Embedded Files
November 18: Monster shocks and transient black hole pulsars from black hole–neutron star mergers
November 18: Monster shocks and transient black hole pulsars from black hole–neutron star mergers
Speaker: Yoonsoo Kim (Caltech)
Abstract: A majority of black hole-neutron star mergers are expected to leave little or no tidally disrupted remnant material. Magnetic field of the neutron star will transition onto the black hole while leaving a clean magnetosphere during and after the merger. I will present our latest simulations of a black hole-neutron star merger with consistently tracking the common magnetosphere of the binary in a nearly force-free limit, uncovering two types of transients with magnetospheric origins. Fast magnetosonic waves, excited during the late inspiral and merger phases, nonlinearly steepen into radiative monster shocks. The post-merger magnetosphere rearranges into a split monopole geometry shortly after the merger, then the spin of the remnant black hole powers a transient `black hole pulsar' state. For inclined magnetic field configurations, a rotating split monopole field launches a pulsar-like striped wind. Our analysis suggests this black hole pulsar to emit a blueing gamma-ray transient. The exponential decay of the magnetic field at the horizon happens on resistive timescales, where we find that the ringdown of the remnant black hole drives a more rapid shedding of magnetic fields in certain cases.
November 1: Nuclear reactions induced by the passage of radiation mediated shocks in BNS mergers
Speaker: Amir Levinson (Tel Aviv University)
Abstract: The propagation of a radiation mediated shock (RMS) in the ejecta of BNS merger is likely to affect the composition of r-process elements and, potentially, the early kilonova signal. The reason is that the merger ejecta contains a large number of r-process isotopes with different charge-to-mass ratio, which when crossing the shock experience different deceleration rates, ultimately leading to a velocity separation between the different isotopes, and their subsequent transmutations through inelastic collisions. Capture of free neutrons that cross the shock should also induce fission behind the shock. In this talk, after a brief introduction to the physics of RMS, I’ll discuss a multi-fluid model that incorporates electrostatic coupling between the different plasma constituents, as well as Coulomb and neutrino friction, in a self-consistent manner, and show that a considerable composition change via inelastic collisions of alpha particles with heavy elements downstream is expected at shock velocities > 0.25 c, if the He abundance is large enough. If the shock is produced prior to freeze-out, neutron capture by heavy nuclei behind the shock can also lead to a significant composition change.
October 18th: Plasma Astrophysics of Fast Radio Burst
Speaker: Maxim Lyutikov (Purdue University)
Abstract: --
October 14th: AGN jets in merging galaxy clusters
Speaker: Paola Domínguez Fernández (Center for Astrophysics | Harvard & Smithsonian).
Abstract: Radio lobes from active galactic nuclei (AGN) are a source of fossil cosmic-ray (CR) electrons within the intracluster medium (ICM), which may be re-accelerated and emit in the radio band. The re-acceleration of this fossil material is often linked to shocks, which may explain certain observed radio relics, and to turbulence, which may account for radio halos. In my talk, I will discuss magnetohydrodynamic simulations of binary galaxy cluster mergers, which include the injection of CR by jets from a central AGN. I will discuss our findings on the diverse morphologies produced by the CR as the merger evolves and the lobes are disrupted. I will also focus the discussion on the potential for this fossil material to be re-accelerated by shocks and turbulence, highlighting its significance in the diffuse radio emission observed in radio relics and radio halos.
October 11th: Decoding the Mysteries of Fast Radio Bursts: Diverse Origins and Future Prospects
October 11th: Decoding the Mysteries of Fast Radio Bursts: Diverse Origins and Future Prospects
Speaker: Mohit Bhardwaj (Carnegie Mellon University).
Abstract: Fast radio bursts (FRBs) are among the most intriguing mysteries in modern astronomy, with their origins remaining elusive even 17 years after their discovery. Theories explaining FRBs span a wide range, from catastrophic events to more persistent, non-cataclysmic origins. In recent years, magnetar-based models have gained traction, yet questions persist about whether most FRBs are the result of rapid, post-supernova processes or if they emerge from recycled compact objects over longer timescales. Recent findings support young progenitor models, underpinned by host galaxy demographics and the recognition of significant selection biases. However, the detection of FRBs in green-valley and quiescent galaxies (albeit from a small sample size) suggests that a more complex and diverse range of formation pathways may be at play. This raises critical questions: Can core-collapse supernovae alone account for the observed host galaxy distribution? Do the data indicate multiple formation channels within the prompt category, beyond just massive star explosions? And how do selection biases influence our understanding of these mechanisms? In this talk, I will explore these questions through the lens of observations from over two dozen well-localized FRBs in the local Universe. I will present new insights into the potential origins of these enigmatic bursts, emphasising the role of host galaxy environments and the influence of selection effects. Finally, I will discuss what the next few years may hold for FRB research, highlighting how upcoming observations could bridge critical gaps in our understanding of stellar evolution and the lifecycle of compact objects.
October 4th: Spectral Broadening of Fast Radio Bursts in Magnetar Magnetospheres
Speaker: Siddhant Solanki (University of Maryland)
Abstract: Fast Radio Bursts (FRBs) are energetic, millisecond to microsecond duration radio transients with cosmological origins. Landmark observations of SGR 1935+2154 showed that at least some FRBs originate in highly magnetized neutron stars, or magnetars. However, the exact site and mechanism of the FRB generation remains unknown. If produced in the inner magnetospheres of the magnetars, the FRB signal can be modeled as linear waves described by force-free electrodynamics (FFE). It has been recently proposed that such waves can be completely lost to the background via the weak turbulence cascade in the force-free regime. I show via direct numerical simulations that this is indeed the case, and the energy of the signal is lost to spectral broadening, which is in agreement with the previous work. Additionally, I highlight the relevant cascades that are responsible for this process and discuss the appropriate timescales for the FRB problem.
September 27th: Nonthermal Electron Acceleration via Stochastic Shock Drift Acceleration: Theory, Simulation, and Observation
September 27th: Nonthermal Electron Acceleration via Stochastic Shock Drift Acceleration: Theory, Simulation, and Observation
Speaker: Takanobu Amano (University of Tokyo)
Abstract: Electron injection into diffusive shock acceleration has been a topic of great interest over the decades. One of the prime candidate mechanisms, stochastic shock drift acceleration (SSDA), has recently been discussed based on fully kinetic simulations and spacecraft observations. In this talk, I will discuss the present status and future perspectives of SSDA as a solution to the electron injection problem.
August 9th: Probing Pulsar Winds and Particle Acceleration with Spider Pulsars
August 9th: Probing Pulsar Winds and Particle Acceleration with Spider Pulsars
Speaker: Andrew Sullivan (Stanford University)
Abstract: Rapidly spinning millisecond pulsars represent some of the most impressive astrophysical machines. A unique class of these objects known as spider pulsars are found in tight binaries with low mass companions. The pulsar emission irradiates the companion star, driving off a massive stellar wind, which can collide with the pulsar wind to produce an intrabinary relativistic shock. Divided into black widows and redbacks, spider pulsars can have their intrabinary shocks wrap around either the pulsar or the companion. These shocks represent sites of magnetic reconnection and high energy particle acceleration which manifests in X-ray emission. In this talk I will discuss how spider pulsars represent excellent probes of the pulsar wind and astrophysical shock-driven magnetic reconnection through recent X-ray analyses of the intrabinary shock of the massive redback pulsar J2215+5135 and future polarization measurements enabled by IXPE.
June 17th: Fully kinetic shearing box simulations of magnetorotational instability in electron-ion plasma
June 17th: Fully kinetic shearing box simulations of magnetorotational instability in electron-ion plasma
Speaker: Evgeny Gorbunov (KU Leuven)
Abstract: Accretion disks are widely appearing in the universe around massive compact objects such as black holes (BHs), and often consist of relativistically hot, radiative electron-ion plasma in a turbulent state. Global general relativistic magnetohydrodynamics (GRMHD) has been widely and successfully employed to study the global structure of accretion disks, as well as to interpret recent observational data obtained by Event Horizon Telescope (EHT) collaboration. However, it is commonly accepted that low-luminosity targets of EHT are essentially collisionless, with the lack of Coulomb interactions between ions and electrons potentially leading to a two-temperature state of the plasma. The interpretation of observations via GRMHD simulations, which is dependent on the temperature ratio between ions and electrons determined by kinetic physics, is therefore fundamentally limited. Thus, it is critical to more accurately determine the ion-to-electron heating ratio. To improve current existing heating ratio prescriptions and account for a collisionless regime, the kinetic particle-in-cell (PIC) approach is essential to model plasma around BHs. To keep first-principles simulations computationally accessible, we propose a novel shearing-box [1,2] approach, in which only a small region within the accretion disk is modelled. The primary driver for turbulence in this case is the magnetorotational instability (MRI), which significantly amplifies magnetic fields and injects energy at large scales, which can cascade and energize particles. Using this approach, we aim to provide a more accurate prescription for ion-electron heating ratios for more precise global simulations in future.
May 17th: Investigating turbulence effects on the magnetic reconnection rate through high-resolution resistive 3D MHD simulations
Speaker: Giovani Heinzen Vicentin (Ph.D. candidate at University of São Paulo; and VRSC at Princeton University)
Abstract: Accretion disks are widely appearing in the universe around massive compact objects such as black holes (BHs), and often consist of relativistically hot, radiative electron-ion plasma in a turbulent state. Global general relativistic magnetohydrodynamics (GRMHD) has been widely and successfully employed to study the global structure of accretion disks, as well as to interpret recent observational data obtained by Event Horizon Telescope (EHT) collaboration. However, it is commonly accepted that low-luminosity targets of EHT are essentially collisionless, with the lack of Coulomb interactions between ions and electrons potentially leading to a two-temperature state of the plasma. The interpretation of observations via GRMHD simulations, which is dependent on the temperature ratio between ions and electrons determined by kinetic physics, is therefore fundamentally limited. Thus, it is critical to more accurately determine the ion-to-electron heating ratio. To improve current existing heating ratio prescriptions and account for a collisionless regime, the kinetic particle-in-cell (PIC) approach is essential to model plasma around BHs. To keep first-principles simulations computationally accessible, we propose a novel shearing-box [1,2] approach, in which only a small region within the accretion disk is modelled. The primary driver for turbulence in this case is the magnetorotational instability (MRI), which significantly amplifies magnetic fields and injects energy at large scales, which can cascade and energize particles. Using this approach, we aim to provide a more accurate prescription for ion-electron heating ratios for more precise global simulations in future.
May 17th: Investigating turbulence effects on the magnetic reconnection rate through high-resolution resistive 3D MHD simulations
Speaker: Giovani Heinzen Vicentin (Ph.D. candidate at University of São Paulo; and VRSC at Princeton University)
Abstract: We study turbulence's effect on magnetic reconnection via high-resolution 3D MHD simulations across a large parametric space, considering Lundquist number values ever assumed, as high as S = 10⁶. We inject an initial multi-mode perturbation in a current sheet, producing self-generated turbulence that is sustained in the system, resulting in fast magnetic reconnection rates of V_rec/V_A ~ 0.03 - 0.08. These rates surpass those driven solely by resistive tearing mode (plasmoid) instability. Reconnection rates remain independent of the Lundquist and magnetic Prandtl numbers, aligning with the theory of turbulent reconnection proposed by Lazarian and Vishniac (1999), as well as with solar observations and previous simulations of accretion flows and relativistic jets. The plasma-β parameter shows mild influence, decreasing V_rec from 0.036 to 0.029 as β increases from 2.0 to 64.0 for simulations with Lundquist number S = 10⁵.
May 3rd: Simulating Supermassive Black Holes Feeding and Feedback at the Meso-scale
Abstract: We study turbulence's effect on magnetic reconnection via high-resolution 3D MHD simulations across a large parametric space, considering Lundquist number values ever assumed, as high as S = 10⁶. We inject an initial multi-mode perturbation in a current sheet, producing self-generated turbulence that is sustained in the system, resulting in fast magnetic reconnection rates of V_rec/V_A ~ 0.03 - 0.08. These rates surpass those driven solely by resistive tearing mode (plasmoid) instability. Reconnection rates remain independent of the Lundquist and magnetic Prandtl numbers, aligning with the theory of turbulent reconnection proposed by Lazarian and Vishniac (1999), as well as with solar observations and previous simulations of accretion flows and relativistic jets. The plasma-β parameter shows mild influence, decreasing V_rec from 0.036 to 0.029 as β increases from 2.0 to 64.0 for simulations with Lundquist number S = 10⁵.
May 3rd: Simulating Supermassive Black Holes Feeding and Feedback at the Meso-scale
Speaker: Trung Ha (University of North Texas)
Abstract: Supermassive black holes' (SMBHs) feeding and feedback cycles play a vital role in galaxy formation and evolution. While current state-of-the-art numerical simulations can successfully reproduce the multiphase gas at the galaxy-scale, it is not well understood how this multiphase gas is connected to the gas at the accretion-disk-scale. Such an understanding is crucial for setting up general-relativistic magneto-hydrodynamical (GRMHD) simulations of the accretion disk. Our work bridges this gap by providing theoretical explanations for the meso-scale accretion flows and their implications for galaxy evolution. To achieve this goal, we employ (M)HD simulations with Athena++, motivated by observations of the elliptical galaxy M87 at the center of the Virgo cluster. Our galaxy-scale simulations include an active-galactic-nuclei (AGN) jet feedback prescription that balances cooling on large scale. Then, we utilize a nested zoom-in technique to construct meso-scale simulations of the central parsecs of the galaxy, and model the dynamical evolution of multiphase accretion flow at this scale. These simulations ultimately provide boundary and initial conditions for future GRMHD simulations of the accretion disk at the horizon-scale, which can be directly compared to observations made with the Event Horizon Telescope. This work aims to improve our understanding of SMBH feeding, AGN jet feedback and their impact on galaxy evolution, and also to inform physically motivated sub-grid models in large-scale simulations.
March 22nd: Understanding the cyclotron-line formation in accreting-magnetic neutron stars via relativistic Monte Carlo radiative transfer simulations
Abstract: Supermassive black holes' (SMBHs) feeding and feedback cycles play a vital role in galaxy formation and evolution. While current state-of-the-art numerical simulations can successfully reproduce the multiphase gas at the galaxy-scale, it is not well understood how this multiphase gas is connected to the gas at the accretion-disk-scale. Such an understanding is crucial for setting up general-relativistic magneto-hydrodynamical (GRMHD) simulations of the accretion disk. Our work bridges this gap by providing theoretical explanations for the meso-scale accretion flows and their implications for galaxy evolution. To achieve this goal, we employ (M)HD simulations with Athena++, motivated by observations of the elliptical galaxy M87 at the center of the Virgo cluster. Our galaxy-scale simulations include an active-galactic-nuclei (AGN) jet feedback prescription that balances cooling on large scale. Then, we utilize a nested zoom-in technique to construct meso-scale simulations of the central parsecs of the galaxy, and model the dynamical evolution of multiphase accretion flow at this scale. These simulations ultimately provide boundary and initial conditions for future GRMHD simulations of the accretion disk at the horizon-scale, which can be directly compared to observations made with the Event Horizon Telescope. This work aims to improve our understanding of SMBH feeding, AGN jet feedback and their impact on galaxy evolution, and also to inform physically motivated sub-grid models in large-scale simulations.
March 22nd: Understanding the cyclotron-line formation in accreting-magnetic neutron stars via relativistic Monte Carlo radiative transfer simulations
Speaker: Nick Loudas (Princeton University)
Abstract: Accretion-powered X-ray Pulsars (XRPs) are strongly magnetized neutron stars (NSs), that accrete matter from a donor companion star, often exhibiting a cyclotron resonant scattering feature (CRSF) in their X-ray spectra. Accretion onto their magnetic poles is responsible for the emergence of X-rays, while the quantization of electrons motion perpendicular to the NS's magnetic field results in the emergence of absorption/emission lines (i.e., CRSFs) via matter-radiation resonant scattering. The CRSF encodes important information about the magnetic field in the line-forming region. Nevertheless, the site of the CRSF formation is still an open puzzle. For low-/sub-critical luminosity sources, the CRSF is believed to form above the magnetic pole's hotspot, while for the high-luminosity (L_X >~ 1e37 erg/s) ones, two places are suspect for the formation of a CRSF: the surface of the neutron star and the radiative shock in the accretion column arising above the magnetic pole.
In this talk, I'll give an overview of the proposed cyclotron-line formation scenarios, introduce the fundamental Physics involved, and briefly discuss the observational signatures/correlations used to constrain the current viable models. Subsequently, I'll focus on the high-luminosity sources in which the existence of a radiative shock in the accretion column is feasible. I'll demonstrate that the observed properties of CRSFs in these sources cannot be explained by the scenario where the site of the CRSF formation is the surface of the NS. Next, I'll describe our relativistic Monte Carlo Radiative Transfer (MCRT) code developed to study the spectral formation in the remaining candidate site of cyclotron-line formation, i.e., the radiative shock in the accretion column. I'll present the derived emergent spectra, show that a power law, hard X-ray continuum, and a prominent CRSF are naturally produced by the first-order Fermi energization as the photons criss-cross the shock, and thus provide concrete evidence that the radiative shock is the site of CRSF formation. Finally, I'll conclude that the combination of bulk-, thermal-Comptonization, and resonant Compton scattering of photons by electrons in a radiative shock is efficient in producing spectra with prominent CRSFs similar to the ones observed in high-luminosity XRPs.
March 8th: Fast magnetic reconnection experiments at the National Ignition Facility: flux annihilation and electron heating
March 8th: Fast magnetic reconnection experiments at the National Ignition Facility: flux annihilation and electron heating
Speaker: Vicente Valenzuela Villaseca (Princeton University)
Abstract: Magnetic reconnection is a fundamental plasma process whereby two merging flows with oppositely oriented magnetic fields drive the reconfiguration of the field topology inside a current sheet, which rapidly converts magnetic into kinetic energy. This ubiquitous process is key to understanding a wide range of systems, such as coronal mass ejections in the Sun, Active Galactic Nuclei flares, and sawtooth crashes in tokamaks. One of the outstanding questions in magnetic reconnection regards the energy partition between thermal heating, bulk acceleration, and non-thermal particles.
In this talk, I will present results from a dedicated laser-driven magnetic reconnection laboratory platform fielded at the National Ignition Facility (NIF). Two highly extended plasma plumes are produced by tiling a total of 40 laser beams which become self-magnetized through the Biermann battery effect. As they collide, they form a reconnecting current sheet in the interaction region with a high aspect ratio ~100.
The magnetic field structure and evolution is probed using proton radiography with inertial confinement fusion capsules as backlighters, which show a peak plume line-integrated magnetization ~ 7.5 T⋅mm. The out-of-plane dynamics was studied using gated X-ray detectors, finding significant electron heating; however we also find that conversion of magnetic to thermal energy is insufficient to explain the observed temperature increase. I will discuss the role of electron-ion Coulomb collisions in a background of free-streaming ions and show that they may account for the anomalous heating found in these experiments.
February 23rd: Metastability of magnetohydrodynamic equilibria and their relaxation
February 23rd: Metastability of magnetohydrodynamic equilibria and their relaxation
Speaker: David N. Hosking, PCTS
Abstract: Motivated by explosive releases of energy in fusion, space and astrophysical plasmas, this talk considers the nonlinear stability of stratified magnetohydrodynamic (MHD) equilibria in 2D. I demonstrate that, unlike the Schwarzschild criterion in hydrodynamics (“entropy must increase upwards for stability”), the modified Schwarzschild criterion for 2D MHD (or any kind of fluid dynamics with more than one source of pressure) is a guarantee only of linear stability. As a result, in 2D MHD (unlike HD) there exist metastable equilibria that are unstable to nonlinear perturbations despite being stable to linear ones. I show that the available energy of these equilibria under non-diffusive reorganization of flux tubes can be calculated by solving a combinatorial optimization problem. The reorganized states with minimum energy are, to good approximation, the final states reached by simulations of destabilized equilibria at small Reynolds number. To predict the state reached by turbulent relaxation at large Reynolds number, I construct a statistical mechanical theory based on the maximization of Boltzmann’s mixing entropy (this is analogous to the Lynden-Bell statistical mechanics of stellar systems and collisonless plasmas and the Robert-Sommeria-Miller theory of 2D vortices). I show that the predictions of this statistical mechanics are in remarkable agreement with numerical simulations.
February 16th: Cosmic ray pressure anisotropy instability and general discussions on CR feedback microphysics
February 16th: Cosmic ray pressure anisotropy instability and general discussions on CR feedback microphysics
Speaker: Xiaochen SUN, Princeton University
Abstract: Cosmic ray (CR) feedback significantly influences galaxy evolution by shaping the dynamics of interstellar and intergalactic media. To comprehend the microphysics underlying CR feedback, kinetic numerical simulations are imperative. I employ magnetohydrodynamic particle-in-cell (MHD-PIC) simulations are to explore the saturation of the CR pressure anisotropy instability. This instability arises from background expansion/compression and is counterbalanced by ion-neutral damping. I quantify the effective scattering rate and CR anisotropy level at saturation, scaling with environmental parameters. These findings hold promise for potential implementation as subgrid physics in CR hydrodynamics. Nonetheless, several enigmatic aspects of CR feedback in microphysics remain unresolved. Towards the conclusion of this discourse, potential avenues for kinetic simulations exploring the CR streaming bottleneck and resonant curvature will be deliberated.
February 9th: A review on pulsar gamma-ray halos
Speaker: Luca Orusa, Princeton University
Abstract: Pulsar halos are extended gamma-ray structures formed by electrons and positrons that have escaped from the central pulsar wind nebulae (PWNe), interacting with photons of the interstellar radiation fields. Pulsar halos serve as unique probes for investigating CR propagation in specific regions of the Galaxy and can provide indirect information about the progenitor electrons and positrons from a region that is close to the source. The inferred cosmic-ray diffusion coefficient from pulsar halos is smaller than the average value in the Galaxy, leading to various proposed interpretations in recent years. In this presentation, I will review recent developments in pulsar halo studies, from the characteristics of these sources, to potential multiwavelength investigations, proposed mechanisms for slow diffusion or other possible explanations, and the crucial role of pulsar halos in understanding cosmic-ray propagation and electron injection from PWNe.
January 26th: Understanding the Dynamics of Neutron Star Accretion Columns by Radiative Relativistic MHD Simulations
January 26th: Understanding the Dynamics of Neutron Star Accretion Columns by Radiative Relativistic MHD Simulations
Speaker: Lizhong Zhang, IAS
Abstract: Accretion-powered X-ray pulsars are neutron stars that accrete matter from a companion star in a binary system, which exhibit fascinating dynamics. The strong magnetic field of the neutron star guides material onto the magnetic poles, which are generally misaligned from the spin axis, resulting in pulsating X-rays as the star rotates. At high accretion rates, a magnetically confined accretion flow forms a column structure near the magnetic poles, supported against gravity by radiation pressure. This column structure is highly dynamical, displaying kHz quasi-periodic oscillations. Simulated accretion columns reveal the presence of the photon bubble instability, although it is not the trigger for the oscillatory behaviors. Instead, the oscillations originate from the inability of the system to resupply heat and locally balance sideways cooling. The column structure is very sensitive to the shock geometry because it directly determines the cooling efficiency. The simulated time-averaged column structures can be approximately reproduced by a corrected 1D stationary model, accounting for the actual 2D/3D shape of the time-averaged column. The scattering opacity can be significantly reduced in a strong magnetic field, potentially altering the column structure and variability. However, pair production appears to boost the opacity above ~4e8 K and would likely introduce additional effects.
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