Cosmic Coffee (Howey IZ)
2024-09-18 15:30 - 16:30, 2024-09-25 15:30 - 16:30, 2024-10-02 15:30 - 16:30 (America/New_York) | 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: 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, 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 | 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.

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.
