arXiv subjects

Ioannis Dimitropoulos

Publications and source records attributed to Ioannis Dimitropoulos.

4 recordsLinked to original sources

arXiv2025

High-energy radiation from the pulsar Equatorial Current Sheet

Pulsars emit beams of radiation that reveal the extreme physics of neutron star magnetospheres. Yet, their understanding remains incomplete. Recent global Particle-in-Cell (PIC) simulations have raised several questions that led us to question their validity and their extrapolation to realistic particle Lorentz factors, electric and magnetic fields. We want to generate realistic sky maps of high-energy radiation from first principles. We propose a novel method to study the Equatorial Current Sheet (ECS) where most of the particle acceleration and the high-energy radiation is expected to originate. We first determine its shape and external magnetic field with a steady-state ideal force-free solution. Then, we consider the extra electric and magnetic field components that develop when dissipation is considered. Finally, we study the particle acceleration and radiation that is due to these extra field components for realistic field and particle parameters. We generate realistic sky maps of high-energy radiation and compare them with those obtained via PIC simulations. These sky maps may also be closely reproduced using the ECS of the split-monopole solution beyond the light cylinder. The ECS is probably stabilized by the normal magnetic field component that is due to the global magnetospheric reconnection. Our method helps us better understand the origin of the pulsed high-energy radiation in the pulsar magnetosphere.

astro-ph.HE↗

arXiv2024

The 3D pulsar magnetosphere with machine learning: first results

All numerical solutions of the pulsar magnetosphere over the past 25 years show closed-line regions that end a significant distance inside the light cylinder, and manifest thick strongly dissipative separatrix surfaces instead of thin current sheets, with a tip that has a distinct pointed Y shape instead of a T shape. We need to understand the origin of these results which were not predicted by our early theories of the pulsar magnetosphere. In order to gain new intuition on this problem, we set out to obtain the theoretical steady-state solution of the 3D ideal force-free magnetosphere with zero dissipation along the separatrix and equatorial current sheets. In order to achieve our goal, we needed to develop a novel numerical method. We solve two independent magnetospheric problems without current sheet discontinuities in the domains of open and closed field lines, and adjust the shape of their interface (the separatrix) to satisfy pressure balance between the two regions. The solution is obtained with meshless Physics Informed Neural Networks (PINNs). In this paper we present our first results for an inclined dipole rotator using the new methodology. We are able to zoom-in around the Y-point and inside the closed-line region, and we observe new interesting features. This is the first time the steady-state 3D problem is addressed directly, and not through a time-dependent simulation that eventually relaxes to a steady-state. We have trained a Neural Network that instantaneously yields the three components of the magnetic field and their spatial derivatives at any given point. Our results demonstrate the potential of the new method to generate new solutions of the ideal pulsar magnetosphere.

astro-ph.HE↗

arXiv2024

A new solution of the pulsar equation

We present the first new type of solution of the pulsar equation since 1999. In it, the whole magnetosphere is confined inside the light cylinder and an electrically charged layer wraps around it and holds it together. The reason this new solution has never been obtained before is that all current time-dependent simulations are initialized with a vacuum dipole configuration that extends to infinity, thus their final steady-state solution also extends to infinity. Under special conditions, such a confined configuration may be attained when the neutron star first forms in the interior of a collapsing star during a supernova explosion, or when it accretes from an external wind or disk from a donor star. It is shown that this new maximally closed non-decelerating solution is the limit of a continuous sequence of standard magnetospheres with open and closed field lines when the amount of open field lines gradually drops to zero. The minimum energy solution in this sequence is a standard magnetosphere in which the closed field line region extends up to about 80% of the light cylinder. We estimate that the released energy when the new solution transitions to the minimum energy one is enough to power a fast radio burst.

astro-ph.HE↗

arXiv2024

The Pulsar Magnetosphere with Machine Learning: Methodology

In this study, we introduce a novel approach for deriving the solution of the ideal force-free steady-state pulsar magnetosphere in three dimensions. Our method involves partitioning the magnetosphere into the regions of closed and open field lines, and subsequently training two custom Physics Informed Neural Networks (PINNs) to generate the solution within each region. We periodically modify the shape of the boundary separating the two regions (the separatrix) to ensure pressure balance throughout. Our approach provides an effective way to handle mathematical contact discontinuities in Force-Free Electrodynamics (FFE). We present preliminary results in axisymmetry, which underscore the significant potential of our method. Finally, we discuss the challenges and limitations encountered while working with Neural Networks, thus providing valuable insights from our experience.

astro-ph.HE↗