Center for Relativistic Astrophysics

Cosmic Coffee
2022-09-21 15:30 - 16:30, 2022-09-14 15:30 - 16:30 (America/New_York) | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States

Welcome to the CRA
2022-09-07 15:30 - 16:30 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Please join us for this year’s introductory event for the Center for Relativistic Astrophysics. Meet the people who make up the CRA, and learn about the science & organization for this new year.

CRA Seminar: Yubo Su (Princeton University)
2022-08-25 15:00 - 16:00 | Bogg 1-44 VizLab
Title: Tilted Planets: Exciting Exoplanetary Obliquities via Spin-Orbit Resonances
Abstract: The obliquity of a planet, the tilt between its spin and orbital axes, reflects the evolutionary history of the planet. In our own Solar System, the 98 degree obliquity of Uranus is hypothesized to be the result of giant impacts during its formation, and the 4 and 26 degree obliquities of Jupiter and Saturn are thought to be the result of spin-orbit resonances due to the gravitational influence of Uranus and Neptune respectively. While there have been few direct constraints on the obliquities of sub-stellar-mass objects beyond our Solar System, there are prospects for better constraints on exoplanetary obliquities in the coming years. Such measurements are important for informing the surface conditions and potential habitability of exoplanets. In this talk, I will describe some mechanisms by which exoplanetary obliquities can be excited due to the rich dynamics resulting from the interaction between tides and spin-orbit resonances. I will use these results to assess the prospects of substantially oblique exoplanets in a few important systems of interest. I will also discuss my results on spin evolution in dynamically-formed binary black hole mergers, which are similar in many ways to the planetary dynamics.

Cosmic Coffee
2022-04-20 15:20 - 16:20 | Bogg 1-44 VizLab
Last Cosmic Coffee for the semester, Spring 2022.

Cosmic Coffee
2022-04-13 15:20 - 16:20, 2022-04-06 15:20 - 16:20, 2022-03-30 15:20 - 16:20, 2022-03-16 15:20 - 16:20, 2022-03-09 15:20 - 16:20, 2022-03-02 15:20 - 16:20, 2022-02-23 15:20 - 16:00, 2022-02-16 15:20 - 16:00 and 6 more dates | Bogg 1-44 VizLab

CRA Seminar: Dr. John Christian
2021-12-09 15:00 - 16:00 | GA
There will be an in-person viewing of the online seminar held in the CRA Visualization Lab (Boggs 1-44).
Title: Astrometric Methods for Star-Based Navigation
Abstract: Humans have used stars for navigation since antiquity. It should come as no surprise, therefore, that star sightings were amongst the first observables used to navigate exploration spacecraft (both crewed and robotic). Today, star observations remain central to the navigation of almost every modern spacecraft. However, as space exploration missions become more ambitious and the technologies for star-based navigation improve, we begin to run into limitations of existing methods for observing and cataloging stars. In this seminar we will discuss a variety of recent advancements at the intersection of astronomy and navigation, such as generalized astrometric calibration and velocity estimation from stellar aberration. The objective is to familiarize the CRA community with current problems at the intersection of astrophysics, astrometry, and spacecraft engineering — with the hope of fostering an a growing dialog between physics and aerospace engineering at Georgia Tech.

Cosmic Coffee
2021-12-01 15:00 - 16:00, 2021-11-17 15:00 - 16:00, 2021-11-10 15:00 - 16:00, 2021-10-27 15:00 - 16:00, 2021-10-20 15:00 - 16:00, 2021-10-13 15:00 - 16:00 | GA

CRA Seminar: Intae Jung, PhD
2021-11-11 15:00 - 16:00 | GA
Learn more about Dr. Intae Jung
Title: Probing the Era of Reionization with Lyman-alpha Observations
Abstract: Lyman-alpha emission has played a pivotal role in constraining the neutral hydrogen (HI) fraction of the intergalactic medium (IGM) during the epoch of reionization thanks to the resonant nature of Lyman-alpha scattering with HI. Initial Lyman-alpha studies of multi-object spectroscopy (MOS) have shown decreasing Lyman-alpha emission from z ~ 6 to 7, which implies an increasing IGM HI fraction. Furthermore, with a rapid increase of collected galaxy spectra over a wide field, recent Lyman-alpha studies have revealed significant spatial variation in the IGM HI fraction, indicating inhomogeneity in reionization. Combining the HST slitless grism data and the Keck multi-slit spectroscopic data, we analyze spectra of 275 z ~ 6 – 8 galaxies to investigate Lyman-alpha transmission in the IGM depending on galaxy UV luminosities. I will present the results of the spectroscopic analysis, which imply that reionization proceeds more rapidly and/or earlier around UV brighter galaxies. I will also discuss future studies for improving the use of Lyman-alpha as a probe of reionization.

CRA Seminar: Steve Sclafani
2021-10-28 15:00 - 16:00 | GA
Title: Search for Southern Sky Neutrino Sources with IceCube
Abstract: The IceCube Neutrino Observatory has measured a flux of astrophysical neutrinos. More recently, it has used muon-neutrino datasets to present evidence for one source, a flaring blazar known as TXS 0506+056. However, the sources responsible for the majority of the astrophysical neutrino flux remain elusive. Galactic sources and the galactic plane have long been hypothesized as potential neutrino sources, however searches in the southern sky are limited due to the high background of downgoing muons produced in the atmosphere. In this work we present a new neutrino dataset relying heavily on Deep-Neural-Networks (DNN) to select cascade events produced from neutral-current interactions of all flavors and charged-current interactions with flavors other than muon-neutrino. The speed of DNN based selections allows the event selection to be performed in near-realtime with a single GPU. Cascade events have reduced angular resolution when compared to muon-neutrino events, however the resulting dataset has a much lower energy threshold in the southern sky and a lower background rate. These benefits lead to an improved sensitivity to sources in the southern sky when compared to muon-neutrino datasets. This improvement is particularly promising for identifying galactic sources including the galactic plane itself.

CRA Seminar: Mathew Potts (University of Utah)
2021-10-21 15:00 - 16:00 | GA
Title: Ultra-High Energy Cosmic Ray Energy Spectrum using Hybrid Analysis with TAx4
Abstract: Cosmic rays are subatomic particles of extraterrestrial origin and at the highest energies, they are not well understood. The variation of the cosmic ray flux with energy is referred to as the “Energy Spectrum.” The measurement of the cosmic ray energy spectrum is important because it may give a hint of where cosmic rays come from. The Telescope Array (TA) Cosmic Ray Observatory, located in Millard County Utah, is the largest cosmic ray detector in the Northern hemisphere. Following evidence for a hotspot in the arrival directions of the highest energy cosmic rays, TA underwent the TAx4 upgrade to expand the area of Surface Detectors (SD) by a factor of 4. The upgrade included new Fluorescence Detector (FD) stations to view over the expanded SD arrays. In this work, I will present a preliminary cosmic ray energy spectrum using hybrid events fromTAx4, the hybrid resolutions of the detector, and data-MC comparisons. I will show this work’s preliminary hybrid energy spectrum is in agreement with previous TA measurements, which is a key first step towards future composition and anisotropy studies.

CRA Seminar: Dr. Edwin Chan (CRA)
2021-10-14 15:00 - 16:00 | GA
Title: Tidal disruption events in active galactic nuclei
Abstract: A fraction of tidal disruption events (TDEs) occur in active galactic nuclei (AGNs); distinguishing TDE flares in these hosts from common AGN variability is an unsolved challenge. We took the first step toward characterizing the appearance of TDEs in AGNs with the help of hydrodynamic simulations. The AGN accretion disk does not hinder the initial disruption, but its subsequent interaction with the tidal debris dictates the electromagnetic output of the event. I will present our predictions of the possible observational signatures, from the optical to soft gamma rays. TDEs in AGNs emphatically resemble neither TDEs nor AGNs; therefore, current observational strategies may miss these events altogether.
