Center for Relativistic Astrophysics

Leo Tsukada -Penn State Un
2024-04-18 15:30 - 16:30 (America/New_York) | 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.

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.

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

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

Cosmic Coffee (Howey IZ)
2024-09-04 15:30 - 16:30, 2024-09-11 15:30 - 16:30, 2024-09-18 15:30 - 16:30, 2024-09-25 15:30 - 16:30, 2024-10-02 15:30 - 16:30 | GA

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.

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: 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)
2024-10-09 15:30 - 16:30, 2024-10-16 15:30 - 16:30, 2024-10-23 15:30 - 16:30, 2024-10-30 15:30 - 16:30, 2024-11-06 15:30 - 16:30, 2024-11-13 15:30 - 16:30, 2024-11-20 15:30 - 16:30 | 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 - Maximiliano Isi - Flatiron Inst. NYC
2024-11-14 15:30 - 16:30 | 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.

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 | 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 | GA

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.
