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# Strong Gravity

This series consists of talks in areas where gravity is the main driver behind interesting or peculiar phenomena, from astrophysics to gravity in higher dimensions.

## Seminar Series Events/Videos

jan 28 2021 - 1:00pm
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## Coherent Gravitational Waveforms and Memory from Cosmic String Loops

Jeudi jan 21, 2021

We construct, for the first time, the time-domain gravitational-wave strain waveform from the collapse of a strongly gravitating Abelian Higgs cosmic string loop in full general relativity. We show that the strain exhibits a large memory effect during merger, ending with a burst and characteristic ringdown as a black hole is formed. Furthermore, we investigate the waveform and energy emitted as a function of width, radius and string tension Gμ.

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## The parents of LIGO black holes and their hometown

Jeudi jan 14, 2021
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Two of the dominant channels to produce black-hole binary mergers are believed to be the isolated evolution of stellar binaries in the field and dynamical formation in star clusters. Pair instabilities prevent stellar collapse from generating black holes more massive than about 45-60 solar mass. This  “mass gap” only applies to the field formation scenario: repeated mergers in clusters can fill the gap. A similar reasoning applies to the binary’s spin parameters.

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## Black hole perturbations and gravitational wave source modelling

Jeudi jan 07, 2021
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Abstract: TBD

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## Photon Emission from Circular Equatorial Orbiters around Kerr Black Holes

Jeudi déc 17, 2020
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We consider monochromatic and isotropic photon emission from circular equatorial Kerr orbiters. We calculate the critical curve delineating the region of photon escape from that of photon capture in each emitter’s sky, allowing us to derive analytic expressions for the photon escape probability and the redshift-dependent total flux collected on the celestial sphere as a function of emission radius and black hole parameters. This critical curve generalizes to finite orbital radius the usual Kerr critical curve and displays interesting features in the limit of high spin.

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## Self-torque and frame nutation in binary black hole simulations

Jeudi déc 10, 2020

We investigate the precession of the spin of the smaller black hole in binary black hole simulations. By considering a sequence of binaries at higher mass ratios, we approach the limit of geodetic precession of a test spin. This precession is corrected by the self-torque'' due to the smaller black hole's own spacetime curvature. We find that the spins undergo spin nutations which are not described in conventional descriptions of spin precession, an effect that has been noticed previously in simulations.

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## QED-mediated plasma processes in compact objects: magnetic reconnection and beyond

Jeudi déc 03, 2020
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In compact astrophysical objects, such as neutron star magnetospheres, black-hole accretion disk coronae and jets, the main energy reservoir is the magnetic field. The plasma processes such as magnetic reconnection and turbulence govern the extraction of that energy, which is then deposited into heat and accelerated particles and, ultimately, the observed emission.

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## GRB Jets At All Angles

Jeudi nov 26, 2020
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Abstract: Gamma-ray bursts (GRBs) associated with gravitational wave events are, and will likely continue to be, viewed at a much larger inclination than GRBs without gravitational wave detections.  Viewing GRBs and their afterglows at large inclination can massively affect the observed electromagnetic emission, as dramatically demonstrated by the binary neutron star merger event GW170817.  Analyzing this event and future ones requires an extension of the common GRB afterglow models which typically assume emission from a structureless (top-hat) jet viewed on-axis.

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## Effective Fly-Bys: Gravitational Waves from Dynamical Capture Binaries

Jeudi nov 19, 2020
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Compact binaries may be formed dynamically in globular clusters, with large (close to unity) orbital eccentricity and emitting gravitational waves within the detection band of ground based detectors. The gravitational waves from such sources resemble more a discrete set of bursts than the continuous signal of their quasi-circular counterparts. I here discuss the construction of new analytic waveforms for such systems, which rely on treating the problem as a perturbation of a parabolic fly-by.

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## Disappearing stars without a trace: what is their maximum angular momentum?

Jeudi nov 12, 2020

We have tentative evidence of massive stars that disappear without a bright transient. It is commonly argued that this massive stars have low angular momentum and can collapse into a black hole without significant feedback. In this talk I will make use of general-relativistic hydrodynamical simulations to understand the flow around a newly-formed black hole. I will discuss the angular momentum needed in order for the infalling material to be accreted into the black hole without forming a centrifugally supported structure, thus generating no effective feedback.

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## GW190521 may be an intermediate mass ratio inspiral

Jeudi nov 05, 2020
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