Magnetic Fields of Relativistic Jets in Supermassive Black Holes

Dahale, Rohan Arun (2022) Magnetic Fields of Relativistic Jets in Supermassive Black Holes. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Rohan Arun Dahale (17MS194)) 17MS194_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (49MB)

Abstract

Because of the supermassive black hole at the galaxy’s center, which is accreting matter, Active Galactic Nuclei (AGN) are unique and good laboratories for a range of extreme phenomenon and processes. A small percentage of these AGN are radio-loud, producing collimated, relativistic jets that radiate brightly across the electromagnetic spectrum. The jet formation and stability are not completely understood. Large-scale magnetic fields extract energy from the central black hole, while the rotating inner accretion disk creates an outflowing wind that helps collimate the jet. Magnetic fields are anchored in the acceretion disk or the ergosphere of a black hole. As a result, the jet launches as a magnetically dominated outflow, with high magnetic fields accelerating the flow to relativistic speeds. The general relativistic magnetohydrodynamic (GRMHD) simulations demonstrate that if the black hole is threaded by dynamically relevant magnetic fields, the jet generation efficiency can be quite high. The structure of the magnetic field at the emission point is one key unanswered question. Some data suggest that the magnetic field is helical at sub-parsec scales but chaotic at larger scales, while others suggest that it could be helical even at parsec scales. This prompted the study of magnetic field of parsec-scale jets, which may aid in our understanding of jet formation processes. This work used a cutting-edge observational technique known as Very Long Baseline Interferometry for this aim (VLBI). In astronomy, this approach delivers the best spatial resolution conceivable. The radio galaxy 3C,120 and the blazar 0716+714 were observed simultaneously with the Very Long Baseline Array (VLBA) on January 21, 2014, and June 2, 2014. At 5, 8, 15, 22, 43, and 86 GHz, observations were made toward these sources. The radio galaxy 3C,120 is a fascinating source with a FRI morphology and a blazarlike radio jet with several superluminal components seen at parsec scales. 0716+714, on the other hand, is a BL Lac object with a jet pointing directly at the observer, causing strong doppler boosting. We compare the jet properties of these different types of AGN. The structure of the magnetic field is determined using VLBA multi-frequency polarimetric measurements, total intensity, linearly polarized maps, and rotation measure (RM) maps. This provide us the platform to investigate the magnetic field in the jets of several AGNs at various spatial scales. In addition, spectral index maps for all adjacent frequency pairs were created by comparing the total intensity maps to analyze opacity in different parts of the jet. They confirm that until the turnover frequency, the VLBI core is optically thick and the jet is optically thin in both sources. It was also able to compute rotation measure (RM) maps to analyze the magnetic field line of sight for specific frequencies of 3C 120. The jet features a helical magnetic field structure, as evidenced by the RM gradient. The intensity in each frequency (band) is modeled with a set of circular Gaussian components to explore the link between diverse features in the jet. The brightness temperature for each of them was calculated (with a maximum of TB ≈ 1×10¹² K and a minimum of TB ≈ 1×10⁸ K). As a function of distance from the core, the brightness temperature has a 1/r² distribution. The equipartition doppler factor and the magnetic field were computed using two distinct models. Results show that the core doesn’t follow equipartition magnetic field and implying high energy processes. On the other hand, the magnetic field in the jet region is found to be in equipartition implying minimum energy state.

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