High-resolution VLBI Studies of the Blazars TXS 2013+370, OJ 287, and 3C 454.3

Blazars are the most luminous sub-class of active galactic nuclei (AGN). Powered by an accreting supermassive black hole (SMBH), these systems are characterized by an axisymmetric pair of powerful relativistic jets that emanate from their central region and are closely aligned to the line of sight of the observer. Thanks to this geometrical coincidence, blazars constitute a unique case of astrophysical objects in which we can study the extreme physical conditions associated with the jet launching region and the strong magnetic fields in the vicinity of the central engine. To date, even though numerous observational and theoretical studies have been established for an improved understanding of the underlying jet physics, a number of questions are pending to be answered. The origin of the high-energy emission and the seed photon field, the true nature of the observed jet base, and the jet phenomenology are investigated in this thesis. For this purpose, we employ a state-of-the-art observational technique called very-long baseline interferometry (VLBI). Simultaneous radio observations at 86 GHz using the largest radio antennas in the world, as well as complementary observations of ground array elements with the space radio telescope RadioAstron, give us the unique opportunity to focus on three case studies: the blazars TXS 2013+370, OJ 287 and 3C 454.3. In the blazar TXS 2013+370, quasi-simultaneous VLBI observations at 15, 43 and 86 GHz, along with space-VLBI data at 22 GHz, allowed us to investigate the jet base with an angular resolution of ≥ 0.4 pc. In combination with broad-band variability observations and γ-ray data, the high-resolution VLBI imaging revealed the ejection of new jet features, accompanied by flaring activity in radio/mm- bands and γ rays. The analysis of the transverse jet width profile constrained the mm-VLBI core to be located within ≤ 2 pc downstream of the jet apex, and also showed the existence of a transition from parabolic to conical jet expansion at a deprojected distance of ~54 pc from the core. The estimation of the intrinsic jet parameters allowed us to determine the magnetic field strength in the 22 GHz VLBI core region to be B(SSA)= 0.36 ± 0.16 G. Cross-correlation analysis of the broad-band variability revealed a strong correlation between the radio-mm and γ-ray data, with the 1 mm emission lagging ~49 days behind the γ rays. Based on this, we infer that the high energy emission is produced at a distance of the order of ~1 pc from the jet apex, suggesting that the seed photon field for the external Compton mechanism originates either in the dusty torus or in the broad-line region. The study of the blazar OJ 287 was based on a unique data set, comprising 15, 43 and 86 GHz VLBI observations along with contemporaneous space-VLBI data at 22 GHz, which enabled us to reconstruct for the first time the fine structure of the innermost compact region with the record-resolution of 10 μas. Total intensity and polarization images of the source revealed a complex structure, with the brightest core feature being located a ~10 pc downstream from the innermost jet component. The source modeling enabled us to perform a spectral decomposition analysis of the VLBI knots, which led us to the determination of the synchrotron turnover parameters. Additionally, through spectral index mapping, we studied the spectral evolution along the radio jet. The results of the spectral fitting were used to obtain the magnetic field strength of the innermost jet features. We calculated the equipartition Doppler factor to be ≥ δ(eq)=2.9±0.26. A variation of the Doppler factor along the jet was also observed, and its origin is unclear. The most likely explanation is the presence of a strong jet bending towards the line of sight, a local change of the plasma speed, or a superposition of both. Polarization imaging and Faraday rotation analysis based on two independent methods helped us to probe the magnetic field topology on sub-parsec scales. A rotation measure analysis in the core region revealed a rotation between -440 to -1100 rad/m^2 (obtained from a pixel-based analysis and single-dish flux density variability measurements). By combining the imaging and the rotation measure (RM) analysis, we report indications of the existence of a helical magnetic field in the OJ 287 core region, which is in agreement with similar polarization studies. Furthermore, during our observational interval, a prominent flaring event took place, which allowed us to study the observed brightness temperature evolution in the VLBI core region. We report a rising trend for the innermost VLBI feature during the flare evolution, rising from T(b)=(33.6±0.8)x10^11 K up to 5.5±0.9x10^12 K. Using the computed δ(eq), we estimated that the intrinsic brightness temperature is T(int)≈10^12 K in the core region, which is significantly above from the equipartition limit of ~5x10^10 K. This implies that the VLBI core in OJ 287 is particle-dominated. Lastly, for the blazar 3C 454.3 we analyzed 24 epochs of VLBI data at 43 and 86 GHz. In this thesis, we present for the first time a five-year structural and kinematic study of the innermost jet region of this source, probed by the ultra-high resolution of 50 μas (≥0.4 pc). The results of the analysis revealed that the flux density distribution along the jet is described by components that move with apparent speeds between 6 c to 26 c, as well as by stationary features. We trace the appearance of 7 new VLBI components during the observing interval of 2013-2017. The detected knots show unusual behavior in their velocity pattern, with a transition from fast motion to apparent stationarity and merging when they reach a radial distance of 0.5 mas from the core. The nature of this region was investigated by studying the spectral index variability and the linear polarization of an indicative epoch; however, the physical interpretation remains ambiguous, with possible scenarios involving local jet bending, standing shock or plasma instabilities. Also, indications of trailing jet components have been found, which are related to rarefaction in the near wake of the most prominent propagating disturbances. Lastly, we report that the newly detected features are ejected at different position angles, pointing to more complicated ejection scenarios related to accretion disk precession, jet nozzle precession, or jet instabilities

Multi-Wavelength and Multi-Messenger Studies Using the Next-Generation Event Horizon Telescope

The next-generation Event Horizon Telescope (ngEHT) will provide us with the best opportunity to investigate supermassive black holes (SMBHs) at the highest possible resolution and sensitivity. With respect to the existing Event Horizon Telescope (EHT) array, the ngEHT will provide increased sensitivity and uv-coverage (with the addition of new stations), wider frequency coverage (from 86 GHz to 345 GHz and higher), finer resolution (<15 micro-arcseconds), and better monitoring capabilities. The ngEHT will offer a unique opportunity to deeply investigate the physics around SMBHs, such as the disk-jet connection, the mechanisms responsible for high-energy photon and neutrino events, and the role of magnetic fields in shaping relativistic jets, as well as the nature of binary SMBH systems. In this white paper we describe some ngEHT science cases in the context of multi-wavelength studies and synergies.https://www.mdpi.com/2075-4434/11/1/17Published versionPublished versio

A Near Magnetic-to-kinetic Energy Equipartition Flare from the Relativistic Jet in AO 0235+164 during 2013-2019

We present the multiwavelength flaring activity of the blazar AO 0235+164 during its recent active period from 2013 to 2019. From a discrete correlation function (DCF) analysis, we find a significant (>95%) correlation between radio and $\gamma$-ray light curves with flares at longer wavelengths following flares at shorter wavelengths. We identify a new jet component in 43 GHz VLBA data that was ejected from the radio core on MJD $57246^{+26}_{-30}$ (2015 August 12), during the peak of the 2015 radio flare. From the analysis of the jet component, we derived a Doppler factor of $\delta_{\rm var}=28.5\pm8.4$, a bulk Lorentz factor of $\Gamma=16.8^{+3.6}_{-3.1}$, and an intrinsic viewing angle of $\theta_{\rm v}=1.42^{+1.07}_{-0.52}\textrm{ degrees}$. Investigation of the quasi-simultaneous radio data revealed a partially absorbed spectrum with the turnover frequency varying in the range of $10-70$ GHz and the peak flux density varying in the range of $0.7-4$ Jy. We find the synchrotron self-absorption magnetic field strength to be $B_{\rm SSA}=15.3^{+12.6}_{-14.0}$ mG at the peak of the 2015 radio flare, which is comparable to the equipartition magnetic field strength of $B_{\rm EQ}=43.6^{+10.6}_{-10.4}$ mG calculated for the same epoch. Additional analysis of the radio emission region in the relativistic jet of AO 0235+164 suggests that it did not significantly deviate from equipartition during its recent flaring activity.Comment: 13 pages, 11 figures, 4 tables; Accepted for publication in MNRA

Resolving the inner parsec of the blazar J1924-2914 with the Event Horizon Telescope

The blazar J1924-2914 is a primary Event Horizon Telescope (EHT) calibrator for the Galactic Center's black hole Sagittarius A*. Here we present the first total and linearly polarized intensity images of this source obtained with the unprecedented 20 $\mu$as resolution of the EHT. J1924-2914 is a very compact flat-spectrum radio source with strong optical variability and polarization. In April 2017 the source was observed quasi-simultaneously with the EHT (April 5-11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline Array (April 28), giving a novel view of the source at four observing frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties from the subparsec to 100-parsec scales. We combine the multi-frequency images of J1924-2914 to study the source morphology. We find that the jet exhibits a characteristic bending, with a gradual clockwise rotation of the jet projected position angle of about 90 degrees between 2.3 and 230 GHz. Linearly polarized intensity images of J1924-2914 with the extremely fine resolution of the EHT provide evidence for ordered toroidal magnetic fields in the blazar compact core

The Repeating Flaring Activity of Blazar AO 0235+164

Context. Blazar AO 0235+164, located at redshift z = 0.94, has undergone several sharp multi-spectral-range flaring episodes during the last decades. In particular, the episodes peaking in 2008 and 2015, that received extensive multi-wavelength coverage, exhibited interesting behavior. Aims. We study the actual origin of these two observed flares by constraining the properties of the observed photo-polarimetric variability, those of the broad-band spectral energy-distribution and the observed time-evolution behavior of the source as seen by ultra-high resolution total-flux and polarimetric Very-long-baseline interferometry (VLBI) imaging. Methods. The analysis of VLBI images allows us to constrain kinematic and geometrical parameters of the 7 mm jet. We use the Discrete Correlation Function to compute the statistical correlation and the delays between emission at different spectral ranges. Multi-epoch modeling of the spectral energy distributions allows us to propose specific models of emission; in particular for the unusual spectral features observed in this source in the X-ray region of the spectrum during strong multi spectral-range flares. Results. We find that these X-ray spectral features can be explained by an emission component originating in a separate particle distribution than the one responsible for the two standard blazar bumps. This is in agreement with the results of our correlation analysis that do not find a strong correlation between the X-rays and the remaining spectral ranges. We find that both external Compton dominated and synchrotron self-Compton dominated models can explain the observed spectral energy distributions. However, synchrotron self-Compton models are strongly favored by the delays and geometrical parameters inferred from the observations

A ring-like accretion structure in M87 connecting its black hole and jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation^{1,2}. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole^3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.Comment: 50 pages, 18 figures, 3 tables, author's version of the paper published in Natur

A ring-like accretion structure in M87 connecting its black hole and jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation1,2. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of [Formula: see text] Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition\ua0to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow

A Ring-Like Accretion Structure in M87 Connecting Its Black Hole and Jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation1,2. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4+0.5−1.1 Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow

Monitoring the Morphology of M87* in 2009–2017 with the Event Horizon Telescope

The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009–2017 to be consistent with a persistent asymmetric ring of ~40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin