Periodic Reporting for period 1 - EMAXSHOCK (On the maximum energy of particles accelerated in astrophysical jets)
Período documentado: 2021-03-01 hasta 2023-02-28
The problem being addressed is the efficiency of particle acceleration and magnetic field amplification in mildly relativistic and low velocity shocks, in radiogalaxy and protostellar jets, respectively. In the former case I am focused on the study of magnetic field amplification in active galactic nuclei (AGN) jets and the maximum energy that particles can achieve. In the later case we aim to address if protostellar jets can accelerate TeV particles, and therefore be a potential new kind of gamma-ray sources for future gamma-ray facilities like the Cerenkov Telescope Array.
Understanding the micro-physics of mildly relativistic and low velocity plasma has important implications for different fields in Astrophysics. In particular, one of the most exciting and unsolved problems is the origin of the Ultra High Energy Cosmic Rays. These particles have an energy of about 100 EeV and arrive on the Earth from outside the Galaxy, but their origin remains unknown. At lower energies (< 3 PeV) cosmic rays are likely galactic. With that respect, a large fraction of gamma-ray sources in the Galaxy remain unidentified, and jets from protostars are among candidates to be the hidden CR accelerators and gamma-ray emitters.
The overall objective of this project is the investigation of the maximum energy that particles can achieve when they are accelerated via diffusive shock acceleration in both mildly-relativistic and low-velocity shocks, in AGN and protostellar jets, respectively. Particles are accelerated at shocks and the magnetic field is likely amplified by the cosmic-ray streaming instability due to the fast drift of the particles in the background plasma. We want to find the maximum energy that particles can achieve when they are accelerated in shocks with different velocities in plasma with different densities. We also investigate the micro-physics of particle acceleration in collision-less shocks generated by intense lasers.
Understanding the micro-physics of mildly relativistic and low velocity plasma has important implications for different fields in Astrophysics. In particular, one of the most exciting and unsolved problems is the origin of the Ultra High Energy Cosmic Rays. These particles have an energy of about 100 EeV and arrive on the Earth from outside the Galaxy, but their origin remains unknown. At lower energies (< 3 PeV) cosmic rays are likely galactic. With that respect, a large fraction of gamma-ray sources in the Galaxy remain unidentified, and jets from protostars are among candidates to be the hidden CR accelerators and gamma-ray emitters.
The overall objective of this project is the investigation of the maximum energy that particles can achieve when they are accelerated via diffusive shock acceleration in both mildly-relativistic and low-velocity shocks, in AGN and protostellar jets, respectively. Particles are accelerated at shocks and the magnetic field is likely amplified by the cosmic-ray streaming instability due to the fast drift of the particles in the background plasma. We want to find the maximum energy that particles can achieve when they are accelerated in shocks with different velocities in plasma with different densities. We also investigate the micro-physics of particle acceleration in collision-less shocks generated by intense lasers.
1) By using the PIC-MHD code newly developed by F. Casse, A. van Marle and A. Marcowith (van Marle et al. 2018) we calculated the magnetic field power spectrum downstream and upstream of the mildly-relativistic shock. Supra-thermal particles are injected in the shock downstream region following the recipe from particle-in-cell simulation carried out by our collaborator Artem Bohdan (Max Planck Institute for Plasma Physics, Garching). Different Alfvènic Mach numbers were considered in order to account for a variety of jet densities and unperturbed magnetic fields. In the figure attached we show the magnetic field strength relative to the original magnetic field (top) and thermal gas mass density relative to the upstream density (bottom) at the end of the simulation. Axis are normalized to the gyroradius of injected particles in the upstream medium.
2) Although the formation of low-mass stars is well known, it is not the case for high-mass stars. The main difficulty of this study is to parameterize the jet properties. Several processes play a role in launching and collimating the jets from high-mass protostars and therefore a deep study of the literature (including numerical ans observational studies) was needed in order to characterize the jets properly. The code describing the evolution and emission of non-thermal electrons is ready, and the hadronic component implementation is currently under completion. We are using a mass distribution model to derive the fraction of the total jet power imparted into cosmic rays and evaluate the particle escape and gamma-ray emission produced when they interact with the dense gas in the molecular cloud. These calculations are used in a CTA consortium paper under preparation.
3) As a side project, we started a new collaboration to study in deep particle acceleration in laser-induced collisionless shocks. The National Ignition Facility at Livermore has indeed observed the production of non-thermal electrons in collisionless shocks (Fiuza et al. 2020). We propose a theoretical interpretation of the experimental result (Tikhonchuk et al., in preparation).
Preliminary results were presented in the International Cosmic Rays Conference and the Texas symposium on Relativistic Astrophysics, as planned.
A.Araudo delivered 4 invited and 4 contributed talks during the period of the fellowship.
2) Although the formation of low-mass stars is well known, it is not the case for high-mass stars. The main difficulty of this study is to parameterize the jet properties. Several processes play a role in launching and collimating the jets from high-mass protostars and therefore a deep study of the literature (including numerical ans observational studies) was needed in order to characterize the jets properly. The code describing the evolution and emission of non-thermal electrons is ready, and the hadronic component implementation is currently under completion. We are using a mass distribution model to derive the fraction of the total jet power imparted into cosmic rays and evaluate the particle escape and gamma-ray emission produced when they interact with the dense gas in the molecular cloud. These calculations are used in a CTA consortium paper under preparation.
3) As a side project, we started a new collaboration to study in deep particle acceleration in laser-induced collisionless shocks. The National Ignition Facility at Livermore has indeed observed the production of non-thermal electrons in collisionless shocks (Fiuza et al. 2020). We propose a theoretical interpretation of the experimental result (Tikhonchuk et al., in preparation).
Preliminary results were presented in the International Cosmic Rays Conference and the Texas symposium on Relativistic Astrophysics, as planned.
A.Araudo delivered 4 invited and 4 contributed talks during the period of the fellowship.
Mildly-relativistic shocks are not as well studied as other shock speed regimes like non relativistic shocks in supernova remnants and relativistic shocks in pulsar winds. Because the approximations usually done in relativistic plasma cannot be used, the treatment of mildly relativistic plasma remains under consideration. One important complication is the magnetic field obliquity. Finding the Rankine-Hugoniot jump conditions in mildly-relativistic shocks with an arbitrary obliquity is non-trivial.
There are no analytical approximations and the solution of the full set of MHD equations lead to a system of coupled equations that need to be solved numerically. We have introduced this procedure in the relativistic PIC AMRVAC code. This allows us to perform simulations beyond the state of the art in mildly-relativistic plasma as we combine them with results obtained using pure PIC simulations. The latter provide us with the fraction of particles reflected upstream. The simulations generalize the results obtained in the non-relativistic regime by van Marle et al. (2022). We perform the same kind of parametric survey but in the mildly-relativistic regime including long time 2D simulations, longer than in Crumley et al. (2019).
We have shown that the combination of adiabatic and radiative shocks is of great relevance for enhancing the gamma-ray emission in low velocity shocks in protostellar jets. In Araudo et al. (2021) we have studied, for the first time, the ability of low velocity adiabatic shocks to accelerate protons and amplify the magnetic field through the non-resonant hybrid instabilities. Relativistic protons accelerated in the adiabatic shock diffuse up the dense layer in the contact discontinuity and radiate there more efficiently than in the adiabatic shock downstream region. In del Valle, Araudo, and Suzuki-Vidal (2022) we focused on the instabilities in the contact discontinuity, both in astrophysical and laser plasma.
In a follow up study, and because the poor angular resolution of current and upcoming gamma-ray detectors, we compute the collective emission from all the massive protostars embedded in a single molecular cloud. This study is of great relevance for detectability studies by CTA. (The paper is under preparation.) In addition, by considering low-mass protostars we investigate in deep the injection of low energy cosmic rays in the molecular cloud. Low energy Cosmic Rays are essential to ionize matter at high density column.
Particle acceleration in laser-plasma interactions. Laser plasma shocks are growing in relevance for understanding cosmic rays physics. In particular the injection problem.
Motivated by the experiment performed by Fiuza et al. (2020), where they succeeded in creating a collision-less shock in the laboratory and accelerate electrons up to 500 keV, we are performing a theoretical study of electron acceleration in laser plasma in the same conditions of the experiments in order to shed light on the acceleration mechanism. (Paper in preparation)
There are no analytical approximations and the solution of the full set of MHD equations lead to a system of coupled equations that need to be solved numerically. We have introduced this procedure in the relativistic PIC AMRVAC code. This allows us to perform simulations beyond the state of the art in mildly-relativistic plasma as we combine them with results obtained using pure PIC simulations. The latter provide us with the fraction of particles reflected upstream. The simulations generalize the results obtained in the non-relativistic regime by van Marle et al. (2022). We perform the same kind of parametric survey but in the mildly-relativistic regime including long time 2D simulations, longer than in Crumley et al. (2019).
We have shown that the combination of adiabatic and radiative shocks is of great relevance for enhancing the gamma-ray emission in low velocity shocks in protostellar jets. In Araudo et al. (2021) we have studied, for the first time, the ability of low velocity adiabatic shocks to accelerate protons and amplify the magnetic field through the non-resonant hybrid instabilities. Relativistic protons accelerated in the adiabatic shock diffuse up the dense layer in the contact discontinuity and radiate there more efficiently than in the adiabatic shock downstream region. In del Valle, Araudo, and Suzuki-Vidal (2022) we focused on the instabilities in the contact discontinuity, both in astrophysical and laser plasma.
In a follow up study, and because the poor angular resolution of current and upcoming gamma-ray detectors, we compute the collective emission from all the massive protostars embedded in a single molecular cloud. This study is of great relevance for detectability studies by CTA. (The paper is under preparation.) In addition, by considering low-mass protostars we investigate in deep the injection of low energy cosmic rays in the molecular cloud. Low energy Cosmic Rays are essential to ionize matter at high density column.
Particle acceleration in laser-plasma interactions. Laser plasma shocks are growing in relevance for understanding cosmic rays physics. In particular the injection problem.
Motivated by the experiment performed by Fiuza et al. (2020), where they succeeded in creating a collision-less shock in the laboratory and accelerate electrons up to 500 keV, we are performing a theoretical study of electron acceleration in laser plasma in the same conditions of the experiments in order to shed light on the acceleration mechanism. (Paper in preparation)