Strong Gravity

In the last couple of years it has been demonstrated that black hole spacetimes contain many more marginally outer trapped surfaces (MOTS) than had been previously recognized. For example, there is an infinite family of axially symmetric MOTS even in the Schwarzschild solution, of which the apparent horizon is only the first element. In a recent series of papers (arXiv: 2104.10265, 2104.11343, 2104.11343) we demonstrated that these exotic new MOTS play a key role in black hole mergers and, in fact, are the missing pieces needed to complete the apparent horizon “pair of pants” diagram. This merger turns out to be far richer than that of event horizons and, staying with clothing analogies, is better represented by a rococo ball gown than a pair of pants. 

In this talk, I will overview the techniques that we used to find these surfaces, explain the role played by the exotic MOTS in resolving the merger of apparent horizons during a (head-on, non-rotating) black hole merger and also show how the stability operator brings order to what initially appears to be a chaotic melee of (often self-intersecting!) MOTS as they create, annihilate and weave through time. The stability operator was initially introduced by Andersson, Mars and Simon to characterize when a MOTS will smoothly evolve in time, but I will show that it can also be fruitfully understood as the Jacobi operator for MOTS.

We discuss how we can use numerical-relativity simulations to derive gravitational-wave and electromagnetic models describing the binary  neutron star coalescence. We show how these models can be used within a multi-messenger framework to derive new constraints on the neutron-star equation of state and the Hubble constant. For this purpose, we analyze the gravitational wave signal GW170817 and its electromagnetic counterparts AT2017gfo and GRB170817A, together with X-ray observations by NICER, radio observations of massive pulsars, and nuclear theory computations. Similarly, we also discuss that a non-detection of a kilonova for the second binary neutron-star merger detection GW190425 placed constraints on the properties of the system.

Zoom Link: https://pitp.zoom.us/j/99911822549?pwd=NXNNMWJVOUhMTGJSSGYwWUN2NEcxQT09

Gravitational wave observations are beginning to reveal the nature of the dark side of our universe. The Advanced LIGO and Virgo detectors have observed dozens of binary black hole mergers during the recent third observing run and, with planned sensitivity improvements, expect to observe significantly more binary black hole mergers in future observing runs. The combination of the increased number of detections  and the sheer volume of data associated with each detection provides a significant data analysis challenge. In recent years, various machine learning approaches such as convolutional neural networks have been explored as a basis for rapid analyses for gravitational wave data. This seminar will give a brief introduction to current transient gravitational wave data analysis methodology and highlight novel applications of machine learning for rapid detection of binary black holes and rapid inference of their astrophysical properties. The use of generative machine learning algorithms for transient gravitational wave signal generation will also be discussed.

Since their first discovery in 2015, gravitational-wave observations yielded several "surprises." The LIGO and Virgo observatories detected more and heavier black holes than anticipated; the first object in the lower mass gap was found; and LIGO announced the discovery of a particularly heavy black hole that could have not come from stellar core collapse. The surprises point to the possibility that some of LIGO/Virgo's black hole mergers occurred in the dense accretion disks of active galactic nuclei (AGNs). AGNs act like black hole assembly lines, resulting in multiple consecutive mergers that create heavier and faster-spinning black holes. I will discuss what we currently know about AGN-assisted mergers and which of LIGO/Virgo's events are suspects. I will finally discuss the prospects of multi-messenger observations from AGN assisted mergers.

Zoom Link: https://pitp.zoom.us/j/93121526365?pwd=c1VCTjBEZnlXYk5HVTFObVBadHlxQT09

Gravitational waves provide a unique observational handle on the properties of strong, dynamical gravity. Ringdowns, in particular, cleanly encode information about the structure of black holes, allowing us to test fundamental principles like the no-hair theorem and the area law. In this talk, I will review the status of this effort, including recent observational results and remaining challenges.

Over forty detections of binary-black-hole mergers have been made during the first three observing runs of the LIGO and Virgo detectors. With this larger number of measurements of increasing accuracy, many of the remarkable predictions of general relativity for strongly curved, dynamical spacetimes will be able to be studied observationally. In this talk, I will discuss one class of strong-gravity phenomena, called gravitational-wave memory effects, which are predictions of general relativity that are most prominent in systems with high gravitational-wave luminosities, like binary black holes. Memory effects are characterized by changes in the gravitational-wave strain and its time integrals that persist after a transient signal passes by a detector. I will summarize the computation of these effects and the prospects for current and planned future gravitational-wave detectors to detect memory effects from black-hole mergers; in particular, there could be evidence for the memory effect in just a few years of advanced LIGO, Virgo, and KAGRA data at their design sensitivities. I will also review what observing gravitational-wave memory effects can teach us about the symmetries and conserved quantities around isolated systems like binary-black-hole mergers. Time permitting, I will present results on memory effects in scalar-tensor theories of gravity and on subleading memory effects.

Core-Collapse Supernovae are a fantastic laboratory to study fundamental physics.  The messengers from the core, neutrino and gravitational waves, carry a wealth of information about the dynamics, thermodynamics, underlying physics, and structure of massive stars at the end of their lives.  In this talk, I will discuss some of the ways we can use supernovae to probe this physics.  From quark-hadron phase transitions emitting unique gravitational wave and neutrino signals, to neutrinos telling us precise information about the distance to and progenitor mass (!) of galactic supernovae, even before the optical signal is emitted.  I will also talk about current efforts to model the population of core-collapse supernovae using parameterized 1D models.

 

The X-ray source Cygnus X-1 was discovered by sounding rockets in 1964 and is perhaps the oldest and best known "black hole candidate".   Its celestial coordinates were poorly known until 1971, when simultaneous radio flares and X-ray flares were observed which enabled the X-ray source to be associated with a 9th magnitude O-star known as HDE 226868.  This star was subsequently shown to be a single-lined spectroscopic binary with an orbital period of about 5.6 days.  However, owing to difficulties with measuring the system geometry and the distance to the source, the mass of the dark companion remained elusive.  In 1974, Stephen Hawking and Kip Thorne famously made a wager on the nature of the dark companion in Cygnus X-1, with Hawking wagering that the dark companion was not a black hole.  Eventually the consensus was that Cygnus X-1 does indeed contain a black hole, and Hawking conceded the bet in 1990.    In the following years many estimates for the black hole mass appeared in the literature, and most of them had large uncertainties, mainly owing to the difficulty of measuring the distance to the source.  In this talk I will describe how we were able to use VLBI radio observations to measure a precise distance to the source, and how, in turn, this precise distance allowed us to obtain precise masses for the component stars in Cygnus X-1.  Using the refined distance and geometry of the binary, it is possible to measure the relativistic spin parameter a* of the black hole using the X-ray continuum fitting technique.  We find that the black hole spin is nearly maximal with  a* > 0.9985 (3σ).

LIGO's successful detection of gravitational waves has revitalized the theoretical understanding of the angular momentum carried away by gravitational radiation. An infinite-dimensional supertranslation ambiguity has presented an essential difficulty for decades of study. Recent advances were made to quantify the supertranslation ambiguity in the context of binary coalescence. In this talk, we will present the first definition of angular momentum in general relativity that is completely free from supertranslation ambiguity. The new definition of angular momentum is derived from the limit of the quasilocal angular momentum we defined previously.

The talk will be based on my latest two papers 2103.02616 and 2103.05700. 
In the first part, I will present my GRMHD simulation of a neutron-star post-merger disk with neutrino fast flavour conversion included dynamically. The fast conversion of neutrinos can lead to flavour space equipartition ubiquitously on the time scale as short as 1ns. Due to the reduction of the number density of electron and anti-electron neutrino, the ejecta becomes more neutron rich. The final r-process nucleosynthesis sees an enhanced abundance of heavy elements close to the solar values. A similar effect may allow for increased lanthanide production in collapsars.
In the second part, I will present fast magnetic energy dissipation through the collision of Alfven waves with anti-aligned magnetic fields. The collision produces a current sheet sustained by an electrical field breaking the MHD condition. Particles entering the current sheet are accelerated following a relativistic variation of Speiser orbit and escape with higher energy. This mechanism can dissipate a large fraction of wave energy, nearly 100% when the wave magnetic field equals the background magnetic field. The fast dissipation may occur in various objects, including magnetars, jets from accreting black holes, and pulsar wind nebulae.

Combining information from the first gravitational wave detected gamma-ray burst, GRB 170817 with observations of cosmological GRBs holds important lessons for understanding the structure of GRB jets and the required conditions at the emitting region. It also re-frames our understanding of more commonly observed phenomena in GRBs, such as X-ray plateaus, and sets our expectations for future observations. I will present different lines of argument suggesting that efficient gamma-ray emission in GRBs has to be restricted to material with Lorentz factor > 50 and is most likely confined to a narrow region around the core. GRB jets viewed slightly beyond their jet cores, result in X-ray plateaus that are consistent with observed light-curves and naturally reproduce correlations between plateau and prompt emission properties. For jets viewed further off-axis (that are expected to be detected as future GW triggered events) we provide new analytical modelling that reveals two different types of light-curves that could be observed (single or double peaked) and outlines how the underlying physical properties can be recovered from such observations.