Center for Relativistic Astrophysics

CRA Seminar - Maximiliano Isi - Flatiron Inst. NYC
2024-11-14 15:30 - 16:30 (America/New_York) | Bogg 1-44 VizLab
Title: Gravitational waves: from black holes to the cosmos
Gravitational waves reveal the universe through an entirely unique spectrum, complementary to electromagnetic and neutrino observations. Although the field is young, it has already revolutionized our understanding of black holes and neutron stars, with implications for gravity, stellar and galactic astrophysics, nuclear physics and cosmology. In this talk, I will review some of these results with a focus on black hole mergers—currently the most abundant source of gravitational waves. I will outline the astrophysical puzzle of heavy black-hole mergers, and their use to study physics and cosmology. This is an exciting, observationally-driven field with abundant open questions, which will continue to grow in the following decades thanks to new and improved instruments. With the right analysis tools and theoretical foundation, the potential for discovery is unprecedented.

Cosmic Coffee (Howey IZ)
2024-11-20 15:30 - 16:30 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Seminar - Yuguang Chen UC-Davis
2024-12-05 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Galaxy Evolution and the Cosmic Baryon Cycle in 3D
Abstract
Galaxies are intricately linked to the gas in and around them, making the characterization of gas flows essential for directly constraining star formation, gas flow rates, and feedback processes, as well as for testing modern theories of galaxy evolution. The recent deployment of highly sensitive integral field units (IFUs) on large telescopes, such as the Keck Cosmic Web Imager (Keck/KCWI), has provided a novel 3D perspective to probe this diffuse gas. In this talk, I will present recent findings from the KBSS-KCWI and CIRAS surveys. The KBSS-KCWI survey, a ~200-hour program utilizing KCWI, aims to understand the nature of Lyα emission around star-forming galaxies at cosmic noon. Through realistic radiative transfer modeling, we have found that CGM Lyα emission is consistent with resonant scattering in a clumpy medium powered by central ISM emission. Meanwhile, the ongoing CIRAS (Comprehensive IFU Research on Accurate Abundance Studies) survey uses the newly commissioned KCWI red channel, alongside Herschel, SOFIA, and JWST data, to construct a reference sample that establishes an accurate abundance scale in star-forming regions throughout cosmic history.

CRA Seminar - Rhiannon Udall, Caltech
2024-12-12 15:30 - 16:30 | Bogg 1-44 VizLab
Title: The anti-aligned spin of GW191109: Glitch Mitigation and its Implications
Abstract: One of the most interesting events in the third LVK observing run was GW191109, which was found to have spins which were confidently anti-aligned with the orbital angular momentum – one of only two such cases which has been released – and high masses. This implies GW191109 may be of dynamical origin, making it our first probe of this formation channel. However, GW191109 was coincident with a transient noise source known as a “glitch,” which may affect these conclusions dramatically. I will discuss the origins and modelling of the glitch in question, and work I have done to fully characterize the astrophysical properties of GW191109 in the presence of this glitch. If time allows I will also discuss additional aspects of glitch modeling, and statistical tests to identify the impact of glitches on parameter estimation.
Bio:
Rhiannon is a fifth year graduate student at Caltech studying gravitational wave astrophysics, and a member of the LIGO-Virgo-Kagra collaboration. As an undergraduate she attended Georgia Tech, where she did research with Laura Cadonati and Deirdre Shoemaker on the inference of compact binary source properties. At Caltech she joined the LIGO Lab and is advised by Alan Weinstein. Much of her work has focused on modeling glitches in the LIGO detectors, understanding how glitches impact the inference of source properties, and developing techniques to mitigate them. Additionally, she has done substantial work on the development of data management infrastructure for the LVK collaboration, and the preparation of the forthcoming gravitational wave transient catalogs.

Cosmic Coffee - [Howey IZ]
2025-01-22 14:00 - 15:00, 2025-01-29 14:00 - 15:00, 2025-02-05 14:00 - 15:00, 2025-02-19 14:00 - 15:00, 2025-02-26 14:00 - 15:00, 2025-03-05 14:00 - 15:00, 2025-03-12 14:00 - 15:00, 2025-03-26 14:00 - 15:00 and 3 more dates | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Seminar - Vasilis Paschalidis, U. of Arizona [Host Tamara Bogdanovic]
2025-01-23 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Probing neutron star physics via multimessenger astronomy
Abstract: In this talk I will review recent work that allowed us to obtain a better understanding of the neutron star equation of state and neutron star properties by use of multimessenger observations of (binary) neutron stars. I will also discuss recent and current efforts of modeling binary neutron stars mergers that will help enable constraints on the unknown finite temperature nuclear equation of state by use of third generation gravitational wave observatories.

CRA Seminar - Emrah Kalemci [Host Matthew Liska]
2025-01-30 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Dust Scattering Halo of black hole binary 4U 1630-47 Observed with Chandra and APEX: Using mm and X-ray Imaging to Constrain Source and Molecular Cloud Distances
Abstract: Dust scattering halos have been utilized to study spectral variations during eclipses, physical properties and distribution of dust grains in the interstellar medium (ISM), X-ray extinction, and source distance determination using X-ray flux evolution and radial distribution of X-ray rings. We have obtained a Chandra image of the dust scattering halo of 4U 1630-47 and used it together with low-resolution 12CO images and determined the source to be 11.5 kpc in Kalemci et al. 2018. We recently obtained very high-resolution mm images (12CO and 13CO) with the Atacama Pathfinder Experiment (APEX) of the region. We used machine learning tools to obtain a 3-dimensional mapping of clouds in the line of sight, however, the clouds could be at their near, or far distance estimates based on their radial velocities. We generated many halo images placing the clouds at near and far distances and compared them with the actual halo image to constrain not only the source distance but also the distribution of molecular clouds in the line of sight. This method has the potential to constrain distances to compact objects behind large column densities for which the traditional methods might fail.

Cosmic Coffee - [Howey IZ]
2025-02-12 14:00 - 15:00

CRA Seminar - Tolga Guver [Host Matthew Liska]
2025-02-13 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Thermonuclear X-ray Bursts in the NICER Era
Abstract: Thermonuclear X-ray bursts are sudden flashes observed from some low-mass X-ray binaries. Time-resolved spectral analysis and long-term observations have revealed that they occur due to the unstable thermonuclear burning of the accreted matter on the surface of the neutron star. They typically last a few tens of seconds, and the recurrence of these events seems to track the amount of matter accreted. Observations of thermonuclear X-ray bursts provide a wealth of information about thermonuclear burning, accretion physics, and the properties of the host systems, while also playing a crucial role in measurements of neutron star masses and radii. In this talk, I will focus on X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) since its launch in 2017. With its large effective area in the soft X-ray band, NICER has allowed us to probe the interaction of X-ray bursts with their surroundings in a systematic way for the first time. I will summarize our findings on the excess soft X-ray emission observed during the bursts and how it can be interpreted in terms of changes in the mass accretion rate during the bursts and/or the reflection of the X-ray burst emission off the accretion disk.

CRA Seminar - Adi Foord, U. of Maryland Baltimore County [Host Tamara Bogdanovic]
2025-02-27 15:30 - 16:30 | Bogg 1-44 VizLab
Title: The Growth and Evolution of SMBHs via Galaxy Mergers
Abstract:
After decades of SMBH observations, the connection between AGN triggering and galaxy mergers remains incomplete, although AGN are likely key players in the evolution of massive galaxies. Theoretically, there are many reasons to expect a link between galaxy mergers and the accretion of material onto at least one of the central supermassive black holes. Yet, observationally, varied results have led to uncertainty in whether AGN triggering is dependent on the environment. One of the best ways to analyze the possible ties between merger environments and SMBH activity is to study systems with unique observational flags of merger-driven SMBH growth — or, dual AGN. I will present my work quantifying the dual AGN fraction at both high redshift, and as a function of redshift, via a large and uniform study of dual AGN in X-rays, up to z=3.5. By analyzing available data in wide and deep public Chandra surveys, the dual AGN fraction at both the high-redshift (2.5 < z < 3.5) and low-redshift (z < 0.03) regime can be better constrained. Pairing X-ray results with available multi-wavelength data (WISE, HST), we gain insight on how the X-ray activity of interacting AGN depends on their environments. Lastly, I will highlight the capabilities of future high spatial-resolution X-ray observatories, which will revolutionize the field of dual AGN detectability, and our understanding of the role mergers play in AGN triggering.

CRA Seminar: Ethan Partington, IA-FORTH (Host Laura Cadonati)
2025-03-06 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Probing the inner geometry of AGN accretion flows and searching for SMBHB candidates in large optical sky surveys
Abstract: Observations of the AGN Fairall 9 with NICER and Swift revealed a strong relationship between the flux of the UV continuum and the X-ray soft excess, indicating the presence of a “warm” Comptonized region which may act as a second reprocessor between the “hot” X-ray corona and the accretion disk. This has previously been suggested as an explanation for the weak X-ray/UV correlation observed in many AGN, and it can be confirmed using NICER’s unprecedented spectral coverage of X-ray variability on timescales of days to years. Our separate NICER reverberation mapping campaign on the AGN NGC 7469 provides no evidence for a warm corona, suggesting that there is no uniform prescription for the geometry of the inner accretion flow across all AGN. In this seminar, I will present the X-ray spectral modeling results from both AGNs in the context of understanding their geometries and energetic properties. I will also discuss the Null-Signal Template (NST) method, a new data-driven approach to testing the significance of possible periodic signals in quasar light curves. NST employs Bayesian statistics to differentiate between supermassive black hole binary (SMBHB) signals and the complex noise of individual quasars. I will share new tests on the validity of the NST method using simulated LSST-like light curves and discuss the prospects for the application of NST to SMBHB detection in both existing and upcoming quasar surveys.

Special CRA Seminar - Jonathan C. Tan [Host John Wise]
2025-03-07 13:00 - 14:00 | Bogg 1-44 VizLab
Title: The Origin of Supermassive Black Holes from Pop III.1 Seeds
Abstract: The origin of supermassive black holes (SMBHs) is a key open question for contemporary astrophysics and cosmology. Here we discuss the predictions of a model of SMBH formation from Pop III.1 protostars, i.e., metal-free stars forming in locally isolated dark matter minihalos, where dark matter annihilation has a chance to alter the structure of the star allowing growth to supermassive scales (Banik, Tan & Monaco 2019; Singh, Monaco & Tan 2023; Cammelli et al. 2025). The model predicts that all SMBHs form very early in the Universe (within a few 100 Myr, i.e., by z ~ 20) with a spatial distribution that is initially relatively unclustered. We also present predictions for SMBH occupation fractions, host galaxy properties, frequency of binary SMBHs and the gravitational wave background from this SMBH population. These predictions are compared to latest results from the Hubble Space Telescope, James Webb Spact Telescope and pulsar timing array observations.

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

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 | Howey IZ, School of Physics, Atlanta, GA, 30332, United States
