About

Charles F. Gammie is a professor in the Department of Astronomy at the Illinois College of Liberal Arts & Sciences. His research interests include black holes, the formation of the Moon, planet formation, star formation, cosmic ray transport, and interstellar turbulence. He has been recognized as an excellent teacher, receiving the Teacher Ranked as Excellent award in Fall 2018. Gammie holds additional campus affiliations as the Richard and Margaret Romano Professorial Scholar in Astronomy, University Scholar in Astronomy, and is a professor in both Physics and Astronomy. He is also associated with the National Center for Supercomputing Applications (NCSA) and holds the Stanley O. Ikenberry Endowed Chair in Physics. His recent publications involve studies on supermassive binary black hole candidates, AGN jets, and black hole imaging, contributing to the understanding of phenomena observed with the Event Horizon Telescope.

Research topics

• Physics
• Astrophysics
• Astronomy
• Computer Science
• Optics
• Geology
• Quantum mechanics
• Remote sensing
• Computational physics

Selected publications

• Locating the missing large-scale emission in the jet of M87* with short EHT baselines

  arXiv (Cornell University) · 2026-01-19

  preprintOpen access

  In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.

  PublisherDOI

• ESO White Paper on Intensity Interferometry: Cosmology, Fundamental Physics, Quantum Optics

  Open MIND · 2026-02-13

  preprint

  In this whitepaper, we outline how recent technological advances and ongoing developments open qualitatively new science opportunities in cosmology, fundamental physics, and quantum astrophysics. First, intensity interferometry can contribute to one of the most foundational observables in cosmology: the expansion rate of the Universe. Its angular resolution allows it to resolve the angular extent of extragalactic objects such as supernovae or quasars; combined with a physical scale local to the source, this yields an angular diameter distance and hence a 'Hubble diagram'. Second, the nature of dark matter can be probed via the astrometric lensing signatures of tiny dark matter halos. Third, intensity interferometry gives direct access to second-order coherence properties of astrophysical emission, opening a window onto genuinely quantum aspects of astrophysical light.

  DOI

• Ring Asymmetry and Spin in M87*

  arXiv (Cornell University) · 2026-01-01

  preprintOpen access

  Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

  PublisherDOI

• ESO White Paper on Intensity Interferometry: Cosmology, Fundamental Physics, Quantum Optics

  arXiv (Cornell University) · 2026-02-13

  articleOpen access

  In this whitepaper, we outline how recent technological advances and ongoing developments open qualitatively new science opportunities in cosmology, fundamental physics, and quantum astrophysics. First, intensity interferometry can contribute to one of the most foundational observables in cosmology: the expansion rate of the Universe. Its angular resolution allows it to resolve the angular extent of extragalactic objects such as supernovae or quasars; combined with a physical scale local to the source, this yields an angular diameter distance and hence a 'Hubble diagram'. Second, the nature of dark matter can be probed via the astrometric lensing signatures of tiny dark matter halos. Third, intensity interferometry gives direct access to second-order coherence properties of astrophysical emission, opening a window onto genuinely quantum aspects of astrophysical light.

  PublisherOA PDF

• Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign

  arXiv (Cornell University) · 2026-04-11

  preprintOpen access

  Sagittarius A* (Srg A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. We investigated the variability and polarization properties of Srg A* using ALMA observations during the 2018 Event Horizon Telescope campaign. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. We find low variability in total intensity ($σ/μ< 10\%$), but significantly higher variability in linear and circular polarization (~ 30% and ~ 50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between -2 and -3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Our results offer new constraints on the physical mechanisms driving variability in Srg A*, and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.

  PublisherDOI

• Ring Asymmetry and Spin in M87*

  ArXiv.org · 2026-01-01

  articleOpen access

  Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

  PublisherOA PDF

• Data for "Event Horizon Telescope Pattern Speeds in the Visibility Domain”

  Illinois Data Bank · 2026-01-01

  datasetOpen accessSenior author

  Visibility Amplitude Pattern Speeds from the v3 Sgr A* Library. Data for "Event Horizon Telescope Pattern Speeds in the Visibility Domain” (Conroy et al.). Data are provided in 2 file formats: a TXT table which is a standard format for the Astrophysical Journal (ApJ) where the paper is submitted and the original NPY format.

  PublisherDOI

• Locating the missing large-scale emission in the jet of M87* with short EHT baselines

  ArXiv.org · 2026-01-19

  articleOpen access

  In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.

  PublisherOA PDF

• Ring Asymmetry and Spin in M87*

  The Astrophysical Journal · 2026-03-25

  articleOpen access

  Abstract Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring’s peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin a * . We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor ∣ a * ∣ ≲ 0.2. This is consistent with the Blandford–Znajek model for M87’s jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

  PublisherDOI

• Relativistic treatment of accretion disk torques on extreme mass-ratio inspirals around nonspinning black holes

  Physical review. D/Physical review. D. · 2025-11-14 · 5 citations

  article

  We develop a relativistically accurate formalism to model the interaction between stellar mass compact objects embedded in thin accretion disks around a nonspinning supermassive black hole, using tools from self-force theory and Hamiltonian perturbation theory. We then apply this formalism to analyze the evolution of a compact object on a nearly circular and equatorial orbit interacting with a thin equatorial disk. We provide analytic and relativistically accurate expressions for the rates of energy and angular momentum exchanged during interactions due to Lindblad and corotation resonances. Our results show that relativistic corrections can enhance the magnitude of the torque by 1--2 orders of magnitude compared to purely Newtonian expressions when the orbit of the compact object is smaller than ten Schwarzschild radii of the supermassive black hole. We also demonstrate that strong relativistic effects shifts the inner Lindblad resonances closer to the compact object than the outer Lindblad resonances when the compact object is closer than four Schwarzschild radii to the supermassive black hole, potentially leading to a reversal in the direction of the torque acting on the compact object. Finally, we provide a dephasing estimate and show that using the relativistic torque formula is crucial to obtain reliable estimates for extreme mass-ratio inspirals in orbits closer than five Schwarzschild radii to the supermassive black hole. Our results highlight the importance of using relativistically accurate models of environmental interactions in extreme mass-ratio inspirals close to a supermassive black hole.

  PublisherDOI

Recent grants

• Collaborative Research: Black Hole Accretion Theory and Computation Network

  NSF · $515k · 2013–2018

• PECASE: Theory of Black Hole Accretion Flows

  NSF · $570k · 2001–2008

• Collaborative Research: Predicting the Observational Appearance of Accreting Black Holes

  NSF · $436k · 2017–2023

• Precision Bothrology: Toward a Physical Theory of Sgr A*

  NSF · $726k · 2007–2014

• ITR: MHD Simulations in Full General Relativity

  NSF · $2.3M · 2002–2009

Frequent coauthors

• G. Desvignes

  375 shared

• Jonathan Weintroub

  Center for Astrophysics Harvard & Smithsonian

  300 shared

• Ue‐Li Pen

  289 shared

• Shiro Ikeda

  The University of Tokyo

  277 shared

• Kazunori Akiyama

  273 shared

• R. P. J. Tilanus

  Dutch Research Council

  235 shared

• Jordy Davelaar

  211 shared

• H. J. van Langevelde

  210 shared

Labs

• Computational Astrophysics GroupPI

  Not provided

Awards & honors

• Teacher Ranked as Excellent, Fall 2018