Universität zu Köln

Boccardi, Biagina (2015). The two-sided relativistic outflow in Cygnus A: extragalactic jet physics at extreme spatial resolution. PhD thesis, Universität zu Köln.

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Abstract

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.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Boccardi, Biagina bboccardi@mpifr-bonn.mpg.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-70250
Date:	31 August 2015
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Natural sciences and mathematics Physics
Uncontrolled Keywords:	Keywords Language Extragalactic astronomy English Radio Interferometry English Relativistic jets English Cygnus A English Jet formation English
Date of oral exam:	20 October 2015
Referee:	Name Academic Title Zensus, J.A. Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/7026