Session E15: Astrophysical Source Simulations and Modeling

Exploring Physics Informed Deep Learning for Resolving Subgrid-Scale Effects in Binary Neutron Star Simulations

We explore the promise of physics informed deep learning to capture the physics of subgrid-scale magnetohydrodynamic turbulence in simulations of the magnetized Kelvin-Helmholtz instability (KHI).  The KHI creates general relativistic magnetohydrodynamic turbulence that amplifies the magnetic field at a smaller scale than binary neutron star simulations are capable of resolving.  We develop physics informed artificial neural network models and evaluate their ability to resolve similar subgrid effects and other relevant phenomena.  Specifically, we use models to reproduce the results of various simulated test problems involving a variety of initial conditions, shocks, and coupled fields.  Finally, we discuss the feasibility of using such methods to capturing subgrid-scale general relativistic magnetohydrodynamic turbulence in numerical relativity simulations of binary neutron star mergers.   

The effect of correlations in models of the nuclear equation of state on neutron star inference

Astrophysical probes of neutron stars such as gravitational-wave and x-ray pulse profile modeling have improved prospects for inferring the nuclear equation of state at high densities.  To connect these observations and infer the properties of dense matter and neutron stars simultaneously, models for the equation of state are introduced. One class, parametric models, such as the commonly used piecewise-polytope and spectral parameterizations, rely on carefully engineered functional forms that reproduce a large array of realistic equations of state. In this talk I will compare these parametric models to a novel model based on Gaussian processes and show that parametric models impose unphysical, and sometimes opaque, correlations between density scales. Such inter-density correlations result in tighter constraints that are unsupported by data, and can lead to biased inference of the equation of state and of individual neutron star properties.    

Chiral anomaly effects in pulsar magnetospheres

In the gap regions of pulsar magnetospheres, the chiral anomaly can be activated by the strong magnetic field and the parallel component of the electric field. The estimates show that a substantial chiral charge density is produced in the electron-positron plasma. The resulting chiral charge should trigger the chiral plasma instability, which, in turn, should lead to the emission of helical electromagnetic modes. The frequencies of the relevant modes lie in the radio to the near-infrared range. It is argued that helical modes have a substantial amount of energy and, thus, can modify observable features of pulsars.   

Developing the QED plasma framework for magnetar magnetospheres

Magnetars -- neutron stars with magnetic fields of ~1e15 Gauss or greater -- do exist in the universe. These fields exceed the Schwinger (critical) field, hence quantum electrodynamics (QED) effects on magnetospheric plasmas cannot be ignored. Furthermore, advances in laser-plasma experiments bring would soon allow us to experimentally study ultra-magnetized plasmas too. Therefore, there is a great demand in systematic studies of the interplay of QED and plasma physics effects in ultra-strong-field environments. Here we present the 'QED plasma framework' which will allow for such studies. We present the derivation of the general equation of linear plasma modes in QED-plasma with arbitrarily strong magnetic field. We also precent an illustrative example of low-frequency modes in the ultra-magnetized cold plasma. These results can be important for understanding of a magnetospheric pair plasma of a magnetar and for future laser-plasma experiments.   

Dynamical screening of the electric field in pair discharges and its implications for pulsar radio luminosity and spectrum

Pulsar radio luminosity is a factor of $10^{-6}$ to $10^{-4}$ smaller than spindown luminosity, but has negligible dependence on spindown luminosity. The radio spectrum is roughly $S_\omega \sim \omega^{-1.4 \pm 1.0} $.  In this talk, we will discuss how these characteristics may arise partially from the nonlinear, relativistic, collisionless physics of the pair plasma discharge that screens the polar cap electric field. During the early stages of the discharge, the electric field experiences strong damping due to large displacements in the momenta of newly added pairs. This strong damping ceases when the displacement of newly injected particles in the electric field is small.  Quantitative statement of the resulting condition on electric field amplitude yields an expression for radio luminosity that is roughly consistent with observation.  After this point, the discharge experiences a linear phase which only slightly decreases the emission amplitude but creates a relationship between emission amplitude and frequency which may contribute to the observed radio spectrum.   

nuSpaceSim: A Comprehensive Simulation Package for Modeling Extensive Air Shower Cherenkov and Radio Signals from Cosmic Neutrinos

nuSpaceSim is a comprehensive simulation built to model the optical and radio signals from extensive air showers (EAS) induced by cosmic neutrinos. It is designed to aid the community in the development of sub-orbital and space-based experiment, and v1.0,  available via Github & pip on a HEASARC webpage, models the upward-moving EASs sourced from tau neutrino interactions within the Earth to determine diffuse flux sensitivity using user-defined input.  nuPyProp is used as the Earth-emergent lepton generator and models the above a PeV neutrino interactions and energy loss.  nuSpaceSim uses a vectorized, multi-tread Python implementation to efficiently simulate all aspects of the processes that lead to the EAS signal at a detector at a specific altitude, including modeling of the tau lepton decays and subsequent EAS, the air optical Cherenkov and radio signals, atmospheric propagation, and detector response while providing event-by-event analysis variables. Further development is in progress to model the sensitivity and sky-coverage for transient sources, improve models of the atmosphere (including clouds, aerosols, and ozone) and add enhancements to the optical EAS signals. The software and physics modeling in nuSpaceSim will be discussed along with example comparisons.   

The NuPyProp module of NuSpaceSim: neutrino and tau propagation in the Earth

NuSpaceSim is designed to simulate optical and radio signals of upward-going extensive air showers caused by neutrino interactions inside the Earth. NuPyProp, a simple-to-use python-based simulation package, generates look-up tables for nuSpaceSim and can be used as a stand-alone program. It simulates and models UHE neutrino interactions and charged lepton energy loss inside the Earth to provide exit probabilities and energy distributions for tau-leptons from tau neutinos and muons from muon neutrinos. Several neutrino cross-section models and charged lepton photonuclear energy loss models are provided, along with a template for custom inputs. Results from nuPyProp are compared with other tau neutrino and tau propagation codes, including from NuTauSim and NuPropEarth.   

Primordial black hole abundance calculation in concert with optimized peaks theory-- what changes compared to traditional Press-Schechter?

The abundance of primordial black hole (PBH) formation in the early Universe has been long studied using the Press-Schechter formalism. Although very easy to apply, the theory is built on a rather flimsy theoretical framework (despite making robust predictions). Peaks theory is an alternative to the study of matter condensation in the Universe (PBHs by extention) which was developed under stronger theoretical foundations while avoiding certain pitfalls of Press-Schechter like the window function ambiguity. Here, I will talk about PBH abundance calculated using the optimized peaks theory. This particular variant introduces a more accurate expression for PBH mass, giving rise to an O(10) increase compared to the traditional calculation. This has very interesting consequences in model building, viz a viz newer limits to where the peak in the curvature power spectra can be placed in order to evade evaporation constraints. Furthermore, peaks theory can help model builders evade certain future, small scale constraints on P_ζ arising from LISA. Applying these on a PBH forming α-attractor model, it can be shown that PBHs of mass M~10¹⁷ g can be formed from the collapse of smaller scale modes k~8x10¹⁴ Mpc^-1 which would have previously predicted M<10¹⁶ g. I also comment on the collapse threshold and show that the numerically favored δ_th≈0.5 can be achieved using smaller peaks in P_ζ.