The two-sided relativistic outflow in Cygnus A: extragalactic jet physics at extreme spatial resolution

Extragalactic jets are collimated and magnetized outflows of relativistic plasma produced by accreting supermassive black holes. Although their understanding has significantly progressed over the years, the physical processes driving their launching, acceleration and collimation are still unclear. One reason is that the crucial mechanisms for jet formation take place on extremely small scales, and the angular resolution achievable in observations is only recently becoming sufficient. This thesis aims at investigating the nuclear regions of the jet in the radio galaxy Cygnus A through Very Long Baseline Interferometry (VLBI) observations down to millimeter wavelengths. The large viewing angle of this jet (θ ∼ 75 deg) facilitates the study of the intrinsic properties of the flow thanks to the reduced geometrical and relativistic effects, and allows a counter-jet to be detected. At the source redshift (z=0.056), mm-VLBI can achieve a resolution of only ∼200 Schwarzschild radii (Rs ). This thesis starts with an introduction to the physics of active galactic nuclei (AGN), with an emphasis on the radio-loud class (Chapter 1). A review of the currently most favored jet launching models is given in Chapter 2, complemented by a comparison with results from state-of-the-art observational studies. A large part of the work for this thesis was spent for the calibration and imaging of VLBI data, therefore Chapter 3 is dedicated to the description of this elegant and challenging observational technique. Chapter 4, 5 and 6 are focused on the study of Cygnus A, and the main findings are summarized in Chapter 7. Monitoring of the source at 43 GHz (Chapter 4) reveals that the flow is parabolic in its acceleration region, which extends for ∼10^4 Rs. Results are in agreement with predictions for a magnetically-driven jet confined by an external medium with mild pressure gradient. The presence of a speed transverse gradient with spine-sheath structure, likely giving rise to the limb brightening observed, is also inferred at 43 GHz. Its origin is further investigated in Chapter 5 at 86 GHz. At this frequency, the improved resolution enables to trace the limb-brightened structure down to the base of the two-sided flow, suggesting that stratification is a direct result of the jet launching mechanism. The minimum jet transverse size of 135±27 Rs is much larger than the radius of the Innermost Stable Circular Orbit (1–9 Rs), implying that the launching region must involve also the outer part of the accretion disk. A spectral analysis in the frequency range 8–86 GHz (Chapter 6) constrains the location of the true central engine to coincide with a prominent gap of emission seen at 43 GHz and at lower frequencies, at a distance of ∼1000 Rs from the 86 GHz core. The gap is a synchrotron self-absorbed region which becomes partially optically thin at 86 GHz. The counter-jet exhibits a flat or inverted spectrum up to high frequencies, compatible with the presence of a compact absorber extending between ∼0.5 and 1.5 pc from the central engine. Spectral fitting indicates that the absorber may have a second, more diffuse component spread over the entire source

Millimeter-VLBI observations of extragalactic jets

Over the past few decades, very-long-baseline interferometry observations have greatly improved our understanding of extragalactic jets. Nevertheless, the physical mechanisms involved in the formation of the plasma flow, as well as those driving its exceptional radiative output up to TeV energies, remain to be clarified. Due to their high synchrotron opacity, the dense and magnetized regions in the vicinity of the central engine – where such mechanisms are likely to take place – can only be penetrated when observing at short radio wavelengths, in the millimeter and sub-millimeter regimes. In this talk I will report on the current capabilities of millimeter-VLBI arrays, and I will present an overview of the main scientific results produced so far

The magnetic field structure in CTA 102 from high-resolution mm-VLBI observations during the flaring state in 2016-2017

Context. Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to understanding the crucial physical processes giving rise to jet formation, as well as to their extraordinary radiation output up to γ-ray energies. Aims. We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm VLBI polarimetric observations, reaching an unprecedented resolution (∼50 μas). We also investigate the variability and physical processes occurring in the source during the observing period, which coincides with a very active state of the source over the entire electromagnetic spectrum. Methods. We perform the Faraday rotation analysis using 3 and 7 mm data and we compare the obtained rotation measure (RM) map with the polarization evolution in 7 mm VLBA images. We study the kinematics and variability at 7 mm and infer the physical parameters associated with variability. From the analysis of γ-ray and X-ray data, we compute a minimum Doppler factor value required to explain the observed high-energy emission. Results. Faraday rotation analysis shows a gradient in RM with a maximum value of ∼6 × 10 4 rad m -2 and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7 mm EVPA orientation is different when the component is exiting the core or crossing a stationary feature at ∼0.1 mas. The interaction between the superluminal component and a recollimation shock at ∼0.1 mas could have triggered the multi-wavelength flares. The variability Doppler factor associated with such an interaction is large enough to explain the high-energy emission and the remarkable optical flare occurred very close in time.Peer reviewe

VLBI20-30: a scientific roadmap for the next decade -- The future of the European VLBI Network

This white paper describes the science case for Very Long Baseline Interferometry (VLBI) and provides suggestions towards upgrade paths for the European VLBI Network (EVN). The EVN is a distributed long-baseline radio interferometric array, that operates at the very forefront of astronomical research. Recent results, together with the new science possibilities outlined in this vision document, demonstrate the EVN's potential to generate new and exciting results that will transform our view of the cosmos. Together with e-MERLIN, the EVN provides a range of baseline lengths that permit unique studies of faint radio sources to be made over a wide range of spatial scales. The science cases are reviewed in six chapters that cover the following broad areas: cosmology, galaxy formation and evolution, innermost regions of active galactic nuclei, explosive phenomena and transients, stars and stellar masers in the Milky Way, celestial reference frames and space applications. The document concludes with identifying the synergies with other radio, as well as multi-band/multi-messenger instruments, and provide the recommendations for future improvements. The appendices briefly describe other radio VLBI arrays, the technological framework for EVN developments, and a selection of spectral lines of astrophysical interest below 100 GHz. The document includes a glossary for non-specialists, and a list of acronyms at the end

VLBI20-30: a scientific roadmap for the next decade -- The future of the European VLBI Network

This white paper describes the science case for Very Long Baseline Interferometry (VLBI) and provides suggestions towards upgrade paths for the European VLBI Network (EVN). The EVN is a distributed long-baseline radio interferometric array, that operates at the very forefront of astronomical research. Recent results, together with the new science possibilities outlined in this vision document, demonstrate the EVN's potential to generate new and exciting results that will transform our view of the cosmos. Together with e-MERLIN, the EVN provides a range of baseline lengths that permit unique studies of faint radio sources to be made over a wide range of spatial scales. The science cases are reviewed in six chapters that cover the following broad areas: cosmology, galaxy formation and evolution, innermost regions of active galactic nuclei, explosive phenomena and transients, stars and stellar masers in the Milky Way, celestial reference frames and space applications. The document concludes with identifying the synergies with other radio, as well as multi-band/multi-messenger instruments, and provide the recommendations for future improvements. The appendices briefly describe other radio VLBI arrays, the technological framework for EVN developments, and a selection of spectral lines of astrophysical interest below 100 GHz. The document includes a glossary for non-specialists, and a list of acronyms at the end