2014-flare and optical polarization behavior are the result of the shock exiting the 43 GHz radio core, suggesting that shock crossings are one of the possible mechanisms for EVPA rotations.
MNRAS 492 (2020) 3829 We report the results of decade-long (2008-2018) $\gamma$-ray to 1 GHz radio
monitoring of the blazar 3C 279, including GASP/WEBT, $\it{Fermi}$ and
$\it{Swift}$ data, as well as ...polarimetric and spectroscopic data. The X-ray
and $\gamma$-ray light curves correlate well, with no delay > 3 hours, implying
general co-spatiality of the emission regions. The $\gamma$-ray-optical
flux-flux relation changes with activity state, ranging from a linear to a more
complex dependence. The behaviour of the Stokes parameters at optical and radio
wavelengths, including 43 GHz VLBA images, supports either a predominantly
helical magnetic field or motion of the radiating plasma along a spiral path.
Apparent speeds of emission knots range from 10 to 37c, with the highest values
requiring bulk Lorentz factors close to those needed to explain $\gamma$-ray
variability on very short time scales. The Mg II emission line flux in the
`blue' and `red' wings correlates with the optical synchrotron continuum flux
density, possibly providing a variable source of seed photons for inverse
Compton scattering. In the radio bands we find progressive delays of the most
prominent light curve maxima with decreasing frequency, as expected from the
frequency dependence of the $\tau=1$ surface of synchrotron self-absorption.
The global maximum in the 86 GHz light curve becomes less prominent at lower
frequencies, while a local maximum, appearing in 2014, strengthens toward
decreasing frequencies, becoming pronounced at $\sim5$ GHz. These tendencies
suggest different Doppler boosting of stratified radio-emitting zones in the
jet.
The \(\gamma\)-ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the ...blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales. VLBI observations at 86 GHz and space-VLBI at 22 GHz allowed us to image the jet base with an angular resolution of \(\sim\)0.4 pc. By employing CLEAN imaging and Gaussian model-fitting, we performed a thorough kinematic analysis, which provided estimates of the jet speed, orientation, and component ejection times. Additionally, we studied the jet expansion profile and used the information on the jet geometry to estimate the location of the jet apex. VLBI data were combined with single-dish measurements to search for correlated activity between the radio and \(\gamma\)-ray emission. The high-resolution VLBI imaging revealed the existence of a spatially bent jet, described by moving and stationary features. New jet features are observed to emerge from the core, accompanied by flaring activity in radio bands and \(\gamma\) rays. The analysis of the transverse jet width profile constrains the location of the mm core to lie \(\leq\) 2 pc downstream of the jet apex, and also reveals the existence of a transition from parabolic to conical jet expansion at a distance of \(\sim\)54 pc from the core, corresponding to \(\sim\)1.5\(\times\)10\(^{\rm 6}\) Schwarzschild radii. The cross-correlation analysis reveals a strong correlation between the radio and \(\gamma\)-ray data, with the 1 mm emission lagging \(\sim\)49 days behind the \(\gamma\) rays. Based on this, we infer that the high energy emission is produced at a distance of \(\sim\)1 pc from the VLBI core, suggesting that the seed photon fields for the external Compton mechanism originate either in the dusty torus or in the broad-line region.
We report the results of decade-long (2008-2018) \(\gamma\)-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, \(\it{Fermi}\) and \(\it{Swift}\) data, as well as polarimetric ...and spectroscopic data. The X-ray and \(\gamma\)-ray light curves correlate well, with no delay > 3 hours, implying general co-spatiality of the emission regions. The \(\gamma\)-ray-optical flux-flux relation changes with activity state, ranging from a linear to a more complex dependence. The behaviour of the Stokes parameters at optical and radio wavelengths, including 43 GHz VLBA images, supports either a predominantly helical magnetic field or motion of the radiating plasma along a spiral path. Apparent speeds of emission knots range from 10 to 37c, with the highest values requiring bulk Lorentz factors close to those needed to explain \(\gamma\)-ray variability on very short time scales. The Mg II emission line flux in the `blue' and `red' wings correlates with the optical synchrotron continuum flux density, possibly providing a variable source of seed photons for inverse Compton scattering. In the radio bands we find progressive delays of the most prominent light curve maxima with decreasing frequency, as expected from the frequency dependence of the \(\tau=1\) surface of synchrotron self-absorption. The global maximum in the 86 GHz light curve becomes less prominent at lower frequencies, while a local maximum, appearing in 2014, strengthens toward decreasing frequencies, becoming pronounced at \(\sim5\) GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet.
