2024
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.