Strong Gravity

Highly magnetized neutron stars are a source of extreme transients observed in different bands, like the fast radio burst (FRB) and associated hard X-ray burst from the Galactic magnetar SGR 1935+2154. The origin of such outbursts, hard X-rays on the one hand and millisecond duration FRBs on the other hand, is still unknown. We present a global model for various kinds of such magnetar outbursting activities. Crustal surface motions are expected to twist the inner magnetar magnetosphere by shifting the frozen-in footpoints of magnetic field lines. We discuss criteria for the development of various instabilities of 3D twisted flux bundles in the force-free dipolar magnetospheres and compare their energetic properties to observations of magnetar X-ray flares. We then review a recently developed FRB generation mechanism in the outer magnetosphere of a magnetar. The strong magnetic pulse induced by a magnetar flare collides with the current sheet of the magnetar wind, compresses and fragments it into a self-similar chain of magnetic islands. Time-dependent plasma currents created during their collisions produce relatively narrow-band GHz emission with luminosities sufficient to explain bright extragalactic FRBs.

Zoom ink:  https://pitp.zoom.us/j/96469987183?pwd=N1UzVkh6RGZUeWV6TFZJLzk0M3VWZz09

After discussing the basics of Carrollian physics, we shall revisit the motion  of massless particles with anyonic spin in the black hole horizons. As recently shown, such particles can move within the horizon of the black hole due to the  coupling of charges associated with a 2-parametric central extension of the 2-dimensional Carroll group to the magnetic field around the black hole -- the so called "anyonic  spin-Hall effect". We shall study several examples of such motions. Of these, the most  interesting is the motion in misaligned (asymptotically uniform) magnetic field around Kerr,  which results in a time dependent motion of Carrollian particles that is reminiscent  of "monarch migration".

Zoom link:  https://pitp.zoom.us/j/97094479078?pwd=SHN6NEU5aE93OXVQYk9aWTVOdzRRUT09

The geometry of kilonovae is a key diagnostic of the physics of merging neutron stars with current hydrodynamical merger models typically showing aspherical ejecta. Previously, Sr II was identified in the spectrum of the only well-studied kilonova AT2017gfo, associated with the gravitational wave event GW170817. In this talk, we show that combining the strong Sr II P Cygni absorption-emission spectral feature and the blackbody nature of the kilonova spectrum, to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source also shows the same sphericity independently. The near-spherical geometry suggests early spectra of kilonovae may provide excellent precision cosmic distance measurements using the Expanding Photosphere Method.

Zoom link:  https://pitp.zoom.us/j/97287055430?pwd=bFVyeTRuZHVLcGlDdnhlK1d0OWE1Zz09

Fast Radio Bursts (FRBs) are extremely energetic, millisecond-duration radio bursts coming outside our galaxy. The burst arrival time at different frequencies implies the number of electrons it has encountered in the foreground. Therefore, FRB is considered a promising new cosmology probe. The uncertainty of the circum-burst environment is one of the biggest concerns regarding its potential as a probe. In this talk, I will review the current understanding of the FRB progenitor and the recent study of the circum-burst environment. I will also discuss the many remaining mysteries, including the seemingly diverse nature of the sources, the magneto-environment, and the 16-day periodicity I found with one source. With the current and upcoming instruments, there will be more FRBs with orders of magnitude better spatial resolution detected in the next few years. The result will be an explosion of opportunity for understanding the burst origin and probing cosmic matter distribution at various spatial scales.

Zoom link: https://pitp.zoom.us/j/92030175800?pwd=dWlGVjV0Qnh5bGhLMk1pd01yRmNKQT09

Gamma ray bursts (GRBs) are the most luminous electromagnetic events in the universe. Short GRBs, typically lasting less than 2 seconds, have already been associated with binary neutron star (BNS) mergers, which are also sources of gravitational waves (GWs). The ultimate fate of a BNS, after coalescence, is usually expected to be a black hole (BH) with 2-3 solar masses. However, numerical relativity simulations indicate the possible formation of a short-lived hypermassive neutron star (HMNS), lasting for tens to hundreds of milliseconds after the BNS merger and before gravitational collapse forms a BH. The HMNS is expected to emit GWs with kHz frequencies that will be detectable by third generation ground-based GW detectors in the 2030s. I will present results from a recent analysis that revealed evidence for HMNSs by looking for kHz quasiperiodic oscillations in gamma-ray observations obtained in the 1990s with the Compton Gamma Ray Observatory.

Zoom link:  https://pitp.zoom.us/j/96687956901?pwd=MkgrUGlqY3IyRCs2bXJYVkhUVEpPZz09

No-hair theorems prevent black holes in General Relativity (GR) from being characterized by any property other than their mass, electric charge, and spin. Scalar fields provide perhaps the simplest way to generalize GR, and it turns out that cases exist where the no-hair theorems can be evaded and black holes with scalar hair may emerge. In this talk I will examine the notion of nontrivial scalar field configurations arising in the strong regime of gravity near black holes, but also compact neutron stars and wormholes. I will go over the concept of spontaneous scalarization of compact objects and discuss the viability of scalarized solutions against observations.

Zoom link:  https://pitp.zoom.us/j/92273469560?pwd=elpDRzliOHFaaUh3YVhzbC92NFlmUT09

The formation of Primordial black holes is naturally enhanced during the quark-hadron phase transition, because of the softening of the equation of state: at a scale between 1 and 3 solar masses, the threshold is reduced of about 10% with a corresponding abundance of primordial black significantly increased by more than 100 times. Performing detailed numerical simulation we have computed the modified mass spectrum for such black holes. Making then a confutation with the LVK phenomenological models describing the GWTC-3 catalog, it is  shown that a sub-population of primordial black black holes formed in the solar mass range is compatible with the current observational constraint and could explain some of the interesting sources emitting gravitational waves detected by LIGO/VIRGO in the black hole mass gap, such as GW190814, and other light events.

Zoom Link:https://pitp.zoom.us/j/94805744664?pwd=Z3oyd05YWVlwenI5NFV6bFZOYUR0dz09

Numerical relativity continues to play a crucial role in interpreting gravitational wave detections as well as the first multi-messenger detection of GW170817. More so, state-of-the-art models for kilonvae, gravitational waves, and more rely on the thousands of numerical relativity simulations that have taken place over more than 20 years.  Simulations of binary systems including neutron stars are particularly taxing due to the equation of state of matter being a significant unknown.  In spite of this fact, there exists vast amount of literature on the independent influence mass asymmetry or spin can have on the merger and post-merger dynamics of neutron star binaries across a wide array of possible equations of state.

In this talk I will extend this topic to extremal configurations consisting of binaries that are not only asymmetric, but include appreciable spins on the component neutron stars.  To do so I will give an introduction into the initial data problem for numerical relativity, it's complexities, and its importance to current and future research.  Furthermore, I will discuss a collection of results for extremal binary configurations including neutron stars and why this line of research is important to enable the next generation of multi-messenger models.

Zoom Link: https://pitp.zoom.us/j/99895521696?pwd=T1VtN0RGbjZrVTNleXB3V0FtQjhldz09

Tidal interactions in coalescing binary neutron stars modify the dynamics of the inspiral, and hence imprint a signature on their gravitational-wave (GW) signals in the form of an extra phase shift. We need accurate models for the tidal phase shift in order to constrain the supranuclear equation of state from observations. In previous studies, GW waveform models were typically constructed by treating the tide as a linear response to a perturbing tidal field. In this work, we incorporate non-linear corrections due to hydrodynamic three- and four-mode interactions and show how they can improve the accuracy and explanatory power of waveform models. We set up and numerically solve the coupled differential equations for the orbit and the modes, and analytically derive solutions of the system's equilibrium configuration. Our analytical solutions agree well with the numerical ones up to the merger and involve only algebraic relations, allowing for fast phase shift and waveform evaluations for different equations of state over a large parameter space. We find that, at Newtonian order, nonlinear fluid effects can enhance the tidal phase shift by  >~ 1 radian at a GW frequency of 1000 Hz, corresponding to a  10−20%  correction to the linear theory. The scale of the additional phase shift near the merger is consistent with the difference between numerical relativity and theoretical predictions that account only for the linear tide. Nonlinear fluid effects are thus important when interpreting the results of numerical relativity, and in the construction of waveform models for current and future GW detectors.

Zoom link:  https://pitp.zoom.us/j/91730134841?pwd=U0JXNHhFbWdQYno3aUpDWHRxWEtkUT09

The Event Horizon Telescope has released total intensity images of the Messier 87* and Sagittarius A* accretion flows; polarized images have been released for M 87*, and are imminent for Sgr A*. These images are a rich source of theoretical constraints on the black hole accretion flow system, but a trustworthy measurement of either black hole's spin remains elusive. Spin nonetheless remains a high priority, as the black hole angular momentum is deeply linked to mechanisms of energy extraction and galactic co-evolution. In my talk, I will discuss my work on providing theoretical traction on supermassive black hole spin, and will review the state of spin measurements using existing and future EHT data, including measurements of the black hole photon ring, inference of near-horizon magnetic field structure, and next-generation spacetime/emissivity inference codes.

Zoom:  https://pitp.zoom.us/j/95355525128?pwd=blczM1ZUMGs5RmNxMVNCV3hlRDA4UT09

Fluid mechanics has proven to be remarkably successful in describing a wide variety of substances, both familiar and exotic. The latter category includes relativistic fluids, often arising in the most extreme regimes found anywhere in the universe. One such example is the quark-gluon plasma (QGP) formed in collisions of heavy ions, which exists at temperatures hot enough to “melt” hadrons; another is the matter composing neutron stars, whose density is comparable to that of an atomic nucleus. Beyond the surprising fact that the aforementioned substances act as fluids, they share an additional similarity in that they may both be measurably viscous, a feature accounted for in models of the QGP but almost never in neutron star simulations.

In this talk I will overview progress toward the incorporation of dissipative effects such as viscosity into relativistic fluid models of astrophysical systems. I will begin by reviewing the modern inter- pretation of fluid mechanics as a gradient expansion about thermodynamic equilibrium, and will discuss the nuances of constructing a theory compatible with beyond-equilibrium thermodynamics and general relativity. I will then define and motivate a promising new formulation of relativistic dissipative hydrodynamics known as BDNK theory before summarizing recent work toward its application in models of neutron stars.

Zoom link:  https://pitp.zoom.us/j/99927210105?pwd=aUJWa0NobWFrT0FHMUhqZmRHWlREdz09

Gravitational waves emitted by compact binaries enable unprecedented tests of gravity at highly non-linear regimes, as well as the underlying cosmological model. Going beyond the current null tests of gravity requires accurate theoretical modelling of the waveforms in viable extensions of General Relativity. In the first part of this talk, I will present the recent results and physical insights from analytical modelling of the gravitational waves in the so-called Einstein-scalar-Gauss-Bonnet gravity. Being a sub-class of both Horndeski and quadratic gravity, this theory introduces non-linear curvature corrections to strong-field regime of gravity, allows for hairy-black hole solutions, and scalar-induced tidal deformations. I will present the gravitational-wave signatures of theory’s curvature corrections and the prospects of testing the features of this theory through gravitational wave observations. In the second part of the talk, I will discuss the prospects of using compact mergers for cosmological tests by solely relying on their gravitational wave signals. Using recent constraints on the equation-of-state of neutron stars from multi-messenger observations of NICER and LIGO/Virgo, I show possible bounds on the Hubble constant (H0) found from (single and multiple) neutron star-black hole standard sirens in the next-generation gravitational wave detector era. I show that such systems could enable unbiased 13% - 4% precision measurement of H0 (68% credible interval) within an observation time-frame of hours to a day.

Zoom link:  https://pitp.zoom.us/j/93964588227?pwd=cGsxcEZHRlNjd3R5eHg5dzdtT2lndz09