Universität zu Köln

Saurabh ORCID: 0000-0001-7156-4848 (2026). Fields, Flows, and the Fabric of Spacetime: Probing Horizon-Scale Physics in Active Galactic Nuclei. PhD thesis, Universität zu Köln.

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Abstract

A century after its formulation, the general theory of relativity continues to pass every test the instruments can devise. It constitutes the paradigm within which modern gravitational astrophysics operates. Among its various theoretical predictions, one stands out as having been particularly difficult for the scientific community to assimilate at the time of its inception: black holes. Today, they occupy a central role in gravitational astrophysics, providing a unique laboratory for testing the interplay between strong-field gravity, plasma physics, and radiative processes. Active Galactic Nuclei (AGN) find themselves at the center of this investigation, thought to be powered by accretion onto supermassive compact object at the center---now, \emph{ab initio}, this is a black hole. They are among the most energetically extreme environments the universe offers such as the M87* and Sagittarius A* (center of our Milky Way galaxy). The physics governing their emission is, however, deeply entangled---the observed polarization structure, the disk kinematics, and the spacetime curvature. The EHT observations at 230 GHz of M87* revealed a polarized ring emission structures with substantial central brightness depression, meanwhile very-long baseline interferometric (VLBI) observations at 86 GHz with the Global Millimetre VLBI Array resolved both the ring and the extended jet emission of M87* simultaneously, identifying a connection between the horizon-scale structure and the large-scale outflow. At 230 GHz, however, earlier EHT campaigns lacked the intermediate baselines needed to probe emission at the scales bridging these two regimes. The 2021 EHT campaign closed this gap by introducing NOEMA and the Kitt Peak 12 m telescope. We imaged and analyzed the 2021 data using the regularized maximum likelihood method utilizing the \texttt{eht-imaging} pipeline through an extensive hyperparameter survey resulting in ~600,000 images. We additionally analyzed the closure phases on the new intermediate baselines which recovered a Gaussian component offset from the compact ring, centered at a projected separation of ~5500 AU, identifying the location, with a flux density of ~60 mJy, an upper limit, of the jet base. The data do not favor more complex models, and the recovered flux implies that most of the missing large-scale emission in earlier EHT studies arises from scales beyond those probed here. Interpreting these observations broadly requires connecting the observed polarized emission to the underlying plasma and field configurations through theoretical models. General relativistic magnetohydrodynamic (GRMHD) simulations provide the most physically self-consistent framework for this purpose. Despite their involvement and inference, however, it remains contentious. Both, astrophysics and gravity, are complicit in this act. Semi-analytic models of radiatively inefficient accretion flows (RIAF) provides the understanding for disentangling their individual contributions. Within the black hole framework, we apply the semi-analytic RIAF model to the 2017 EHT polarimetric dataset of M87* across a systematic parameter grid, ray-tracing synthetic images through a range of magnetic field geometries and extracting their polarimetric signatures for direct comparison with the observations. The effect of disk thickness on the observables is found to be limited, while magnetic configurations dominated by toroidal and poloidal fields can be distinguished reliably. The flow dynamics, in particular the degree of radial inflow, significantly affects the EHT observables, pointing to the presence of a partially infalling sub-Keplerian flow in M87*. Each of these investigations operates under the working hypothesis that the central object in M87* is a black hole. GR, however, also admits gravitational end-states that lack event horizons, arising from physically reasonable matter conditions departing from the idealized initial conditions. GR is a falsifiable theory, and the arrival of horizon-scale precision means that the assumption of an event horizon is no longer beyond observational reach. We adopted the Joshi-Malafarina-Narayan 1 (JMN-1) spacetime, a horizonless compact object arising from gravitational collapse with anisotropic pressure---an exact solution of Einstein's field equations and the best mimicker of the black hole---and subjects it first to the semi-analytic RIAF imaging framework and then to the first three-dimensional GRMHD simulation of sustained accretion onto such an object. The semi-analytic model finds that JMN-1 produces synthetic synchrotron images indistinguishable from a black hole at current EHT resolution, with a central brightness excess as the sole discriminant. The GRMHD simulation confirms that this is not a geometric coincidence: JMN-1 reaches a magnetically arrested state with an accretion rate $\sim~(3-6)\times10^{-6}M_\odot {\rm yr}^{-1}$ comparable to the black hole case, and the presence of a smaller inner shadow is a key observational target for the near-future observations by the next-generation EHT (ngEHT) or the Black hole Explorer (BHEX). Collectively, these findings map the boundary of what horizon-scale precision currently establishes and what it does not. We explored various problems dealing with the same underlying question: what governs the behavior of matter, energy, and spacetime in the most extreme environments the universe produces. As the next generation of arrays delivers dynamic reconstructions and multi-epoch movies of M87* and Sgr A*, the discriminants identified here will move from theoretical targets to observational tests, and the nature of the central engine will become a question the data can finally answer.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Saurabh sbhkmr1999@gmail.com https://orcid.org/0000-0001-7156-4848 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-810335
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Active Galactic Nuclei English Black holes English General relativity English Numerical simulations English Very Long Baseline Interferometry English General Relativistic Magnetohydrodynamics English Accretion flow English
Date of oral exam:	17 July 2026
Referee:	Name Academic Title Zensus, Anton Prof. Dr. Riechers, Dominik A. Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/81033