Traianou, Efthalia ORCID: 0000-0002-1209-6500 (2020). High-resolution VLBI Studies of the Blazars TXS 2013+370, OJ 287, and 3C 454.3. PhD thesis, Universität zu Köln.

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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.

Item Type: Thesis (PhD thesis)
CreatorsEmailORCIDORCID Put Code
URN: urn:nbn:de:hbz:38-117059
Date: 6 August 2020
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
Uncontrolled Keywords:
blazars, agn, astrophysical jets, plasma, polarimetry, vlbi, high-resolution, black holeEnglish
Date of oral exam: 9 June 2020
NameAcademic Title
Eckart, AndreasProf. Dr.
Zensus, J. AntonProf. Dr.
Münker, CarstenProf. Dr.
Refereed: Yes


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