Center for Relativistic Astrophysics

Cosmic Coffee (Howey IZ)
2024-11-06 15:30 - 16:30, 2024-10-30 15:30 - 16:30, 2024-10-23 15:30 - 16:30, 2024-10-16 15:30 - 16:30, 2024-10-09 15:30 - 16:30 (America/New_York) | Howey IZ, School of Physics, Atlanta, GA, 30332, United States

CRA Seminar: Fabio Pacucci - Harvard cfa
2024-10-24 15:30 - 16:30 | Bogg 1-44 VizLab
Title: JWST’s Little Red Dots: Masters of Disguise in the High-Redshift Universe
This talk will explore the exciting journey to probe several populations of black holes, spanning from the distant to the local Universe. I will focus primarily on the “little red dots” (LRDs), a newly discovered and puzzling population of compact red sources at redshift z > 4, identified by JWST. These objects challenge our current astrophysical models in several ways. First, I will discuss the detection of overmassive black holes relative to the stellar mass of their host galaxies. Second, I will address the X-ray weakness problem, where these sources remain undetected in deep X-ray surveys. I will use advanced GRRMHD simulations to show how mildly super-Eddington accretion onto slowly spinning black holes can resolve this issue. Third, I will explore the extremely high stellar densities at the cores of these objects and their potential for triggering runaway stellar collisions. Throughout the talk, I will also discuss how the LRDs represent a perfect bridge to study other populations of black holes, with a focus on the role of future observatories such as HWO and AXIS. These include the search for black hole seeds at redshifts z = 20-30 and the detection of quiescent black holes in the local Universe.

CRA Seminar: Tiger Lu - Yale UN
2024-10-03 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Planets are not Points: Structure Evolution and Dynamical Evolution are Intertwined
Abstract: The two subfields of planetary structure evolution and planetary dynamics are two rich and complex fields that have advanced in lockstep, but independently of one another. I will discuss how accounting for minor interior structure evolution of a planet can have a profound impact on its dynamics and the architecture of the resulting system. Using full N-body simulations, I present a detailed case study of the HAT-P-11 system, which is a bizarre system with two misaligned, eccentric planets. I show that a combination of planet-planet scattering and Kozai-Lidov migration is capable of reproducing the present-day configuration of the system, but ONLY if these minute structure effects are accounted for.

Cosmic Coffee (Howey IZ)
2024-10-02 15:30 - 16:30, 2024-09-25 15:30 - 16:30, 2024-09-18 15:30 - 16:30, 2024-09-11 15:30 - 16:30, 2024-09-04 15:30 - 16:30, 2024-04-17 15:30 - 16:30, 2024-04-10 15:30 - 16:30, 2024-04-03 15:30 - 16:30 and 3 more dates | GA

Special CRA Seminar Michael Eracleous - Penn State 11am-12 noon
2024-09-24 11:00 - 12:00 | Bogg 1-44 VizLab
Title: Gravitational Wave and Electromagnetic Signals Associated With the Tidal Disruption of White Dwarfs
Abstract:
I will discuss the gravitational-wave and electromagnetic signals expected to accompany the tidal disruption of a white dwarf by a massive (i.e., “intermediate mass” black hole). Since white dwarfs are very compact, they can only be disrupted outside of the horizon of a black hole that less massive than about 105 M⦿ (if it is not spinning; up to 106 M⦿, if it is maximally spinning). As such, their disruptions can serve as signposts of black holes in this intermediate mass range. The gravitational wave signal from the disruption of a white dwarf in an unbound orbit is a brief burst while the optical spectroscopic signature of such an event is arguably unique and can be used to identify it. Things get more interesting if the white dwarf is captured in a bound orbit before it is disrupted. Such encounters are likely the most common type of an extreme mass ratio inspiral. In such a scenario, the gravitational wave signal is a “slow chirp” that traces the gradual decay of the orbit, it can last for months to years, and is detectable by LISA. Therefore, observers will have ample advance warning to get ready to catch (quasi-)periodic electromagnetic emission that may precede the disruption and the electromagnetic flare that accompanies and follows the disruption.

CRA Seminar: Nick Kaaz - Northwestern UN
2024-09-12 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Challenging the standard accretion disk paradigm with warps, strong magnetic fields and super-Eddington accretion in quasar disks
Abstract: “Classical” accretion disks are geometrically thin, radiatively efficient and mechanized by turbulent viscosity. Yet, many observational and theoretical issues challenge this paradigm, especially in quasars. This is perhaps unsurprising, as classical disks do not take into account certain key physics. For instance, the infalling gas that eventually forms a disk around a black hole has no prior knowledge of the black hole spin axis. Thus, most accretion disks will be at least initially misaligned with respect to the black hole spin axis. As the black hole rotates, it drags the surrounding space-time, inducing ‘Lense-Thirring’ torques which cause the disk to undergo precession and become warped. This can drastically alter the accretion process and drive rapid variability, similar to that which is seen in changing-state active galactic nuclei. Additionally, quasars are generally fed from cold, highly magnetized gas complexes, which determines the initial conditions of the disk. This can result in a disk that is magnetically dominated, enabling high accretion rates that are sometimes above the Eddington limit, which has important consequences for the inner accretion flow. In this talk, I will discuss my recent work on both radiative and non-radiative general-relativistic magnetohydrodynamic simulations of both thin, highly tilted accretion disks and super-Eddington, highly magnetized disks around rapidly rotating black holes. In the former case, I will present novel dissipation mechanisms unique to these systems that drive accretion on timescales much shorter than the usual viscous time. In the latter case, I will discuss the physics that governs the magnetic state of the disk, the resulting emission and outflow properties, and the implications for the cosmological growth of supermassive black holes.

CRA Special Event - John Wise
2024-08-21 08:00 - 17:00 | GA

CRA Seminar: Chris Bambic Princeton UN
2024-05-02 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Insights from Local Simulations of Accretion Disk Coronae: Two-temperature effects, thermal conduction, and radiation transport
Abstract: I will present a series of local accretion disk models, i.e., stratified shearing-box magnetohydrodynamic (MHD) simulations, for radiatively efficient accretion flows (REFs), which for the first time, include the effects of field-aligned free-streaming thermal conduction and radiation transport. The goal of my work is to understand the formation and evolution of hot, magnetically dominated coronae in active galactic nuclei (AGN) and X-ray binary (XRB) accretion flows, relevant for black hole systems accreting from about 1 to 10 percent of the Eddington rate. First, I will show that coronal plasmas should be in the two-temperature regime, where ion and electron/ positron (lepton) temperatures are only weakly coupled such that ions cool primarily through Coulomb collisions with rapidly Compton-cooled leptons. This realization allows me to introduce a simple, two-temperature model for the coronal ions that captures their thermodynamic evolution. The models naturally form temperature inversions, with a hot corona “sandwiching” a colder, thin disk.
Within this framework, I examine the evaporation and truncation of disks through field-aligned ion thermal conduction from the hot corona into the colder disk, which has been invoked as a mechanism to explain soft-to-hard state transitions in XRBs. I will show that, independent of the chosen magnetic field structure in the flow, thermal conduction is unable to evaporate disks and form a radiatively inefficient accretion flow (RIAF) near the black hole. Since thermal conduction alone cannot account for the observed truncation of disks in XRB accretion flows, an alternative physical mechanism is necessary to account for the soft-to-hard state transitions observed in XRBs. The introduction of net-vertical magnetic flux (NF) may provide such a mechanism. NF fields launch magnetocentrifugal outflows, which can deplete the disk’s surface layers near the black hole, allowing the plasma there to become two-temperature and evaporate into a RIAF. To study how NF fields heat coronae and mediate state transitions, I will discuss ongoing work to assemble a suite of local radiation MHD simulations with varying NF that self-consistently solve the radiation transport equation. These calculations will allow me to quantify the dissipation of energy in the optically thin surface layers of our model disks as a function of disk magnetization and surface density. Informed by these local models, I will discuss under what conditions we should expect hard X-ray coronae to form in luminous accretion flows.
Author: Christopher J. Bambic, with Eliot Quataert, Matthew W. Kunz, and Yan-Fei Jiang

Mehr Un Nisa (MSU)
2024-04-25 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Unveiling the TeV Sky in Neutrinos and Gamma rays
Abstract: The search for the origin of cosmic rays as well as the signatures of new physics, are closely tied to precision measurements, of both point and diffuse sources of very-high-energy Galactic and extra-galactic emission. The IceCube Neutrino Observatory has been observing a bright, diffuse flux of neutrinos at TeV—PeV energies for over a decade, but the origin of the majority of this flux remains unknown. At the same time, the High-Altitude Water Cherenkov (HAWC) observatory has been taking continuous high-statistics measurements of TeV gamma rays from two-thirds of the sky since commencing full operations in 2015. With its wide-angle survey capability, and a sensitivity to photon energies reaching over 100 TeV, HAWC is in a unique position probe the acceleration and propagation of cosmic rays in the local universe. I will describe some efforts on both IceCube and HAWC frontiers to resolve the puzzle of the diffuse neutrino background, and discuss the prospects for combined measurements with neutrinos and gamma rays in the era of multi-messenger astronomy.

Leo Tsukada -Penn State Un
2024-04-18 15:30 - 16:30 | GA
Title: Going wider and deeper in the search for gravitational waves
Abstract:
Gravitational waves (GWs) are oscillations of spacetime itself due to strong dynamical gravity. Since its first detection by the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2015, a network of the ground-based GW detectors detected 90 signals from compact binary mergers by 2020. In this seminar, I will give an overview of the GW observations in the context of the ground-based detectors, including the recent results from the ongoing fourth observing run. In particular, I will introduce some developments I have contributed to GW data analysis for compact binary mergers as well as stochastic GW background.
Also, I briefly describe future prospects of new discoveries in the GW observation and how to improve its synergy with other means of astronomy, e.g., electromagnetic waves, in the current multi-messenger era.

CRA Seminar: Mariia Fedkevych (GT)
2024-04-04 15:30 - 16:30 | Bogg 1-44 VizLab
Title: “Trinity neutrino observatory: progress of the Demonstrator phase”
Abstract:
In this seminar, I will present the Trinity’s pioneering efforts in Earth-skimming tau neutrino detection. I delve into the evolution of the Trinity project focusing on the Demonstrator phase, detailing its concept, development, and deployment. I will discuss its operational challenges and highlights the instrument’s sensitivity and its potential to detect transient sources like NGC 1068 and TXS 0506+056. I will outline further steps for both the Demonstrator and the broader Trinity project, envisioning a future of enhanced neutrino astronomy.

CRA Seminar: Zoltan Haiman (Columbia, on sabbatical at GT)
2024-03-28 15:30 - 16:30 | Bogg 1-44 VizLab
TITLE: Gravitational wave and electromagnetic signatures of binary
black holes with circumbinary gas
ABSTRACT: Binary black holes (BHBs) embedded in dense gas hold the
promise of so-called “multi-messenger astrophysics”: when they are
detected both through gravitational waves (GWs) and electromagnetic
(EM) observations, they will enable novel science. This is true both
for massive BHBs, whose GWs will be detectable by the future LISA
satellite and by on-going pulsar timing arrays (PTAs), as well as for
stellar-mass BHBs detected through ground-based GW detectors. In both
cases, identifying the coalescing binaries through their EM signatures
will help clarify their astrophysical origin and yield novel probes of
cosmology, fundamental physics, and accretion physics. In this talk,
I will describe how circumbinary gas may produce characteristic EM
signatures for both massive and stellar-mass BHBs, based on analytic
models as well as hydrodynamical simulations. I will also argue that
in both cases, some coalescing binaries may have already been detected
in optical surveys, providing clues about their origin.

Special CRA Seminar: Christine Lee (AU)
2024-03-25 14:00 - 15:00 | Bogg 1-44 VizLab
Title: Identification and characterization of GW bursts with the BayesWave algorithm for expanded GW detector networks
Abstract: The global gravitational-wave detector network currently consists of four interferometers: LIGO Hanford (H) and Livingston (L), Virgo (V) and KAGRA (K); the commissioning of LIGO India is also well underway. As the detector network continues to expand, it is crucial to understand the performance of existing gravitational-wave detection algorithms with various detector configurations. Our studies focus on comparing the performance of BayesWave, a source-agnostic (unmodelled) gravitational-wave burst characterization algorithm, between detector configurations of different sizes. In this talk, I will be presenting two aspects of BayesWave performance: (i) comparing the overall detection efficiency between HL and HLV in the third observing run, and (ii) comparing the ability of HL, HLV and HLKV to recover gravitational-wave burst polarizations via Stokes parameters.

SPRING BREAK March 18 -March 22
2024-03-18 | GA

CRA Seminar: Yuan-Sen Ting (ANU)
2024-03-14 15:30 - 16:30 | Bogg 1-44 VizLab
Title: Transcending the Limits of Theoretical Physics with Deep Learning
Abstract: Astronomy has recently experienced a profound transformation, driven by the acquisition of extensive datasets generated by increasingly sophisticated instruments. This influx of data has opened up numerous new avenues for exploration in the field. However, this boon is accompanied by its own set of challenges, as astronomical phenomena often exhibit intricate and are inherently high-dimensional. These complexities involve precise imaging, spectral analysis, and time series data.
Traditional statistical methods in astronomy can struggle to effectively handle these complexities. Deep learning offers a solution by effectively addressing the challenges by allowing for a more faithful representation of the intricate astronomical phenomena. In this talk, I will delve into deep learning approaches for characterizing complex astronomical systems. Additionally, it will explore the theoretical underpinnings of deep learning, including its close relationship with symmetry and physics. This exploration will encompass a diverse range of applications in astronomy, including asteroseismology, stellar spectroscopy, weak lensing, reionization, galactic dynamics, galaxy evolution, and quasars. These applications collectively contribute to expanding the boundaries of Bayesian statistics and theoretical astrophysics, all within the framework of deep learning.

CRA Seminar: Labani Mallick (U Manitoba) Virtual
2024-02-29 15:30 - 16:30 | GA
Title: Impact of Central Massive Black Hole Accretion on the Event Horizon, Host Galaxy, and Beyond
Abstract:
Astrophysical black holes are surprisingly simple physical objects. Their gravitational field can be fully described by two parameters: mass and spin. We cannot directly observe black holes as no light escapes from the event horizon. However, we can detect the light from accreting gas, which forms a dense disk around the black hole, known as an accretion disk. The accretion of material by a massive black hole at the center of its host galaxy forms an active galactic nucleus (AGN), the innermost region of which emits X-ray radiation. An AGN is energetically efficient for regulating the growth of galaxies and is crucial for the development of the Universe we see today. One of the most important tools to probe the innermost accretion flow is the detection of X-ray reverberation echoes, where the X-ray photons reflected from the accretion disk are delayed relative to the primary X-ray source. In this talk, I will first discuss how detailed measurements of the reflected X-rays from the accretion disk can be used to probe the innermost regions of accretion flow just outside the event horizon and determine the fundamental properties of the black hole, such as its spin, across the complete mass scales from ∼ 105 −1010 solar masses. Peering into the growth
channels of black holes, I will discuss how we can distinguish accretion vs. merger-dominated black hole growth and probe the cosmological evolution of black hole spins in the last 10 billion years of cosmic history. Finally, I will show how enigmatic relativistic winds or Ultra-Fast Outflows (UFOs) launched from the AGN accretion disk can be used to probe the feedback mechanism connecting the central black holes with their host galaxies.
