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November 14: Global Non-Axisymmetric Whistler Instabilities in a Rotating Plasma
November 14: Global Non-Axisymmetric Whistler Instabilities in a Rotating Plasma
Speaker: Alexandre P. Sainterme (Princeton University)
Abstract: Flow-driven instabilities in magnetized, differentially rotating disk plasmas are studied as a potential mechanism of turbulent viscosity. In addition to the well-understood linear magneto-rotational instability, global eigenvalue calculations also observe large-scale modes that can be interpreted as over-reflected Alfven waves that are trapped either between resonances and/or the domain boundary (Ogilvie, et al. MNRAS 279 1995, Ebrahimi, et al. ApJ 936:145 2022). This work investigates local and global linear stability of non-axisymmetric perturbations in a differentially rotating disc threaded with axial and azimuthal magnetic fields in incompressible Hall-MHD. We find unstable whistler modes, and Alfvenic modes that are modified by a coupling with whistler waves. The Hall term introduces an asymmetry in the linear system based on the relative orientation of the magnetic field and the rotation axis, known in the axisymmetric case (Wardle MNRAS 307 1999, Balbus, et al. ApJ 552 2001). We show that this effect is more pronounced for non-axisymmetric modes. The whistler branch of instability exists in the electron-MHD limit, from which a single second ordinary differential equation for the radial structure of the eigenmodes is derived. Nonlinear Hall-MHD simulations of the differentially rotating system using the NIMROD code quantify the effect of non-axisymmetric whistler modes on angular momentum transport.
November 7: To B or not to B: The Creation and Consequences of Cosmic Magnetic Fields
November 7: To B or not to B: The Creation and Consequences of Cosmic Magnetic Fields
Speaker: Ryan Golant (Columbia University)
Abstract: Over the past half-century, it has become increasingly clear that turbulence is central to two of the biggest unsolved problems in astrophysics: the origins of cosmic magnetism and the nature of cosmic ray transport. However, much of our knowledge of these phenomena comes from studies performed in the magnetohydrodynamic (MHD) regime, which is fundamentally inapplicable in most of our Universe: in the hot and diffuse volume-filling regions of the cosmos, Coulomb collisions are negligible and we must take a kinetic approach, rather than a fluid approach. In this talk, I will discuss recent results from fully kinetic simulations of the collisionless small-scale dynamo and collisionless large-amplitude turbulence, establishing a self-consistent thread from the turbulent generation and amplification of magnetic fields to the scattering and acceleration of cosmic rays. These results may explain several observable phenomena in the interstellar medium of our Galaxy, in the intracluster medium of galaxy clusters, and in cosmic voids.
October 31: The Optical to X-ray Luminosity and Spectra of Supernova Wind Breakouts
Abstract: Over the past half-century, it has become increasingly clear that turbulence is central to two of the biggest unsolved problems in astrophysics: the origins of cosmic magnetism and the nature of cosmic ray transport. However, much of our knowledge of these phenomena comes from studies performed in the magnetohydrodynamic (MHD) regime, which is fundamentally inapplicable in most of our Universe: in the hot and diffuse volume-filling regions of the cosmos, Coulomb collisions are negligible and we must take a kinetic approach, rather than a fluid approach. In this talk, I will discuss recent results from fully kinetic simulations of the collisionless small-scale dynamo and collisionless large-amplitude turbulence, establishing a self-consistent thread from the turbulent generation and amplification of magnetic fields to the scattering and acceleration of cosmic rays. These results may explain several observable phenomena in the interstellar medium of our Galaxy, in the intracluster medium of galaxy clusters, and in cosmic voids.
October 31: The Optical to X-ray Luminosity and Spectra of Supernova Wind Breakouts
Speaker: Tal Wasserman (Weizmann Institute of Science, Israel)
Abstract: I will discuss our recent work on the breakout of SN shock waves through extended dense circumstellar medium (CSM), which observations indicate is common at radii of 10^14 – 10^15 cm around core-collapse SN progenitors. The breakout of the radiation-mediated shock (RMS) through such CSM leads to the formation of a collisionless shock (CLS). We analyzed the evolution of the shock structure and associated radiation field during and after the RMS-CLS transition for non-relativistic breakouts, providing an analytic description of the key properties of the emitted optical to X-ray radiation, supported by numeric radiation-hydrodynamics calculations. I will show that the characteristic photon energy shifts from the UV to X-rays as the shock moves to a few breakout radii on a few breakout timescales (days), and that the X-ray signal is not suppressed by propagation through the upstream wind. Upcoming surveys that will systematically detect large numbers of young SNe will enable the use of these results to infer the pre-explosion mass loss history of the SN progenitor population.
October 24: Can acceleration mechanisms be extracted by particle trajectories without supervision?
Abstract: I will discuss our recent work on the breakout of SN shock waves through extended dense circumstellar medium (CSM), which observations indicate is common at radii of 10^14 – 10^15 cm around core-collapse SN progenitors. The breakout of the radiation-mediated shock (RMS) through such CSM leads to the formation of a collisionless shock (CLS). We analyzed the evolution of the shock structure and associated radiation field during and after the RMS-CLS transition for non-relativistic breakouts, providing an analytic description of the key properties of the emitted optical to X-ray radiation, supported by numeric radiation-hydrodynamics calculations. I will show that the characteristic photon energy shifts from the UV to X-rays as the shock moves to a few breakout radii on a few breakout timescales (days), and that the X-ray signal is not suppressed by propagation through the upstream wind. Upcoming surveys that will systematically detect large numbers of young SNe will enable the use of these results to infer the pre-explosion mass loss history of the SN progenitor population.
October 24: Can acceleration mechanisms be extracted by particle trajectories without supervision?
Speaker: Omar French (University of Colorado)
Abstract: One of the central difficulties in interpreting energetic flaring events in the Universe (e.g., from black hole magnetospheres and pulsar wind nebulae) is our limited understanding of the particle acceleration which powers them. While collective plasma processes such as magnetic reconnection, shocks, and turbulence have been identified as primary engines of particle acceleration, their underlying microphysics—and thus radiative signatures—are poorly understood. A promising path forward is to cast the acceleration of individual particles in terms of the local mechanisms which accelerate them, e.g. magnetic X-points or Fermi reflections. This casting is only possible, however, if such mechanisms can be characterized merely by the “experience” they impart to particles. In this talk, I will argue that particle-in-cell (PIC) studies on particle injection in relativistic magnetic reconnection have supported this assumption directly, via reproducing key theoretical predictions, including the work done by each mechanism and their time-dependent dominance. I will conclude by discussing how unsupervised machine learning approaches, such as a vector-quantized variational autoencoder (VQ-VAE), may be applied to PIC tracer-particle trajectories to extract the underlying acceleration mechanisms, akin to discovering discrete phonetic units from raw audio. This opens the possibility for a descriptive bridge between the particle distribution function and the underlying microphysics to be constructed.
October 3: The Crystallography of Neutron Star Crusts
October 3: The Crystallography of Neutron Star Crusts
Speaker: Matt Caplan (Illinois State University)
Abstract: At sufficiently high temperatures matter becomes fully ionized, but under sufficiently high pressure those plasmas freeze solid. These “strongly coupled” plasmas have Coulomb energies orders of magnitude greater than their thermal energies and can be found in systems spanning electrically charged dusts to the crusts of neutron stars. While many of the physically interesting combinations of temperature and density are inaccessible to laboratory experiments on earth, numerical simulations allow us to study the detailed microphysics of these crystals such as how they break, with implications for starquakes, pulsar glitches, and more.
September 14: Positrons, electrons and γ-ray halos around pulsars
September 14: Positrons, electrons and γ-ray halos around pulsars
Speaker: Fiorenza Donato (Prof. at University of Torino)
Abstract: We discuss possible interpretation of current positron and electron in the Cosmic Rays data in terms of a combination of different sources in the Galaxy. We dig in particular into pulsars as sources of antimatter in Galaxy. We also deal with possible interpretations of gamma-ray halos recently observed around pulsars, and how multi-wavelength studies focused on single sources may help understand their origin and physical properties.
September 12: Deciphering particle acceleration in massive-star environments with gamma-ray modelling and MHD simulations
Abstract: We discuss possible interpretation of current positron and electron in the Cosmic Rays data in terms of a combination of different sources in the Galaxy. We dig in particular into pulsars as sources of antimatter in Galaxy. We also deal with possible interpretations of gamma-ray halos recently observed around pulsars, and how multi-wavelength studies focused on single sources may help understand their origin and physical properties.
September 12: Deciphering particle acceleration in massive-star environments with gamma-ray modelling and MHD simulations
Speaker: Lucia Haerer (Max-Planck-Institut für Kernphysik)
Abstract: Several young star clusters show degree-scale diffuse gamma-ray emission, hinting at ongoing particle acceleration. Despite these detections, acceleration sites remain largely unidentified and transport properties are not well constrained. Addressing both questions can provide essential information on the origin of multi-PeV cosmic rays and on Galactic particle transport. A key difficulty is that the environments of young star clusters are shaped by large-scale (~100 pc) feedback from massive stars. Wind-wind interactions create intricate flow and magnetic field structures, which affect cosmic ray acceleration and transport in non-trivial ways. Here, 3D (M)HD simulations can provide insights, which in turn serve as bases for acceleration and transport models. I present simulations of wind-wind interaction in young star clusters and discuss their implications. In addition, I discuss the recent detection of PeV gamma-rays from the Cygnus region and demonstrate that a 50 kyr-old energetic supernova remnant can accelerate particles to the required energies.
September 5: Neutron stars in Full Color: Unifying Multi-messenger Echoes to Reveal Nature’s Densest Matter
Abstract: Several young star clusters show degree-scale diffuse gamma-ray emission, hinting at ongoing particle acceleration. Despite these detections, acceleration sites remain largely unidentified and transport properties are not well constrained. Addressing both questions can provide essential information on the origin of multi-PeV cosmic rays and on Galactic particle transport. A key difficulty is that the environments of young star clusters are shaped by large-scale (~100 pc) feedback from massive stars. Wind-wind interactions create intricate flow and magnetic field structures, which affect cosmic ray acceleration and transport in non-trivial ways. Here, 3D (M)HD simulations can provide insights, which in turn serve as bases for acceleration and transport models. I present simulations of wind-wind interaction in young star clusters and discuss their implications. In addition, I discuss the recent detection of PeV gamma-rays from the Cygnus region and demonstrate that a 50 kyr-old energetic supernova remnant can accelerate particles to the required energies.
September 5: Neutron stars in Full Color: Unifying Multi-messenger Echoes to Reveal Nature’s Densest Matter
Speaker: Chun Huang (Washington University in St. Louis)
Abstract: What neutron stars are made of remains one of the biggest mysteries in science. Despite a wealth of observations, we still lack a coherent framework that synthesizes these data to probe their interiors. During my doctoral research, I tackled this challenge head-on. I will present a new X-ray analysis framework grounded in modern plasma physics that delivers a more realistic, physically motivated emission model. To break through long-standing computational bottlenecks, I lead the development of an accelerated software package that boosts analysis speed by three orders of magnitude. I also convened a small international collaboration—CompactObject—that bridges cutting-edge nuclear theory with astrophysical observations and experiments. Finally, I will outline a roadmap for the multi-messenger future of neutron-star physics, charting a path toward a rigorous, falsifiable framework capable of unveiling the fundamental makeup of matter at the cores of these extraordinary objects.
June 16: Magnetohydrodynamic turbulence follows Kolmogorov scaling
Abstract: What neutron stars are made of remains one of the biggest mysteries in science. Despite a wealth of observations, we still lack a coherent framework that synthesizes these data to probe their interiors. During my doctoral research, I tackled this challenge head-on. I will present a new X-ray analysis framework grounded in modern plasma physics that delivers a more realistic, physically motivated emission model. To break through long-standing computational bottlenecks, I lead the development of an accelerated software package that boosts analysis speed by three orders of magnitude. I also convened a small international collaboration—CompactObject—that bridges cutting-edge nuclear theory with astrophysical observations and experiments. Finally, I will outline a roadmap for the multi-messenger future of neutron-star physics, charting a path toward a rigorous, falsifiable framework capable of unveiling the fundamental makeup of matter at the cores of these extraordinary objects.
June 16: Magnetohydrodynamic turbulence follows Kolmogorov scaling
Speaker: Mahendra K. Verma (Indian Institute of Technology Kanpur, India)
Abstract: The problem of scaling in isotropic magnetohydrodynamic (MHD) turbulence has remained unresolved, with competing predictions of $k^{-5/3}$ (Kolmogorov) and $k^{-3/2}$ (Iroshnikov-Kraichnan) scalings. In this talk I will present results of high-resolution numerical simulations on $8192^2$ and $1024^3$ grids. The computed energy spectra are closer to $k^{-5/3}$ than $k^{-3/2}$. More importantly, our detailed analyses of structure functions, intermittency exponents, and energy fluxes demonstrate robust support for Kolmogorov scaling. Our findings firmly establish Kolmogorov scaling in MHD turbulence and significantly improve the theoretical foundations for modeling astrophysical turbulence and dynamo processes.
May 23: Electromagnetic Signatures and Simulation Advances in Supermassive Black Hole Binaries
Speaker: Jordy R. Davelaar (Princeton University)
Abstract: Massive black hole binaries (SMBHBs), formed through galaxy mergers, are key targets for multi-messenger astrophysics. While LISA will detect their gravitational waves, electromagnetic (EM) signatures remain essential for identifying host galaxies and constraining the merger environment. In this talk, I will go over the current EM-based detection mechanisms for SMBHBs and highlight our recent efforts to identify candidates using Gaia astrometry, which can reveal nearby binaries through subtle long-term orbital motion. I will also revisit previously proposed tell-tale signatures, such as self-lensing flares, caused by gravitational lensing when one black hole passes in front of the other, and X-ray dimming due to disrupted accretion in the late inspiral phase.
I will then highlight our ongoing development of GRMHD binary simulations within the HAMR code, aimed at capturing the complex dynamics and radiative processes in these systems. As part of this, we are building the infrastructure needed to scale simulations to late-stage binaries and to incorporate radiative transfer efficiently. I will also discuss our use of machine learning to accelerate the conservative-to-primitive variable recovery step, a major bottleneck in radiation GRMHD. Together, these efforts represent our path forward toward bridging simulation and observation in the era of LISA.
April 25: Implications of the proposal that Binary Neutron Star mergers are the source of Ultrahigh Energy Cosmic Rays
April 25: Implications of the proposal that Binary Neutron Star mergers are the source of Ultrahigh Energy Cosmic Rays
Speaker: Glennys Farrar (NYU)
Abstract: I will summarize what is known about UHECRs. The observations are in fact highly constraining. The rigidity distribution (R==E/Ze) of UHECRs is observed to be very narrow which rules out the most popular source scenarios. This feature follows naturally, however, for Binary Neutron Star mergers. I will discuss evidence that UHECR acceleration occurs not in the jets but in the magnetized turbulent outflow, including the successful prediction of the shape and location of the spectral cutoff and smoking-gun tests of the composition
April 4: Universal fluctuation spectrum of Vlasov–Poisson turbulence
Speaker: Michael L. Nastac (Oxford)
Abstract: The thermal fluctuation spectrum of the electric field arising due to particle noise in a quiescent Vlasov-Poisson plasma was derived in the 1960s. Here, we derive the universal fluctuation spectrum of the electric field, at Debye and sub-Debye scales, for a turbulent Vlasov-Poisson plasma. This spectrum arises from what is likely to be the final cascade - a universal regime to be encountered at the extreme small-scale end of any turbulent cascade in a nearly collisionless plasma. The cascaded invariant is C2, the quadratic Casimir invariant of the particle distribution function. C2 cascades to small scales in position and velocity space via linear and nonlinear phase mixing, in such a way that the time scales of the two processes are critically balanced at every scale. We construct a scaling theory of the fluctuation spectrum of C2 and of the electric field in wavenumber space. The electric-field spectrum is sufficiently steep for the nonlinear mixing to be controlled by the largest-scale electric fields, and so the C2 cascade resembles the Batchelor cascade of a passive scalar. Our theory is supported by simulations of a forced 1D-1V plasma. We predict that the cascade is terminated at the wavenumber where the turbulent electric-field spectrum gives way to the thermal noise spectrum. The time scale for this small-scale cutoff to be reached is the dynamical time of phase-space mixing times a logarithmic factor in the plasma parameter - this is the first concrete demonstration of this property of Vlasov-Poisson turbulence, akin to how fluid turbulence dissipates energy at a rate independent (or nearly independent) of molecular diffusion. In the presence of the sub-Debye phase-space cascade - a scenario that may be ubiquitous - standard collisional plasma theory ceases to be valid. This calls for the development of new collision operators suited to such turbulent environments.
March 28: Understanding the Dynamics of Black Hole Accretion in Radiation GRMHD Simulations
March 28: Understanding the Dynamics of Black Hole Accretion in Radiation GRMHD Simulations
Speaker: Lizhong Zhang (IAS)
Abstract: Radiation and magnetic fields play crucial roles in shaping black hole accretion, particularly in near- and super-Eddington regimes. To model these systems, we solve the GRMHD equations coupled with angle-dependent radiation transfer, which enables us to capture the complex dynamics driven by radiation and magnetic fields in extreme environments. In the super-Eddington regime, radiative support causes the accretion disk to thermally expand, forming a narrow conical funnel through which radiation escapes, leading to low radiation efficiency. In the near-Eddington regime, the magnetic field topology strongly influences the resulting disk structure, allowing the system to reach a steady state as either a thin disk with magnetic coronae or a magnetically elevated disk. These simulations are broadly consistent with observational findings and provide predictive diagnostics for future observations, which I will discuss in detail during the talk.
March 3rd: A brief review of shock breakout from cosmic explosions
March 3rd: A brief review of shock breakout from cosmic explosions
Speaker: Udi Nakar (Tel Aviv University)
Abstract: Any stellar explosion triggers a radiation-mediated shock (RMS) that crosses the star and breaks out when the optical depth ahead is too low to sustain the shock. The photons released during shock breakout mark the first electromagnetic signal of any cosmic explosion, and they carry unique information about the medium from which the shock emerged, including geometry, radius, composition, and density profile. In my talk, I will discuss the unique properties of radiation-mediated shocks and describe the resulting breakout signal in a variety of cosmic explosions, including various types of supernovae, gamma-ray bursts, and binary neutron star mergers. I will highlight what we have learned from shock breakout observations until now, and what we expect to learn during the next decade.
February 21st: Nonlinear MHD stability of a plasma pinch
Speaker: David Hosking
Abstract: TBA
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