Center for Relativistic Astrophysics

CRA Seminar: Amy Secunda
2023-04-20 15:30 - 16:30 (America/New_York) | Bogg 1-44 VizLab

Cosmic Coffee
2023-04-19 15:30 - 16:30, 2023-04-12 15:30 - 16:30, 2023-04-05 15:30 - 16:30, 2023-03-29 15:30 - 16:30, 2023-03-22 15:30 - 16:30, 2023-03-15 15:30 - 16:30, 2023-03-08 15:30 - 16:30, 2023-03-01 15:30 - 16:30 and 11 more dates | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States

CRA Seminar: Mariia Fedkevych
2023-04-13 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Studying neutrino mass and dark matter, the key factors of the Universe structure formation
Abstract: The formation of the structure of the Universe observed today is largely determined by the mass of its components. We know that ordinary visible matter makes up only about 20% of the total mass of the Universe. The rest is known as dark matter, and its nature is as yet a mystery. Various particles claim the role of dark matter. One of them is neutrino, a Standard Model particle abundantly present in the Universe. Oscillation experiments demonstrated that neutrinos have tiny mass, which, however, is still not exactly known. In this presentation I will discuss the experimental challenge of determining the exact neutrino mass, which would be an important milestone in particle physics and would greatly clarify the cosmological role of neutrinos. I will pay special attention to the direct measurement of neutrinos using different approaches: the KATRIN and the HOLMES experiments. However, only a small fraction of the total amount of dark matter can exist as light neutrinos in order to allow small-scale structures (stars, galaxies and galaxy clusters) to exist as we see them, so the search for another responsible particle continues. So-called weakly interacting massive particle (WIMP), a hypothetical particle predicted by various extensions of the Standard Model of particle physics, is considered to be a plausible candidate for dark matter. In my talk I will show the current experimental effort to directly detect WIMPs in the LZ experiment.

CRA Seminar: Ibrahim Safa (Columbia)
2023-03-02 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Hunting extragalactic tau neutrinos in mountains, canyons, and volcanoes
Abstract: The IceCube neutrino observatory discovered a high energy astrophysical neutrino flux extending to ~10PeV in energy. A first generation extragalactic neutrino telescope, IceCube gave us a taste of mulitmessenger astronomy with several reported hints thus far. An upcoming generation of telescopes is in the works, promising a further reach into EeV energies. Many of these proposed experiments rely on natural dense media like mountains, volcanoes, and canyons to capture neutrinos. In particular, the tau neutrino has unique propagation properties that boost the prospects of such detections. I will first discuss how a better understanding of astrophysical neutrino flavors provides a smoking gun signature to new physics at yet unexplored energy scales. Then I will present the challenges that lie ahead before such signatures can be detected. I will also briefly describe the proposed experiments and their expected sensitivities.

CRA Seminar: Sophie Hourihane (Caltech)
2023-02-09 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Glitches get stitches: The effect non-transient noise in LIGO detectors on astrophysical inference, and what to do about it
Abstract: Ground-based gravitational-wave detector networks have detected a deluge of compact-binary events in the years since the original 2015 detection. Far more often than these events (approximately every two minutes), detectors experience transient, non-gaussian noise events (glitches). When the noise model is misspecified due to these glitches, parameter estimation results can be biased, or even wrong entirely. I present a method to mitigate this bias by modeling the glitch and the compact binary waveform together. I focus on a case study of GW200129, an event overlapping a glitch with a contentious interpretation of its precession.

CRA Seminar: Tousif Islam (UMass-Dartmouth)
2023-01-26 15:30 - 16:30 | Bogg 1-44 VizLab
Title: High accuracy waveform models for high precision gravitational wave astronomy
Abstract: Characterization of gravitational wave (GW) signals from the merger of binary black holes (BBHs) heavily rely on accurate gravitational waveform models. The most accurate way to compute waveforms is by solving the Einstein equation directly. This can only be done numerically (e.g. numerical relativity) — which is computationally expensive, taking weeks to months for a single simulation. Such a huge computational cost has paved the way for the development of approximate models that are now widely used in data analysis studies. However, these models can compromise accuracy in favor of speed. Recently, data-driven surrogate models have been shown to be capable of generating waveforms that are almost indistinguishable from NR in a fraction of a second. In this talk, I will summarize state-of-the-art NR surrogate models and discuss the application of these models to high-accuracy inference. We find that for many GW events, NR surrogates provide both more accurate and better-constrained parameter estimates in many of the key parameters including sky localization, mass ratio, and spin quantities. Furthermore, using numerical relativity surrogate models, we are able to constrain the properties of the remnant black hole and kick velocity. This information will help understand the environment and evolutionary pathways of black holes.

CRA Seminar: Hans Niederhausen
2022-12-01 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Neutrino astronomy with IceCube Connecting Photoelectrons to Supermassive Black Holes
Abstract: The IceCube Neutrino Observatory, deployed beneath the South Pole, is the largest optical neutrino telescope in the World. IceCube has been studying a diffuse flux of high-energy (>TeV) astrophysical neutrinos, ever since it reported the first evidence in 2013. However, the sources of the vast majority of these neutrinos, likely of extragalactic origin, remain elusive. Recent results, including evidence for neutrino emission from the nearby active galaxy NGC 1068, point to neutrino production in processes related to active galactic nuclei (AGN) powered by supermassive black holes at their centers. I will explain the crucial role of high-energy neutrinos as astrophysical messengers, the detection of neutrinos in IceCube, and recent progress in our searches for their sources. I will then discuss IceCube’s latest NGC 1068 result and what clues it gives us about the origin of the extragalactic neutrino flux.

Cosmic Coffee
2022-11-30 15:30 - 16:30 | Bogg 1-44 VizLab

CRA Seminar: Rixin Li (Cornell)
2022-11-17 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Hydrodynamical Evolution of Binary Black Holes Embedded in AGN Disks
Abstract: Stellar-mass binary black holes (BBHs) embedded in active galactic nucleus (AGN) disks are promising progenitors of black hole mergers detected in gravitational waves (GW) by LIGO-VIRGO-KAGRA. Current observed mergers show distinct properties that may only be explained by this AGN channel, with a few potentially associated electromagnetic counterparts identified. However, the hydrodynamical evolution of stellar-mass BBHs interacting with disk gas was poorly understood and lacked numerical studies. In this talk, I will present our high-resolution simulations of binaries in local disk models, with an extensive coverage of parameter space. We find that circular comparable-mass binaries contract, with an orbital decay rate of a few times the mass doubling rate. I will demonstrate that less massive and wider binaries embedded in disks with a non-isothermal equation of state and a low viscosity are easier to be hardened. In addition, our results suggest that the hardening time-scales of BBHs are much shorter than their migration time-scales in AGN disks. Finally, I will briefly describe recent progresses made in this topic and future prospects.

CRA Seminar: ChongChong He (U. of Maryland)
2022-11-10 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Multiscale Simulations of Star Formation from Molecular Clouds
Abstract: Star formation lies at the center of many fundamental topics in astrophysics. No comprehensive, quantitative theory of star formation has been established due to the complex physics involved and the huge dynamic range of the problem. Using a series of MHD simulations of star formation from turbulent giant molecular clouds (GMCs) with stellar feedback, I try to answer some of the fundamental questions in star formation. I find physically motivated scaling relations between star formation efficiency and cloud properties. I have shown, for the first time, that star formation in GMCs is a purely stochastic process. I will follow up with my findings from a set of novel simulations of the formation and collapse of magnetized prestellar cores. I will also briefly discuss my study’s implications on the epoch of reionization in the JWST era.

CRA Seminar: Meg Millhouse (GaTech)
2022-10-27 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Searches for gravitational-waves beyond compact binary coalescences
Abstract: The Advanced LIGO and Virgo detectors have now completed three observing runs, and reported dozens of detections form compact binary mergers. As we observe more of the gravitational-wave universe, we may detect signals from novel sources. For example, core-collapse supernovae or cosmic string cusps may produce short bursts of gravitational-waves. We could also detect persistent gravitational-wave radiation from non-axisymmetric rotating neutron stars. This talk will discuss gravitational-wave searches for signals beyond compact binary mergers, including searches for generic transient events, and continuous gravitational waves.

CRA Seminar: Andrea Derdzinski (U. of Zurich)
2022-10-13 15:30 - 16:30 | Bogg 1-44 VizLab
Title: The role of AGN accretion disks in the formation of characteristic, low-frequency GW sources
Accretion disks in active galactic nuclei (AGN) can enhance the formation of gravitational wave (GW) sources via the interaction of embedded stars and BHs with the surrounding gas. This can produce a substantial population of ‘gas-embedded’ GW sources across a range of frequencies, including extreme mass ratio inspirals detectable by space-based detectors such as LISA. These sources may occur at a significant rate compared to other EMRI formation channels, and they are particularly interesting: interaction with their environment can imprint signatures in the GW signal, allowing us to independently measure AGN disk properties solely with GWs. I will present an overview of recent results that demonstrate why we expect these sources to occur, how gas interaction manifests in the GW signal, and what we need to figure out in order to prepare for these detections.
