Strong Gravity

Radio pulsars display a range of puzzling long-term variability in their magnetospheric emission and spin-down. In this talk I will discuss the mechanisms for internal magnetic field evolution of pulsars, how it differs from the MHD of classical conducting fluids, and how the neutron star interior couples to the magnetosphere and its radiation.

Numerical modeling of accreting millisecond X-ray pulsars (AMXPs) allows us to understand the physical origin of different observational signatures detected from these systems. Since the birth sites of these signals are strongly influenced by the gravitational potential of the star, magnetohydrodynamic (MHD) simulations in full GR (GRMHD) are essential to accurately capture space-time curvature effects and inherent variations in the X-ray spectra. In this talk, I will present results from 3D GRMHD simulations of accreting neutron stars with oblique magnetospheres. I will discuss the pulse profiles generated from the GRMHD simulations and their implications for mass-radius inference in the accreting sources. Apart from the surface features, AMXPs are also good candidates for studying neutron star jet formation mechanisms. Though there have been extensive investigations into black hole jets, neutron star jets remain highly unexplored. Our 2D axisymmetric study in the quiescent regime suggests that the thick disk collimates the initial open stellar flux, leading to jets like the Blandford-Znajek mechanism proposed for black holes. However, much remains to be done before we can draw a complete picture of jets launched from neutron stars. The global 3D GRMHD simulations of the accreting neutron stars allow us to explore the jet formation mechanisms in these systems in detail for the first time.

We present the first two-dimensional axisymmetric Newtonian magnetohydrodynamic simulations of accretion-induced collapse (AIC) of rotating white dwarfs (WDs) with self-consistent initial magnetic progenitors and neutrino leakage. Our findings show that with initial surface magnetic field strength constrained by isolated WD observations, the protoneutron star can reach field strength consistent with magnetar observations. Our results suggest that single degenerate WDs can form magnetars via AIC.

Accreting supermassive black hole binaries are powerful multimessenger sources emitting both gravitational and electromagnetic (EM) radiation. Understanding the accretion dynamics of these systems and predicting their distinctive EM signals is crucial to informing and guiding upcoming efforts aimed at detecting gravitational waves produced by these binaries. To this end, accurate numerical modeling is required to describe both the spacetime and the magnetized gas around the black holes. In this talk, I will outline two key advancements in this field of research. On the one hand, I will present a novel 3D general relativistic magnetohydrodynamics (GRMHD) framework that combines multiple numerical codes to simulate the inspiral and merger of supermassive black hole binaries starting from realistic initial data and running all the way through merger. Throughout the evolution, we adopt a simple but functional prescription to account for gas cooling through the emission of photons. On the other hand, I will present the application of our new computational method to following the time evolution of a circular, equal-mass, non-spinning black hole binary of total mass ${M}$ for ${\sim\!200}$ orbits starting from a separation of ${20\,r_g\equiv 20\,M}$ and reaching the post-merger evolutionary stage of the system. Our simulation has confirmed the predictions of previous works about the early inspiral phase, but has also revealed phenomena specific to the late-inspiral and merger so far undocumented in the literature. Perhaps our most striking finding is that, although the accretion rate onto the black holes is approximately constant from ${\sim\!3000\,M}$ before merger onward, the EM luminosity undergoes a sharp increase around the time of merger. This effect is caused by the sudden lack of binary torque, which allows the gas in the immediate vicinity of the remnant to quickly fall in, thus compressing and heating up as it shocks. Secondly, the magnetic flux brought to the ${\sim\!0.68\text{-spinning}}$ merger remnant is able to drive a relativistic, Poynting-flux-dominated jet. These dynamics could lead to potentially observable EM signals, supporting upcoming multimessenger observational campaigns.

Amplification of magnetic fields by differential rotation and feedback by magnetic instabilities is one of the main mechanisms for magnetizing a protoneutron star. I will discuss a recent revision of the Tayler instability of strong toroidal fields and its implications for the stably stratified interior of protoneutron stars. If time permits, I will briefly highlight new simulations quantifying the efficiency of the chiral dynamo instability.

Gravitational and electromagnetic signatures of black hole superradiance are a unique probe of ultralight particles that are weakly-coupled to ordinary matter. Through the kinetic mixing with the Standard Model photon, a dark photon superradiance cloud sources a rotating visible electromagnetic field. I will describe how this leads to the production of a turbulent pair plasma, characterized by efficient magnetic reconnection, which radiates large-luminosity high-energy electromagnetic emissions. This enables multi-messenger search strategies to probe unconstrained regions of parameter space.

Magnetic fields play a key role in shaping the dynamics and observational phenomenology of binary neutron star (BNS) mergers. In this talk, I will present results from general relativistic magnetohydrodynamic (GRMHD) simulations performed with the code GR-Athena++, exploring how different initial magnetic field configurations affect the evolution of BNS mergers. We investigated magnetic field amplification, primarily driven by the Kelvin-Helmholtz instability, the post-merger remnant and disk structure, and the characteristics of the ejected material. I will discuss how these processes impact potential electromagnetic counterparts and their detectability. Finally, I will highlight recent advancements in our numerical methods that improve the modeling of magnetized neutron star mergers, paving the way for more accurate predictions of multimessenger signals from these extreme events.

Numerical simulations are essential to understand the complex physics accompanying the merger of binary systems of neutron stars. However, these simulations become computationally challenging when they have to model the merger remnants on timescales over which secular phenomena, such as the launching of magneti- cally driven outflows, develop. To tackle these challenges, we have recently developed a hybrid approach that combines, via a hand-off transition, a fully general-relativistic code (FIL) with a more efficient code mak- ing use of the conformally flat approximation (BHAC+). We here report important additional developments of BHAC+ consisting of the inclusion of gravitational-wave radiation-reaction contributions and of higher-order formulations of the equations of general-relativistic magnetohydrodynamics. Both improvements have allowed us to explore scenarios that would have been computationally prohibitive otherwise. More specifically, we have investigated the impact of the magnetic-field strength on the long-term (i.e., ∼ 200 ms) and high-resolution (i.e., 150 m) evolutions of the “magnetar” resulting from the merger of two neutron stars with a realistic equa- tion of state. In this way, and for sufficiently large magnetic fields, we observe the loss of differential rotation and the generation of magnetic flares in the outer layers of the remnant. These flares, driven mostly by the Parker instability, are responsible for intense and collimated Poynting flux outbursts and low-latitude emissions. This novel phenomenology offers the possibility of seeking corresponding signatures from the observations of short gamma-ray bursts and hence revealing the existence of a long-lived strongly magnetized remnant.

Supermassive black hole mergers represent a spectacular cosmic event with immense energy implications, emitting gravitational waves equivalent to the total light output of stars in the entire Universe within a brief timespan. These mergers play a crucial role in shaping the overall mass distribution of supermassive black holes across the cosmos. However, capturing visual evidence of these mergers remains elusive due to uncertainties surrounding the type of light emissions accompanying gravitational waves during these events. To address this challenge, novel General Relativistic Magnetohydrodynamics (GRMHD) simulations are being conducted to gain detailed insights into the astrophysical environments surrounding supermassive black hole binaries as they progress towards merger. By employing sophisticated computational techniques capable of accurately capturing the intricate dynamics of accretion within circumbinary disks and the relativistic flow of magnetized matter around each black hole, these simulations reveal the behavior of gas flows near binary systems, particularly when both black holes exhibit spin. These simulated scenarios provide critical data for predicting the electromagnetic and gravitational wave signatures produced by supermassive binary black holes, guiding future observational strategies utilizing advanced missions like LISA and other upcoming astronomical facilities. Ongoing initiatives are focused on refining computational tools to deepen our understanding of supermassive black hole behavior within binary systems and its interactions.

In this talk, I will explore key open questions in our understanding of neutron star mergers and their multi-messenger emission, highlighting recent progress made by our group. I will then introduce some of the first applications of AthenaK to compact binary mergers and outline our plans for its future development.