Radio Emission from Pulsars and White Dwarfs

Joonas Nättilä (University of Helsinki) — August 6, 2026

Abstract

Radiative plasma processes in neutron star and white dwarf magnetospheres play a central role in shaping their radio emission and high-energy variability. Magnetospheric gaps — regions of unscreened electric field — are natural sites of extreme particle acceleration, radiation, and electron–positron pair creation. They are thought to trigger pair cascades and regulate magnetospheric activity in radio pulsars, millisecond pulsars, and possibly the white-dwarf-hosting long-period transients.Their time-dependent plasma physics, however, remains poorly understood.

In this talk I will present our recent work on the radiative plasma physics of these discharges. We develop an analytical theory for the coupled evolution of electric field, particle acceleration, radiation, and pair creation in this extreme regime, and complement it with first-principles radiative particle-in-cell simulations that include exact QED processes and realistic plasma parameters, allowing us to test and validate the theoretical predictions.

Cosmology and Astrophysics with SPHEREx

Yun-Ting Cheng (Caltech) — July 8, 2026

Abstract

SPHEREx is a NASA near-infrared satellite mission launched in March 2025, currently conducting the first all-sky near-infrared spectral survey. I will begin with a brief overview of the mission, its current status, and some early science results. Then I will introduce the main scientific goals of SPHEREx, focusing on two: 1) constraining primordial non-Gaussianity (PNG) from inflation, with a target precision of O(1) on the PNG amplitude parameter (f_NL); and 2) studying galaxy formation and evolution through intensity mapping of the extragalactic background light. I will walk through the science motivation, the analysis methodology, and our progress on these analyses within the SPHEREx science team.

Planet formation and evolution around misaligned circumbinary disks

Cheng Chen (Bishop's University) — May 25, 2026

Abstract

Recent observations have revealed that circumbinary disks that are misaligned to the binary orbit could be common in the universe. Dissipation in the disk causes it to move either towards coplanar alignment or polar alignment. Since planets form inside disks, circumbinary planets may also form misaligned to the binary orbit. We explore the orbital evolution of the binary with the misaligned circumbinary disk. The disk-binary interaction leads to the evolution of the binary’s orbit, which could either shrink over time due to the loss of its angular momentum via resonances or expand due to the acquirement of specific angular momentum via accretion. We explore diverse parameter domains of circumbinary systems with polar and retrograde disks. Our results have implications in a variety of astrophysical objects, including the orbital evolution of stellar binaries and the evolution of both stellar and supermassive black hole binaries.

Gravitational Waves from Active Galactic Nuclei

Nicholas Stone (University of Wisconsin) — May 11, 2026

Abstract

The discovery of GW150914 inaugurated the era of gravitational wave (GW) astronomy, opening a new window to study our Universe’s compact objects and through which to test general relativity. Now, a decade later, the LIGO-Virgo-KAGRA (LVK) collaboration has seen hundreds of GW signals, overwhelmingly from mergers of binary stellar mass black holes. Despite the many successes of GW astronomy, a zeroth-order astrophysical question remains unanswered: what astrophysical environments produce the LVK binary black holes, and by what process are they assembled? I will briefly detail the many formation channels that have been proposed, before zooming in on one uniquely testable solution: the “AGN channel” In this scenario, individual black holes pair up and merge in the dissipative gaseous environment of an active galactic nucleus. I will review recent progress in understanding the ecology of stars and black holes embedded in the massive gas disks of AGN, an astrophysics problem not unlike that describing planet formation in protoplanetary gas disks. I will then describe the unique predictions of the AGN channel for the LVK population of binary black hole mergers, focusing both on GW signatures as well as electromagnetic counterparts, both direct (transient) and indirect (statistical).

The challenge and promise of LIGO’s heavy black holes

Maximiliano Isi (Columbia University) — May 4, 2026

Abstract

The LIGO-Virgo instruments have so far detected ~300 mergers of compact objects, most of them black holes. These detections give us an increasingly detailed view of the distribution of black hole masses and spins, with implications for stellar physics and the dynamics of dense astrophysical environments. While many interesting features have emerged, one of the most puzzling is the existence of merging black holes exceeding 60 solar masses, several of which also appear to spin extremely fast. Such systems are difficult to explain with our current theories of stellar binary evolution and lack a clear formation pathway. They also present a big challenge to our modeling and analysis techniques, since their signals are short and dominated by the least-understood portion of the merger process in general relativity. In this talk, I will outline the challenge and promise of understanding LIGO’s heavy black holes from the point of astrophysics, general relativity and data analysis, which, as LIGO continues to observe, may well be the key to some of the most pressing questions in the field.

X-Ray Heartbeats from the Nuclei of Galaxies

Riccardo Arcodia (Harvard University) — April 20, 2026

Abstract

X-ray Quasi-Periodic Eruptions (QPEs) are the latest and perhaps the most peculiar addition to the zoo of extragalactic nuclear transients. They are high-amplitude bursts of X-ray radiation recurring every few hours to few days with a lifetime of years to decades, originating from the central black holes in the nuclei of low-mass galaxies. So far, only a dozen of such events has been found, although with rising interest in the broader community given their observational and theoretical connection with tidal disruption events and as possible low-frequency gravitational wave sources. Some of the latest models suggest that these eruptions occur in extreme mass ratio inspirals (EMRIs) systems, in which the secondary stellar-mass body interacts with the accretion flow of the primary massive black hole. I will outline the observational properties of QPE sources and the latest tests of theoretical models.

Multiscale physics of particle acceleration in collisionless shocks

Anatoly Spitkovsky (Princeton University) — April 13, 2026

Abstract

Shocks in low density plasmas (so-called “collisionless shocks”) are ubiquitous throughout the Universe and are thought to produce nonthermal particles spanning decades of energy (such as cosmic rays). The process of shock acceleration is an intrinsically multiscale problem, connecting plasma microphysics at the shock to self-generated instabilities driven by accelerated particles far from the shock. I will describe the progress in modeling collisionless shock structure and particle acceleration using ab-initio kinetic simulations, focusing on the current understanding of magnetic field amplification mechanisms and the conditions necessary for particle injection into the acceleration process. Our large scale simulations are beginning to show the effects of nonlinear cosmic ray feedback on shock structure and on the resulting cosmic ray spectrum in high-Mach-number shocks. These results help to interpret the emission spectra from young supernova remnants and transient sources.

The Beguiling Mystery of the Little Red Dots

Jenny Greene (Princeton University) — April 4, 2026

Abstract

One of the most surprising results from JWST has been the discovery of a large population of compact red sources at z>4, with very red rest-frame optical colors, blue UV slopes, and broad Balmer lines. The compact sizes and luminous broad lines strongly suggest these objects are powered by accreting supermassive black holes, but their lack of evidence for X-ray emission or hot dust in the mid-infrared calls that conclusion into question. Regardless, their high number densities (~5% of the galaxy population) makes them an important new contribution to the high-redshift galaxy zoo. I will discuss our ongoing efforts to understand the nature of this population, and what they may teach us about the growth of black holes and galaxies.

Reconstructing the early Milky Way using the most metal-poor stars

Akshara Viswanathan (University of Victoria) — March 30, 2026

Abstract

Metal-poor stars are among the oldest surviving objects in the Universe, acting as fossil records of the first generations of star formation and the early assembly of galaxies. In this talk, I will show how these rare stars can be used to reconstruct the Milky Way’s formation history–from its chaotic proto-Galactic phase to the emergence of an ordered disc. Leveraging large datasets from the ESA Gaia survey and the CFHT Pristine survey, along with extensive spectroscopic follow-up, I will first discuss the rarity and spatial distribution of very and extremely metal-poor stars in the Galactic halo, and present new catalogues and methods that significantly expand the known population of these objects. I will then highlight the discovery of a surprising population of extremely metal-poor stars on prograde, disc-like orbits, pointing to the existence of a proto-disc in the early Milky Way. By modeling the kinematics of chemically distinct stellar populations, I show that the old disc undergoes a gradual spin-up with increasing metallicity, revealing a continuous transition from a slowly rotating proto-Galaxy to the present-day disc. Zooming out to the outer Galactic halo, I will present new constraints on its metallicity structure, where a large fraction of stars are very metal-poor, and discuss how tidal streams and substructures encode the remnants of past accretion events of the Galaxy. This includes the confirmation of the most metal-poor stellar stream known to date, as well as the discovery of a very metal-poor stellar counterpart to the Magellanic Stream. Together, these results demonstrate how combining chemistry and dynamics in metal-poor stars allows us to piece together the Milky Way’s earliest history, offering new insight into star formation and galaxy assembly in the first billion years of the Universe using our own cosmic backyard.

The Black Hole Diet: How Accretion Controls Spin, Growth, and Feedback

Alexander Tchekhovskoy (Northwestern University) — March 26, 2026

Abstract

A black hole’s accretion rate controls its destiny, governing its spin, growth, luminosity, and feedback. However, connecting these crucial quantities to the scales we can actually observe, remains a major theoretical hurdle. In this talk, I will present recent simulations of magnetized gas feeding rapidly rotating black holes. These models offer new insights into the mechanics of gas consumption and ejection. I will demonstrate how these findings apply to a vast range of systems, linking the physics of supermassive black holes at the centers of galaxies to stellar-mass black holes born in dying stars.