2023

December 22nd: Comptonization in Black Hole Coronae 

Speaker: Navin Sridhar, Columbia University.

Abstract: What powers the hard, non-thermal X-rays from accreting compact objects has been a longstanding mystery. In my talk, I will address the underlying question of what energizes the particles of the Comptonizing “corona” against the strong inverse Compton (IC) cooling losses with first-principle particle-in-cell simulations of radiative magnetic reconnection (subject to IC cooling) in magnetically dominated (σ>>1) electron-positron plasmas for a wide range of guide field strengths, and for the first time, in mildly-magnetized (σ~1) electron-ion plasmas. I will demonstrate that the electrons in the corona are necessarily cooled down to non-relativistic temperatures (in electron-ion and pair plasmas), due to inefficient energy transfer from hot ions to cold electrons through ion anisotropic instabilities. This renders the traditionally believed paradigm of 'thermal Comptonization' by a cloud of hot electrons unfeasible. Instead, Comptonization might be facilitated by the trans-relativistic bulk motions of turbulent/reconnection plasmoids (with a bulk spectrum resembling a ~100 keV Maxwellian). I will also discuss the potential sites of Comptonization (the corona) using our latest global resistive GRMHD simulations of accreting black holes.

December 8th: Coherent Phenomena in Relativistic Astrophysical Plasmas

Speaker: Alexander (Sasha) Chernoglazov, University of Maryland.

Abstract: In this talk, I will focus on the emission signatures we can expect from magnetospheres of compact objects, with a strong focus on pulsars. In the first part of the talk, I will describe the evolution of a three-dimensional collisionless reconnecting relativistic current sheet with strong synchrotron cooling. I will discuss the acceleration mechanisms that play the most important role, and how their contributions vary with the efficiency of the synchrotron cooling. I will also present the expected signatures of gamma-ray emission, e.g., its spectrum and anisotropy. These results will be (tentatively) applied to the recently observed TeV-emission from the Vela pulsar. In the second part of my talk, I will describe the 2D and 3D models of the polar pair production discharge. These new multi-dimensional models will help to understand how pulsar radio emission is produced, and the mechanisms behind its variability.

December 1st: Tearing-mediated reconnection in magnetohydrodynamic poorly ionized plasmas. I. Onset and linear evolution
Speaker: Libby Tolman (Flatiron Institute's Center for Computational Astrophysics)

Abstract: In high-Lundquist-number plasmas, reconnection proceeds via onset of tearing, followed by a non-linear phase when plasmoids continuously form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. We use theory and computation to study tearing-initiated reconnection in such poorly ionized systems. In this talk, we focus on the onset and linear evolution of this process. In poorly ionized plasmas, magnetic field nulls on scales below v_{An0}/\nu_{ni0}, with v_{An0} the neutral Alfven speed and \ni_{ni0} the neutral-ion collision frequency, will self-sharpen via ambipolar diffusion into CSs which tear. This nonlinear sharpening occurs at an increasing rate, delaying the onset of reconnection by decreasing the CS width at which the tearing growth rate becomes larger than the CS formation rate. Once the CS becomes thin enough, however, ions decouple from neutrals and thinning of the CS slows, allowing tearing onset. We find that the wavelength of the mode that first disrupts the forming sheet decreases as ionization fraction decreases and that onset occurs in a time of order \nu_{ni0}^{-1}.

November 17th:  Large-scale magnetic-field generation in turbulent black-hole accretion disks and its imprint on black hole spin 

Speaker: Jonatan Jacquemin Ide (Northwestern University)

Abstract: Jetted astrophysical phenomena with black hole (BH) engines, including binary mergers, jetted tidal disruption events, and X-ray binaries, require a large-scale vertical magnetic field for efficient jet formation.  I will present a possible mechanism for generating these crucial large-scale magnetic fields, using 3D global general relativistic magnetohydrodynamical (MHD) simulations of accretion disks. I find that the dynamo mechanism can be best understood as a nonlinear outcome of the magnetorotational instability (MRI) and large-scale advection. We characterize the complete dynamo mechanism with two timescales: one for local magnetic field generation and one for large-scale advection. The description and understanding of the dynamo mechanism pave the way toward a better understanding of jet launching. 

I will also describe the consequences of the large-scale vertical magnetic fields on the black hole. I will show that jet launching through a large-scale field leads to efficient spin-down of the central black hole, reducing the dimensionless black hole spin from a=1 to a=0.2 after accreting only 20% of its initial mass. By separating the contributions of the accretion disk and the large-scale magnetic field, we constructed a simplified black hole spin evolution model. I used this model to explore the consequence of black hole spin-down in the context of collapsing stars. My results show that collapsar black holes are born slowly spinning and should remain slowly spinning, consistent with LIGO/VIRGO/KARGA constraints on black hole spin.

November 10th: Cosmic ray acceleration by merger shocks in galaxy clusters
Speaker: Kamlesh Rajpurohit (CfA Harvard & Smithsonian)

Abstract: Galaxy clusters undergoing mergers host spectacular megaparsec-scale, highly polarized diffuse radio emission known as radio relics. With their enormous extent, these sources trace the largest particle accelerators in the Universe. Relics are usually found at the cluster outskirts and illuminated by relativistic electrons accelerated by merger-driven shock fronts in the intracluster medium (ICM). However, the currently proposed particle acceleration mechanisms are not efficient enough to accelerate particles from the thermal pool of the ICM. There is increasing observational evidence that requires us to consider the unknown acceleration processes across the entire lifetime of clusters, not only in the proximity of large merger events. The new generation of radio telescopes permits wideband spectro-polarimetric radio observations with unprecedented sensitivity and spatial resolution, thus allowing the investigation of the finest details. In this talk, I will discuss recent results obtained with high-resolution radio observations of some well-known relics. These findings challenge our understanding of particle acceleration mechanisms in galaxy clusters and highlight the need for additional theoretical work to uncover the underlying physics.

October 20th: Kinetic simulations of strong non-relativistic shocks propagating in a turbulent medium

Karol Fulat (University of Potsdam)

Abstract: Strong non-relativistic shocks are known to accelerate particles up to relativistic energies. However, for Diffusive Shock Acceleration electrons must have a highly suprathermal energy, implying a need for very efficient pre-acceleration. Most published studies consider shocks propagating through homogeneous plasma, which is an unrealistic assumption for astrophysical environments. To address this limitation, we have developed a novel simulation technique that provides a framework for studying shocks propagating in turbulent media. In this talk, I will present results from PIC simulations of non-relativistic high-Mach-number shocks propagating in electron-ion plasma with a turbulent upstream medium. We explore the impact of the fluctuations on electron heating and acceleration, the dynamics of upstream electrons, and the driving of plasma instabilities. I will also discuss our recent findings from oblique shock simulations.

October 6th:  News from phase space: universal equilibria, constant-flux cascades, and enduring mysteries

Alex Schekochihin (Oxford)

September 29: Magnetic orientation effects in 3D simulations of accreting neutron stars

Kyle Parfrey (PPPL)

July 21: Magnetogenesis in a collisionless plasma: from Weibel instability to turbulent dynamo

Muni Zhou (Princeton University)

Abstract: Astronomical observations suggest pervasive micro-gauss magnetic fields in our Galaxy and in the intracluster medium (ICM) of galaxy clusters. It is widely believed that such dynamically important magnetic fields are produced by plasma dynamos acting upon some "seed" magnetic fields. However, a complete understanding of this process in a weakly collisional plasma is still lacking. We report a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of "seed" magnetic fields, the formation of turbulence, and the inductive amplification of fields by fluctuation turbulent dynamo. The driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which in turn leads to the development of thermal pressure anisotropy. The Weibel instability is then triggered and produces filamentary, micro-scale "seed" magnetic fields. The plasma is thereby magnetized, enabling the stretching and folding of the fields by the plasma motions and the development of pressure-anisotropy instabilities. The scattering of particles off these microscale magnetic fluctuations provides an effective viscosity, impacting the field morphology and turbulence. During this process, the seed fields are further amplified by the fluctuation dynamo until they attain equipartition with the turbulent flow. This work has important implications for magnetogenesis in dilute astrophysical systems by demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows.

July 12: PIC simulations of the stratified magnetorotational instability

Mario Riquelme (University of Chile)

Abstract: The magnetorotational instability (MRI) plays a crucial role in regulating the accretion efficiency in various types of astrophysical accretion disks. In very low-luminosity disks around black holes, such as Sgr A* and M87, Coulomb collisions are very infrequent, making the MRI physics effectively collisionless. The collisionless MRI gives rise to kinetic plasma effects that can potentially affect both its dynamic and thermodynamic properties. In this seminar we will present 2D and 3D particle-in-cell (PIC) plasma simulations of the collisionless MRI in stratified disks using shearing-boxes with net vertical field. Our results consider pair plasmas, with initial beta = 100 and concentrate on sub-relativistic plasma temperatures. We these initial conditions, our 2D and 3D runs show disk expansion, significant particle and magnetic field outflows, and a dynamo-like process. They also produce magnetic pressure dominated disks with (Maxwell stress dominated) viscosity parameter alpha ~ 0.5 − 1. These results are fairly consistent with previous 3D MHD simulations. Our simulations also show significant nonthermal particle acceleration, approximately characterized by power-law tails with temperature dependent spectral indices −p. For temperatures in the range T ~ 0.05−0.3 times the particles rest mass energy, p ~ 2.2−1.9. The maximum particle energy in the tails depends on the scale separation between MHD and Larmor-scale (kinetic) plasma phenomena in a way approximately consistent with previous PIC results of magnetic reconnection-driven acceleration. Due to its scale separation dependence, nonthermal acceleration is more clearly observed in our 2D runs, where this separation is sufficiently large. Our study constitutes a first step towards modeling from first principles potentially observable stratified MRI effects in low-luminosity accretion disks around black holes and other compact objects.

July 7: BHXRBs Spectra: The Gateway to New Discoveries

Vladislav Loktev (University of Turku)

Abstract: The diversity and unique variability patterns inherent to Black Hole X-ray Binaries (BHXRBs) illustrate the complex and, as yet, debated physics underlying their many attributes. Historically, X-ray observations were primarily based on spectral data, but the introduction of the X-ray polarimetry mission IXPE has added a new dimension to this data: polarization. This novel perspective grants valuable insights into the geometric structure of these systems and the intricate physical processes at work. This talk will navigate the realm of polarized X-ray observations, demonstrating how IXPE has enriched our understanding of BHXRBs. Moreover, as time permits, I'll present some of my ongoing work on particle-in-cell simulations of magnetized turbulence, a study designed to explore the phenomena of high energy non-thermal particle acceleration, identifiable in the hard X-ray spectra of BHXRBs.

July 5: The evolution of acoustic pulses into shock waves — exact solutions using Einstein’s equivalence principle

Tamar Faran (Princeton University)

Abstract: An acoustic pulse propagating in the decreasing density profile of a stellar envelope is expected to steepen into a weak shock wave, and can potentially eject some of the envelope mass when it approaches the surface. This process is thought to be important in failed supernovae and stars that undergo vigorous core convection, which excites waves. A reliable estimate of the ejected mass, as well as the observational signatures of these events, relies on an accurate description of the transition from a weak to a strong shock. While analytic solutions exist in both the strong and weak limits, this intermediate regime is much harder to treat analytically. In this talk I will describe how we employ Einstein’s equivalence principle to obtain exact, analytic solutions for the hydrodynamics of isentropic, non-linear acoustic waves, and their steepening into shock waves. We then use our solutions to solve for the propagation of the shock (up to where energy dissipation becomes substantial) and capture the weak-to-strong transition, which shows a remarkable agreement with numerical simulations.

May 12: Fully-kinetic Simulations of Pulsar Magnetospheres and magnetic reconnection using WarpX

Revathi Jambunathan (Lawrence Berkeley National Laboratory)

Abstract: Pulsars are rapidly rotating neutron stars immersed in strong electromagnetic fields that emit twin beams of electromagnetic radiation. However, the plasma composition and structure in the region surrounding pulsars, called magnetospheres, and the physical processes that drive particle acceleration leading to the observed spectra are not well understood. Global pulsar magnetosphere simulations are required to answer these questions. However, resolving the current sheet skin-depth which is six orders of magnitude smaller than the pulsar radius for realistic systems, is intractable even on large supercomputers. In this talk, I present a two-pronged approach to study particle acceleration. First, we use a “wide computational lens” to perform global pulsar simulations with scaled-down energies. I will present the effect of magnetization on the plasma structure in the magnetosphere and particle energization. Second, we use a “focussed lens” to study relativistic reconnection in the currents sheets using a Harris-sheet set-up.  For the reconnection simulations, we will also explore the use of ultra high-order spectral methods (PSATD) and compare its accuracy and efficiency with traditional Yee methods. For our simulations, we use WarpX, a highly scalable, electromagnetic PIC code with advanced algorithms to simulate relativistic plasma on large-scale heterogeneous supercomputers.

Apr 28: Select numerical models of magnetic reconnection in black hole magnetospheres and jets

John Mehlhaff (Université Grenoble Alpes)

Abstract: The magnetospheres and jets of black holes host collisionless and highly magnetized plasmas. In such plasmas, relativistic magnetic reconnection may catalyze the release of free magnetic energy, powering quiescent and – especially – flaring emission. Exploring reconnection models in detail requires addressing multiwavelength observations that boast ever-increasing temporal and spatial resolution. This, in turn, demands a fully kinetic plasma description. Only then can one capture the collisionless coupling between position, momentum-space, and time that the observations probe. Thus, in my work, I use kinetic, particle-in-cell (PIC) simulations to model reconnection in regimes relevant to black hole magnetospheres and jets. I summarize results from two projects in this talk. The first project features PIC simulations of reconnection coupled to QED radiative physics. Here, the plasma particles radiate their energy in discrete quanta through Klein-Nishina inverse Compton scattering. Furthermore, the emitted gamma-ray photons can feed back on reconnection through electron-positron pair production. This radiative physics induces distinct observable signatures that can be compared to observations, both current and upcoming, of gamma-ray flares from flat-spectrum radio quasars. In the second part of my talk, I discuss ongoing efforts to model striped-jet launching from black holes using general relativistic PIC simulations. In this work, loops of alternating-polarity poloidal magnetic flux are accreted onto a spinning black hole. The rotational shear between the loop footpoints explosively tears the loops open via magnetic reconnection, potentially yielding observable magnetospheric counterpart emission to the formation of a large-scale striped jet.

Apr 21: Heavy element nucleosynthesis and high-energy neutrinos from magnetized outflows

Mukul Bhattacharya (Pennsylvania State University)

Abstract: While nuclei lighter than iron are fused over the course of typical stellar evolution, almost half of the elements heavier than iron are created through the rapid neutron capture process (r-process). These nuclei are thought to be produced in magnetised outflows from neutron-rich explosive events including compact mergers and core-collapse supernovae. In this talk, I will discuss the potential of neutrino-driven winds from strongly magnetised and rapidly rotating protomagnetars as plausible sites for r-process nucleosynthesis. As heavy nuclei can eventually produce ultra-high energy cosmic rays, we examine the acceleration and survival conditions for these nuclei. We also explore the propagation of these jets within Wolf-Rayet stars and blue/red supergiants. In particular, we analyse the criteria for a successful jet breakout, maximum energy deposited into the cocoon and structural stability of these magnetised jets. We show that high-energy neutrinos can be produced for extended progenitors like blue/red supergiants and estimate the detectability of these neutrinos with IceCube-Gen2.

Apr 7: The nature of filaments in 21 cm channel maps and other maps of diffuse emission

Alexandre Lazarian (University of Wisconsin–Madison)

Abstract: The striation of intensities observed in 21 cm velocity channel maps is frequently associated with filaments. Incidentally, a similar striation is seen in diffuse emission maps, e.g., spectroscopic channel maps, synchrotron polarization maps, and Faraday rotation maps. This suggests a common cause of the striation. I will demonstrate the importance of turbulent velocities in forming the structures in thin channel maps by comparing the results of synthetic observations of multi-phase simulations of magnetized HI with GALFA and FAST data. I will show that the filament-finding algorithms, e.g., RHT, associate these velocity-induced structures with filaments. Such filaments are well aligned with the magnetic field, while the alignment of filaments associated with 3D density enhancements is significantly lower. I shall discuss the implications of this finding to magnetic field studies. Finally, I will briefly demonstrate the magnetic field structure obtained in a new way for molecular clouds, active galaxies, and clusters of galaxies and compare these maps with the available polarization data.

Mar 3: Electron Energization in the Intracluster Medium and Solar Wind Shocks

Aaron Tran (Columbia University)

Abstract: In the weakly-collisional intracluster medium (ICM) and solar wind, the flow of energy from hydrodynamic to electron kinetic scales dictates the light and waves that we see with telescopes or with satellites in the heliosphere. We explore two mechanisms for energy flow in such plasmas.

First, in the high-beta ICM, Megaparsec-scale motions promptly trigger nanoparsec-scale plasma waves, which in turn can interact with a diffuse, long-lived "fossil" population of 1–100 MeV cosmic ray electrons (CRe). We study CRe scattering upon ion cyclotron waves driven by continuous compression in 1D particle-in-cell (PIC) simulations, which leads to energy gain by magnetic pumping. Resonant CRe gain ~10–30% of their initial energy in one compress/dilate cycle with magnetic field amplification ~3–6x, assuming adiabatic decompression without further scattering and averaging over initial pitch angle.

Second, within solar wind shocks, satellites see a zoo of electron Debye-scale structures that are not yet fully reproduced in kinetic shock simulations. We study one such class of waves—electron acoustic waves and electron holes—within the ion-scale, fast-mode wave-train precursor of a weak, low-beta shock. We'll show preliminary results on how the electron-scale wave power varies with simulation parameters.

Feb 3: Predicting emission spectra of X-ray binaries from first-principles kinetic plasma simulations

Daniel Groselj (Columbia University)

Abstract: We report results from the first radiative particle-in-cell simulations of turbulence in plasmas of moderate optical depth. The simulations self-consistently follow the evolution of radiation as it interacts with the turbulent plasma via Compton scattering. Under conditions expected in magnetized coronae of accreting black holes, we obtain an emission spectrum consistent with the observed hard state of Cyg X-1 and find that most of the emitted power comes from Comptonization by the bulk turbulent motions. The method presented here shows promising potential for ab initio modeling of various high-energy astrophysical sources and opens a window into a new regime of kinetic plasma turbulence.

Jan 23: Electron acceleration at non-relativistic shocks: first-principles simulations and a minimal model

Siddhartha Gupta (University of Chicago)

Abstract: The energetic charged particles are the prime sources of nonthermal radio, X-rays, and gamma rays, which are emitted by most astrophysical objects. Although diffusive shock acceleration (DSA) is the promising mechanism for particle acceleration, whether the processes that promote particles to DSA act similarly for electrons and protons/ions is still not well understood. To solve this puzzle, we have performed fully kinetic non-relativistic shock simulations for an unprecedented range of parameters. In this talk, I will demonstrate the results of these simulations with the help of particle tracking and test-particle analysis, and present a minimal model for electron acceleration. Finally, I will discuss the implications of our results to different environments from the interstellar medium to the intracluster medium, and highlight the limitations of our 1D and 2D kinetic simulations.