Thesis title
Constraining the nature of compact objects from interferometric observables of black holes.
Composition of the jury
- Marie-Christine ANGONIN (University Professor, LTE) : Chair
- Eugeny BABICHEV (Research Director, IJCLab) : Examiner
- Julien MALZAC (Research Director, IRAP) : Examiner
- Delilah GATES (Postdoctoral Researcher, CfA) : Examiner
- Frédéric VINCENT (Research Fellow, LIRA) : Thesis Supervisor
- Cédric DEFFAYET (Research Director, LPENS) : Co-supervisor
- Aaron HELD (Research Fellow, LUX) : Guest
Abstract
Recent advances in electromagnetic observations of supermassive black holes (SMBHs) have revolutionised our ability to probe general relativity (GR) in the strong-field regime, where it remains largely untested. Two facilities are at the heart of these studies : the GRAVITY instrument on the Very Large Telescope Interferometer, which monitors the astrometric motion of matter near the supermassive black hole at the centre of our Galaxy, Sgr A*, and the Event Horizon Telescope (EHT), which has provided the first horizon-scale images of Sgr A* and M87*, the latter being situated in the Virgo Cluster. In the near future, improvements in observational capabilities, made possible by the Next Generation EHT and space interferometry projects such as the Black Hole Explorer and SHARP, will enable the acquisition of higher-resolution images and even videos of these extreme environments, thereby reinforcing the need for detailed theoretical predictions and realistic numerical simulations. This doctoral thesis contributes to these efforts by exploring these two emerging avenues, focusing on the study of currently unresolved fine image features produced by strongly curved photonic trajectories, known as ‘photon rings’, as well as on the analysis of temporal variability in image sequences of black holes. Regarding the first research direction, by considering parameterised deviations from general relativity and a simplified model, we show that gravitational deviations in M87* could be detected in measurements of photon rings despite astrophysical uncertainties. We also present the first steps towards extending these results to more complex configurations and to physically better-motivated parameterisations. For the second approach, we describe how differential light-propagation effects shape the intrinsic variability of a compact structure of energetic plasma—a ‘hot spot’—orbiting Sgr A*, and show that these signatures can, in principle, be used to constrain the astrophysical properties of the emitter, the relative orientation of the observer, and the spacetime properties of the black hole. With a view to future applications to real-world data, we also present extensions to the modelling framework, notably the implementation of analytically treatable fluctuating accretion discs, on which similar analyses can be carried out.
