# BHI Colloquium

## Date:

Tuesday, April 7, 2020, 1:30pm to 2:30pm

## Location:

BHI Meeting Room

Daniel Palumbo (BHI and Harvard University)

Title: MAD vs. SANE: Discriminating Magnetic Field Configurations in Polarized Simulations of M87

Abstract: In April 2017, the Event Horizon Telescope observed the shadow of the supermassive black hole at the core of the elliptical galaxy Messier 87. While the original image was constructed from measurements of the total intensity, full polarimetric data were also collected, and linear polarimetric images are expected in the near future. We propose a modal decomposition of polarized images to capture rotational symmetries that may map underlying magnetic field structure in the accretion flow. We find that simulated images with strong rotational symmetric polarization correspond to magnetically arrested flows, while images with more radially directed electric vector position angles correspond to models with higher black hole spin.

Halston Lim (MIT Kavli Institute for Astrophysics and Space Research)

Title: Relativistic three-body effects in hierarchical triples

Abstract: The hierarchical three-body problem has many applications in relativistic astrophysics, and is likely to play an important role in forming binary black hole mergers detected by LIGO/Virgo  However, many studies have only included secular relativistic corrections responsible for the precession of pericenter of the inner and outer binaries, neglecting relativistic interactions between the three bodies. We revisit this problem and develop a fully consistent derivation of the secular three-body problem to first post-Newtonian order. We start with the Einstein-Infeld-Hoffman equations for a three-body system and expand the accelerations as a power series in the ratio of the semi-major axes of the inner ($a_1$) and outer ($a_2$) binary. We then perform a post-Keplerian, two-parameter expansion of the single orbit-averaged Lagrange planetary equations in $\delta = v^2/c^2$ and $\epsilon = a_1/a_2$ using the method of multiple scales. Using this method, we derive previously-indentified secular effects at $\delta \epsilon^{5/2}$ order that arise directly from the equations of motion. We also calculate new secular effects through $\delta \epsilon^4$ order that can lead to eccentricity growth over many Lidov-Kozai cycles when the tertiary is much more massive than the inner binary. In such cases, inclusion of these effects can substantially alter the evolution of three-body systems as compared to an analysis in which they are neglected. Careful analysis of post-Newtonian three-body effects will be important to understand the formation and properties of coalescing binaries that form via three-body dynamical processes.