# BHI Colloquium

## Date:

Tuesday, October 29, 2019, 1:30pm to 3:00pm

## Location:

BHI Meeting Room

David Kaiser (MIT)

Title: The Price of Gravity: Private Patronage and the Transformation of Gravitational Physics after World War II

Abstract: This talk (based on research conducted with Dean Rickles) focuses on how various private patrons intervened to support research in gravitational physics from the late 1940s through the early 1960s. The analysis centers primarily on two wealthy and eccentric entrepreneurs, Roger Babson and Agnew Bahnson, and their efforts to galvanize the study of gravitation. Not only did these patrons provide generous funding at a time when the subject of gravitation received few other institutional sources of support; they also helped to knit together a research community. Yet the arrangements were far from straightforward, and the talk will trace the evolution of the patronage relationships over time. These unusual philanthropic efforts played an outsized role in spurring what has been called the renaissance of general relativity during the middle decades of the twentieth century.

Hung-Yi Pu (Perimeter Institute)

Title: Exploring Horizon-scale Black Hole Image Features: a phenomenological approach

Abstract: Horizon-scale images of astrophysical black holes can reveal several important features of their accretion and jet environment. To explore the colossal parameter space and related black hole shadow image gallery, phenomenological models for accretion/jet  provides a prompt, flexible, and heuristic approach to understand  the key parameters for a black hole and its environment, such as black hole spin, accretion dynamics, flow height, electron thermal dynamics, injection site of energetic electrons and more. In this talk, we will discuss some phenomenological models and their applications.

Robert Wald (University of Chicago)

Title: Black Hole Shadows and Photon Rings

Abstract: The presence of a bright "photon ring" surrounding a dark "black hole shadow" has been discussed as a key feature of the observational appearance of emission originating near a black hole. The photon ring consists of light rays that are nearly trapped by the black hole and emerge to infinity after orbiting the black hole (possibly many times), thereby picking up additional brightness. Using toy models of thin disc emission in Schwarzschild, we analyze the relationship between emission near a black hole and its appearance to a distant observer, with the particular aim of determining the importance of the photon ring. We conclude that the back-side image of the disc (produced by light rays that make half an orbit around the black hole) can make a significant contribution ($\sim 10\%$) to the total luminosity, but far too few light rays orbit the black hole one or more times to contribute significantly to the luminosity. Depending on the emission profile, the observed emission may or may not peak near the photon ring.