Event Horizon Telescope

EHT is a millimeter-wavelength very-long-baseline interferometry (VLBI) experiment with unprecedented micro-arcsecond angular resolution using an array of millimeter telescopes that spans the Earth. (Figure credit: EHT Collaboration)

My research as a graduate student at the University of Arizona focused on new radio instrumentation, observation, and data analysis for the Event Horizon Telescope (EHT) project. EHT VLBI combines a network of widely separated millimeter telescopes to simulate a much larger aperture to study supermassive black holes at the highest resolutions ever achieved. From the EHT observations, we expect to better understand the physics around the black hole, as well as probe General Relativity. In 2019, EHT reported the first-ever picture of the black hole with the observation of the nuclear black hole in the galaxy M87 (EHT Collaboration et al. 2019a,b,c,d,e,f, 2021a,b). Then, EHT presented the first image of the Galactic Center black hole Sagittarius A* in 2022 (EHT Collaboration et al. 2022a,b,c,d,e,f).

A VLBI receiving system for the South Pole Telescope

230 GHz assembly of the SPT VLBI receiver.

In addition to the nuclear black hole in M87, another primary target of the EHT is the Galactic Center black hole Sagittarius A* (Sgr A*). For the observation of Sgr A*, the geographical location of the South Pole provides the longest EHT baselines, thus the highest angular resolution of the global array. I developed a VLBI receiver system operating at 230 and 345 GHz (1.3 and 0.87 mm) for the South Pole Telescope (SPT). The SPT is a 10-meter dish located at the South Pole (Carlstrom et al. 2011), and an essential element of the EHT array. However, a coherent receiver and VLBI signal chain were required because the existing SPT-3G cosmic microwave background (CMB) receiver (Benson et al. 2014) was not suitable for the phase-sensitive VLBI observations. I carried out the following steps of the receiver development from inception to implementation: 1) electromagnetic simulations of the millimeter receiver components, 2) assembly of specially manufactured components, 3) system testing, and 4) the software development.

Time variability of the Galactic Center black hole Sgr A*

The rapid variability of Sgr A* has been reported in multiwavelength observations (e.g., Marrone et al. 2008) as well as in early EHT observation (Fish et al. 2011), and it makes the EHT VLBI analysis challenging. I’m interested in understanding the physics of Sgr A* from its variable emission. In my thesis, I developed statistical techniques that will be particularly useful for the study of Sgr A* using the EHT array: a Bayesian framework to compare the EHT data with time-dependent black hole simulation models using the VLBI observables.


Dan Marrone, Dimitrios Psaltis, Feryal Özel, Lia Medeiros, Chi-Kwan Chan, The South Pole Telescope Collaboration, The Event Horizon Telescope Collaboration

© 2021 Junhan Kim. All rights reserved.