Strong Gravity

Black hole X-ray binaries and Active Galactic Nuclei transition through a series of accretion states in a well-defined order. During a state transition, the accretion flow changes from a hot geometrically thick accretion flow, emitting a power-law–like hard spectrum to a geometrically thin, cool accretion flow, producing black-body–like soft spectrum. The hard intermediate accretion state present in the midst of a state transition is thought to be associated with the presence of both a hot geometrically thick component, termed the corona, and a cool, geometrically thin component of the accretion flow. The details concerning the geometry of the disk in the hard intermediate state are not agreed upon and numerous models have been proposed: In the “truncated disk” model, the accretion flow is geometrically thick and hot close to the black hole, while the outer regions of the flow are geometrically thin and cool. There are many open questions concerning the nature of truncated accretion disks: Which mechanisms generate the truncated disk structure? What sets the radius at which the disk truncates? How is the corona formed and what is its geometry? In this talk I present the first high-resolution 3D General Relativistic Magneto-Hydrodynamic (GRMHD) simulation and radiative GRMHD simulation modelling the self-consistent formation of a truncated accretion disk around a black hole.

Observations of gravitational waves from binary black-hole mergers provide a unique testbed for General Relativity in the strong-field regime. To extract the most information, many gravitational-wave signals can be used in concert to place constraints on theories beyond General Relativity. Although these hierarchical inference methods have allowed for more informative tests, careful consideration is needed when working with astrophysical observations. Assumptions about the underlying astrophysical population and the detectability of possible deviations can influence hierarchical analyses, potentially biasing the results. In this talk, I will address these key assumptions and discuss their mitigation. Finally, I will demonstrate how we can leverage the astrophysical nature of gravitational-wave observations to our advantage to empirically bound the curvature dependence of extensions to General Relativity.

After reviewing the motivation and challenges connected with the dRGT theory of ghost-free massive gravity, we discuss our recent progress in understanding non-linear dynamics of this model. In spherical symmetry, numerical studies suggest the formation of naked singularities during gravitational collapse of matter. Analytically, the same can be seen in the limit where the graviton mass is much smaller than the scales of the matter present. Both of these results underline the need to move beyond spherical symmetry to try and obtain realistic predictions. To that end, we present a new ‘harmonic-inspired’ formulation of the minimal model and argue that it is well-posed, opening the door to full 3+1 numerical simulations. 

Maturing Pulsar Timing Arrays are expected to inaugurate the era of nano-hertz GW astronomy in the coming days under the auspices of the International Pulsar Timing Array. Implications of ongoing IPTA efforts for astrophysics and cosmology will be discussed while focussing on PTA contributions. Ongoing IPTA efforts should lead to persistent multi-messenger GW astronomy with massive BH binaries especially during the Square Kilometre Array era, and its implications will be discussed.

With upcoming LIGO runs and new projects like LSST and ULTRASAT, black hole (BH)-powered multi-messenger events will be at the forefront of astrophysics. A major challenge in studying BH-powered explosions is the vast dynamical range between the BH and the emission site, which has hindered theoretical models from capturing the underlying physics from observations. Using 3D neutrino-general relativistic magnetohydrodynamic simulations, I will present the first such models, introducing an innovative model that now enables us to link GRB classes in mergers to their central engines and binary merger origins. For collapsing stars, I will demonstrate how our simulations open new frontiers in astrophysics, including a novel idea of how nascent BHs acquire their strong magnetic fields, heavy element nucleosynthesis in supernovae, the evolution of relativistic jets, new types of transients, and predictions of new vigorous, coherent, non-inspiral gravitational wave sources that may already be detectable by LIGO. These insights will be crucial for extracting the physics of transients from future gravitational wave and electromagnetic detections.

As endpoints of massive stellar evolution, showcases for the densest matter in the universe, and sites for heavy element nucleosynthesis, neutron star mergers are superb laboratories for astrophysics, strong gravity and nuclear physics. Gravitational-wave observations of these mergers are beginning to reveal neutron stars’ internal structure, provide insight into the astrophysical processes that form them, and expose their role in the chemical evolution of the Galaxy. I will survey some of my recent work in these areas and describe how our theoretical understanding of neutron stars is being shaped by gravitational-wave discoveries.

In this talk, I will provide an overview of neutron star (NS) mergers, highlighting the insights gained through numerical relativity simulations. I will mainly focus on the role of the cocoon shock breakout emission as a key early electromagnetic counterpart of NS mergers, with special relevance to events like GW170817. I will explore how the properties of the merger ejecta and the nature of the central engine influence the resulting emission. Additionally, I will present recent advancements in the development of our new general relativistic magnetohydrodynamics (GR-MHD) code GR-Athena++, and share ongoing research efforts on the evolution of magnetic fields in the post-merger remnant.

I will review various mechanisms and detection strategies of precursor emission to black holes and neutron stars mergers. I will also discuss other peculiar physical processes at the intersection of electromagnetism, classical General Relativity, and the physics of continuous media.

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Fueling and feedback couple supermassive black holes (SMBHs) to their host galaxies across many orders of magnitude in spatial and temporal scales, making this problem notoriously challenging to simulate. We use a multi-zone computational method based on the general relativistic magneto-hydrodynamic (GRMHD) code KHARMA that allows us to span 7 orders of magnitude in spatial scale, to simulate accretion onto a non-spinning SMBH from an external medium with Bondi radius ~ 2e5 G*M/c^2, where M is the SMBH mass. For the classic idealized Bondi problem, spherical gas accretion without magnetic fields, our simulation results agree very well with the general relativistic analytic solution. Meanwhile, when the accreting gas is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic field. The density profile varies as ~ r^(-1) rather than r^(-3/2) and the accretion rate is consequently suppressed by over 2 orders of magnitude below the Bondi rate. We find continuous energy feedback from the accretion flow to the external medium at a level of 1% of the accreted rest mass energy (~ 0.01 Mdot * c^2). Energy transport across these widely disparate scales occurs via turbulent convection triggered by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that accumulate on horizon scales transform the flow dynamics far from the SMBH and naturally explain observed extremely low accretion rates compared to the Bondi rate, as well as at least part of the energy feedback.

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The two body problem in general relativity is of great theoretical and observational interest, and can be studied in the post-Newtonian, post-Minkowskian and small mass ratio approximations, as well as with effective one body and fully numerical techniques. An issue that arises is whether the motion can be decomposed into dissipative and conservative sectors for which the conservative sector admits a Hamiltonian description. This has been established to various orders in the post-Newtonian and post-Minkowskian approximations. In this talk, I will go over recent work where we showed that in the small mass ratio approximation, the motion of a (spinning) point particle under the conservative piece of the first-order self force is Hamiltonian in any stationary spacetime. After this, I describe two issues that arise when attempting to extend these results to subleading order in the mass ratio, namely infrared divergences and ambiguities in the conservative/dissipative splittings. I suggest resolutions of these issues and successfully derive a subleading Hamiltonian conservative sector for the scalar self force, as a toy model for the gravitational case.

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