Center for Relativistic Astrophysics

Cosmic Coffee - [Howey IZ]
2025-03-12 14:00 - 15:00, 2025-03-26 14:00 - 15:00, 2025-04-02 14:00 - 15:00, 2025-04-09 08:00 - 17:00, 2025-04-16 14:00 - 15:00 (America/New_York) | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Seminar - Lizhong Zhang, Flatiron Institute (Host Julia Speicher)
2025-04-03 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Understanding the dynamics of accretion onto black holes in radiation GRMHD simulations
Abstract: Radiation and magnetic fields play crucial roles in shaping the physics of black hole accretion, especially in near- and super-Eddington regimes. To model this process, we solve the GRMHD equations coupled with angle-dependent radiation transfer, which enables us to capture the complex dynamics driven by radiation and magnetic fields in extreme environments. In the super-Eddington regime, the accretion disk thermally expands under radiative support, forming a narrow conical funnel through which radiation escapes, resulting in low radiation efficiency. In the near-Eddington regime, the disk structure strongly depends on the magnetic field topology, allowing the system to reach a steady state as either a thin disk with magnetic coronae or a magnetically elevated disk. These simulation results align with several observational findings and provide predictive diagnostics for future observations, which I will further elaborate on in the talk.

CRA Seminar: Caitlin Rose, Georgia Tech [ Surabhi Sachdev]
2025-04-17 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Facilitating Multi-Messenger Astrophysics with Timely Information about Gravitational Waves
Abstract:
In the age of multi-messenger astrophysics, providing timely information about gravitational-wave events is vital to achieve coincident detections. Electromagnetic counterparts to gravitational waves originating from compact binary coalescences containing at least one neutron star can help answer unsolved questions in physics such as the rate of expansion of the universe and the neutron star equation of state, describing how matter behaves in extreme densities. Rapid PE is a parallelized Bayesian parameter estimation scheme designed to facilitate these follow-up observations by quickly estimating parameters such as the component masses of these binaries with gravitational waves detected by the LIGO-Virgo-KAGRA (LVK) collaboration. In this seminar, I will present our results on the use of this method to provide reliable source classification updates for binary black hole (BBH), binary neutron star (BNS), and neutron star-black hole (NSBH) mergers within a few minutes of detection.

Special CRA Seminar: Afroditi Papadopoulou [LANL]
2025-06-18 15:00 - 16:00 | Howey N110, 837 State Street NW C101, ATLANTA, GA, 30332, United States
Title: First study of neutrino angle reconstruction using quasielastic-like interactions in MicroBooNE
Abstract: We investigate the expected precision of the reconstructed neutrino direction using a muon neutrino-argon quasielastic-like event topology with one muon and one proton in the final state and the reconstruction capabilities of the MicroBooNE liquid argon time projection chamber. This direction is of importance in the context of DUNE sub-GeV atmospheric oscillation studies. MicroBooNE allows for a data-driven quantification of this resolution by investigating the deviation of the reconstructed muon-proton system orientation with respect to the well-known direction of neutrinos originating from the Booster Neutrino Beam. Using simulation studies, we derive the expected sub-GeV DUNE atmospheric-neutrino reconstructed simulated spectrum by developing a reweighting scheme as a function of the true neutrino energy. We further report flux-integrated single- and double-differential cross section measurements of charged-current muon neutrino quasielastic-like scattering on argon as a function of the muon-proton system angle using the full MicroBooNE data sets. We also demonstrate the sensitivity of these results to nuclear effects and final state hadronic reinteraction modeling.

State of the School Address Monday, August 18, 2025
2025-08-18 15:30 - 16:30 | GA

Cosmic Coffee Welcome to CRA [Howey IZ]
2025-08-20 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

Cosmic Coffee [Howey IZ]
2025-09-03 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States
Topic: Open discussion: job search—academia and industry.

Cosmic Coffee [Howey IZ]
2025-09-10 15:30 - 16:30, 2025-09-17 15:30 - 16:30, 2025-09-24 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

Cosmic Coffee [Howey IZ] -Speaker: Vishal Sudhakar
2025-10-01 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Seminar - Dr. Jonathan Zrake [Prof. Tamara Bogdanovic]
2025-10-02 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Variability of binary AGNs
Abstract: I will discuss recent results from numerical modeling of the electromagnetic (EM) variability signatures of accreting supermassive black hole (SMBH) binaries. Orbiting SMBH binaries produce both periodic, and stochastic, variability with potentially tell-tale signatures, especially in the shape of the power spectral distribution (PSD) of the optical continuum emission. Furthermore, the gravitational wave (GW) driven inspiral and merger of the two black holes can induce either of two types of “changing look” AGN events — Type-A events in which the AGN light is dimmest at the time of the merger, and Type-B events in which the AGN light is brightest around the merger time. The two types correspond to the direction of angular momentum flow between the binary orbit and the surrounding gas disk, and produce a correlated phase drifting in the LISA band, creating a new avenue for multi-messenger (EM+GW) study of binary black holes with e.g. LSST and LISA.

Cosmic Coffee [Howey IZ]-Speaker: Eden Schapera
2025-10-08 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

Cosmic Coffee [Howey IZ] Speaker: Sena Ghobadi
2025-10-15 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Special Seminar - Prof. Paola Arias [Prof. John Wise]
2025-10-21 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Ultralight to Heavy: Production Pathways of Axions and Dark Photons as Dark Matter
Abstract:
In this talk, I will review the motivation for axions (and alike particles) and dark photons as dark matter candidates. I will then briefly discuss some popular detection strategies in the ultralight mass regime.
Next, I will present two production mechanisms: freeze-in for the intermediate-to-high mass range, and misalignment plus parametric resonance for the ultralight regime. For both cases, I will highlight the favored regions of parameter space to account for the dark matter paradigm and the associated detection opportunities.

Cosmic Coffee [Howey IZ] Speaker: Noah Zhang
2025-10-22 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Special Seminar Joheen Chakraborty (Host Dr. Matthew Liska)
2025-10-28 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Quasi-periodic Eruptions
Abstract: Quasi-Periodic Eruptions (QPEs) are a newly discovered type of recurring X-ray transient originating from supermassive black holes (SMBHs) in nearby, low-mass galaxy nuclei. They are thought to be the first observed counterparts to extreme mass-ratio inspirals (EMRIs), a long-anticipated gravitational wave source class detectable by future space-based interferometers (e.g. LISA). About a dozen QPEs are known, and in an unexpected twist, several of them have emerged from the nascent accretion disks formed by Tidal Disruption Events (TDEs). I will give an overview of the surprising discoveries, observational properties, and theoretical progress in QPEs since their discovery six years ago, including constraints on their emission mechanisms, prospects for probing EMRI orbital dynamics using QPE timings, and estimates of the QPE rate and their implication for EMRI formation. I will also briefly discuss ongoing work regarding the X-ray variability properties of TDEs as probes of their turbulent accretion flows, and electromagnetic study of evolutionary processes in ultracompact binary LISA sources, putting QPEs in the broader context of time-domain and multi-messenger astronomy.

Cosmic Coffee [Howey IZ] Speaker: Mayra Velazquez
2025-10-29 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

Cosmic Coffee [Howey IZ] Speaker: Julia Esposito
2025-11-05 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA SEMINAR Dr. Christopher Bambic [Prof. John Wise]
2025-11-06 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Coronal Heating and Feedback from Luminous Accretion Flows: Challenges and Insights from Local and Global Simulations
Abstract
Radiatively efficient accretion represents a frontier problem for computational astrophysics. Through leveraging GPU acceleration and exascale supercomputer architectures, the H-AMR Collaboration, led out of Georgia Tech, has been working to understand the structure of accretion flows onto black holes, where strong magnetic fields, two-temperature plasma physics, radiation transport, and general relativity may all imprint themselves on the radiative and kinetic feedback observed from black-hole-powered sources. In this talk, I will discuss our ongoing efforts to model accretion flows across a wide range of accretion rates, from the radiatively inefficient regime powering targets of the Event Horizon Telescope, to the super-Eddington regime, which may be relevant for sources such as tidal disruption events, ultra-luminous X-ray sources, and quasars. Special attention is given to the thin-disk regime at around 1 to a few tens of percent of the Eddington limit, where hard X-ray emission, thought to emerge from a hot (billions of Kelvin) “corona,” is routinely observed. I will discuss two different models for the coronal geometry, the “sandwich” and “truncated disk” models, and what we have learned about possible coronal heating mechanisms based both on the local stratified shearing-box calculations I explored during my Ph.D., and the especially high-resolution global simulations that I have been exploring here at Tech.

Cosmic Coffee [Howey IZ] Speaker: David Raudales
2025-11-12 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA SEMINAR - Dr. Arthur Daniel Adams [Dr. Gongjie Li]
2025-11-13 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Mapping Sub-stellar Companions in Time and Space
Abstract: Companion brown dwarfs and exoplanets are complex objects whose time and spatial variability are now being mapped with high-fidelity infrared observations from JWST. I will begin by presenting our analysis of JWST NIRSpec/IFU PRISM observations of the 2M1207 system that represent the first spectroscopic time monitoring of a planetary-mass companion simultaneously with its host. By monitoring the variability of the system as a whole, we are sensitive both to the time evolution of atmospheric structure — including through hot spots and clouds — as well as external variability such as the accretion of disk material. 2M1207 A’s spectra show multiple time-dependent structures across the near-infrared. While we do not see a full rotation within our observations, we do capture non-linear trends in time across much of the spectra, with localized regions showing light curves that hint at the possibility of variability imparted by the circum-stellar disk. The planetary-mass 2M1207 b’s spectra generally corroborate the interpretation of recent atmospheric characterizations, where a simple cloud evolution model can capture most of the short-term variability. Turning to spatial variability, I will also provide a first look at the eclipse mapping of brown dwarfs in orbit around white dwarfs. These systems provide a unique opportunity to study the spatially-resolved structures of highly-irradiated atmospheres. Finally, I will highlight ongoing student research in brown dwarfs and exoplanets at UVA, which includes the spectral and orbital characterization of the aforementioned brown-dwarf/white-dwarf binaries as a complement to atmospheric studies, as well as classifying the seasonal evolution of Earth’s vegetation as spectral signatures of the response of the majority of Earth’s life by mass to the solar energy budget.

Cosmic Coffee [Howey IZ] Speaker: Neil Baker
2025-11-19 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

Public talk: Through the Kaleidoscope: The Multicolor Music of Supermassive Black Holes
2025-12-03 19:00 - 20:00 | Howey L1, 800 Atlantic Dr, Atlanta, GA, 30332, United States
Speaker: Prof. Krista Lynne Smith (Texas A&M)
Abstract: The space-time distortion caused by supermassive black holes provides a unique laboratory for violent physical processes. Synthesizing observations across many wavelengths and studying their time variability at slow and rapid timescales promises a new, dynamic, thorough understanding of how black holes form and grow, how they consume material, how they launch powerful relativistic jets, and how they bend the evolution of galaxies to a path that matches our observations of the Universe. I will discuss a brief history of our understanding of these fascinating objects, and discuss how multicolor, time-varying data from new missions has opened new windows into their nature. I will also discuss the rich multi-messenger future of black hole observations, including both neutrinos and gravitational waves.
