Relativistic Outflows from Active Galactic Nuclei (AGN) and their connection to Accretion Disks

Abstract

Active galactic nuclei (AGN) are remarkable astrophysical objects characterized by enormous luminosities and large-scale collimated, relativistic outflows. Miniature versions in our galaxy are called galactic microquasars. The general consensus is that AGN and microquasars are powered by the accretion of matter onto the black hole they harbor at their centers. We attempt to interpret observations of such sources that reveal significant dips in the X-ray intensity, followed by ejections of superluminal blobs at radio frequencies. Since Xrays originate from the inner, hot accretion disk and radio emission from the relativistic jet, these observations suggest a “disk-jet connection”, which we explore in this thesis. We address the issue of episodic blob ejection from the inner, hot accretion disk/corona by envisaging the plasmoids as pre-existing current carrying magnetic flux ropes that were initially anchored in the accretion disk-corona. The plasmoids are ejected outwards via MHD instability mechanism called the toroidal instability (TI). The TI, which was originally explored in the context of laboratory tokamak plasmas, has been very successful in explaining coronal mass ejections (CMEs) from the Sun. Our detailed model predictions compare favorably with a representative set of multi-epoch observations of radio emitting knots from the radio galaxy 3C 120 (Shende, Subramanian, and Sachdeva, 2019). On the other hand, the dips in X-ray intensity can be attributed to the rapid collapse of the hot, inner parts of the accretion disk, which can occur over the radial infall time-scale of the inner disk. However, estimates of this time-scale are hindered by a lack of knowledge of the operative viscosity in the collisionless plasma comprising the inner disk. We address this issue by prescribing the microphysical viscosity in hot accretion disks, instead of parametrizing it in terms of Shakura-Sunyaev α parameter. The viscosity is characterized using published estimates of diffusion coefficients of cosmic ray protons in turbulent magnetic fields. The estimates of Shakura-Sunyaev α parameter arising from our viscosity prescription have a range 0.02 to 0.08. We build simplified disk models and estimate the inner disk collapse timescales for AGN 3C 120 and 3C 111, and the galactic microquasar GRS 1915+105. Our inner disk collapse time-scale estimates are in good agreement with those of the observed X-ray dips (Shende, Chauhan, and Subramanian, 2021). We find that the collapse time-scale is most sensitive to the outer radius of the hot accretion disk. We build upon the work of Becker, Subramanian, and Kazanas (2001) to investigate the origin of steady relativistic winds from advection-dominated hot accretion disks. We match the fluid description of self-similar inflow-outflow solutions of advection-dominated disks with the particle distribution function of protons due to second-order Fermi acceleration in the accretion disk. In this way, a concrete disk-wind connection is established, and the results show how the high energy tail of the proton distribution function is ejected outwards as a relativistic wind.