2025

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

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

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

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