THEMIS: A Parameter Estimation Framework for the Event Horizon Telescope

Abstract

The Event Horizon Telescope (EHT) provides the unprecedented ability to directly resolve the structure and dynamics of black hole emission regions on scales smaller than their horizons. This has the potential to critically probe the mechanisms by which black holes accrete and launch outflows, and the structure of supermassive black hole spacetimes. However, accessing this information is a formidable analysis challenge for two reasons. First, the EHT natively produces a variety of data types that encode information about the image structure in nontrivial ways; these are subject to a variety of systematic effects associated with very long baseline interferometry and are supplemented by a wide variety of auxiliary data on the primary EHT targets from decades of other observations. Second, models of the emission regions and their interaction with the black hole are complex, highly uncertain, and computationally expensive to construct. As a result, the scientific utilization of EHT observations requires a flexible, extensible, and powerful analysis framework. We present such a framework, Themis, which defines a set of interfaces between models, data, and sampling algorithms that facilitates future development. We describe the design and currently existing components of Themis, how Themis has been validated thus far, and present additional analyses made possible by Themis that illustrate its capabilities. Importantly, we demonstrate that Themis is able to reproduce prior EHT analyses, extend these, and do so in a computationally efficient manner that can efficiently exploit modern high-performance computing facilities. Themis has already been used extensively in the scientific analysis and interpretation of the first EHT observations of M87.

First M87 Event Horizon Telescope Results. VII. Polarization of the Ring

“…Complex station gains are reconstructed via the Laplace approximation (see Section 6.8 of Broderick et al 2020a). The right-and left-hand complex station gains are constrained to be equal, and permitted to vary independently on every scan.…”

Verification of Radiative Transfer Schemes for the EHT

“…Out of these, BHOSS, IPOLE, RAPTOR, and GRTRANS are coupled to the EHT parameter estimation framework THEMIS (Broderick et al 2020) via a driver routine, while VRT2 and ODYSSEY are both natively included.…”

How narrow is the M87* ring? I. The choice of closure likelihood function

“…As full gain modeling would be computationally intractable, EHTC considered two simpler alternatives. The first used amplitudes and closure phases, including amplitude gain parameters but utilizing a Laplace approximation to marginalize over them instead of fully sampling the likelihood (Broderick et al 2020). The second used closure amplitudes and closure phases without including gain parameters (EHT VI).…”