Skip to main content

Origin and Magnetization of astronomical Jets

Periodic Reporting for period 2 - O.M.J. (Origin and Magnetization of astronomical Jets)

Reporting period: 2020-04-01 to 2021-09-30

The key scientific question that this work addresses is the origin and internal properties of astronomical jets. Relativistic jets are ubiquitous in very many systems on very many scales- from stellar-size objects such as neutron stars or solar-mass black holes, to active galactic nuclei, in the heart of which there are black holes the size of million to billion solar masses.

We aim at understand the underlying mechanism behind the launching of relativistic jets, as well as study their internal properties. A key assumption which is tested is that strong magnetic fields play a key role in this process, as mediators of rotational energy to kinetic energy. Furthermore, magnetic fields may affect the dynamics, radiation, and their interaction with particles (via Alfvenic waves) can provide a natural way of obtaining a population inversion, necessary for the production of maser emission, a possible mechanism for the recently discovered “fast radio bursts” (FRB) phenomenon.

Astrophysics is among the most rapidly evolving field in science, which rightfully enjoys a huge amount of public interest. The work in this project intriguers the imagination of many young kids and students who consider their path in life.
This project combines basic physics of broad nature (relativity, magneto-hydrodynamics, particle-wave interactions and radiation) with observations. This is done, largely, by means of state-of-the-art computational algorithms, which are run on parallel computational facilities. Beyond promoting the understanding of the basic physics involves, this project refines novel algorithms, and is therefore promoting knowledge in both physics and computer science, as well as contributes to the wide-spread of astronomy among the public.
1. Development of new GR-MHD code: (i) we wrote a cuda-version of the publicly available code “HARM” (“cu-HARM”) which enables it to run on graphic processor units [GPUs], thereby speeding up the runs by a factor of >~100. (ii) We improve upon the algorithms for correction of malfunction cells which naturally occur when solving the partial differential equations that determine the evolution of the gas and EM field properties, as well as modified the grid used in the code. These modifications enable to further increase the calculation speed and accuracy of the results.
2. Improving the radiative transfer algorithm, enabled us to propose a new picture for gamma-ray bursts (GRB) prompt emission. In our picture, photons are emitted close to the jet base and then propagate in the virtually empty jet before being scattered off from an expanding cork. The observed signal is therefore due to scattering by the back side of the cork, which, at this stage is already relativistically expanding. This model is the first to provide a physically-motivated, natural explanation to the observed E_peak – E_iso relation.
3. We proposed a novel model for explaining the “plateau” phase seen in many GRBs, which is both natural and at the same time leads to a paradigm shift. We showed that both the X-ray and optical data can be explained as due to an explosion with an initial Lorentz factor of only a few 10s rather than a few 100’s.
4. One of the most important objects studied in recent years are fast radio bursts (FRBs). These show a coherent, bright radio emission. We study the interaction of Alfvenic waves, expected in the vicinity of a strongly magnetized source such as NS or rapidly rotating BH, with relativistic, thermal plasma, as a way of producing a population inversion, thereby provide the necessary condition for a synchrotron maser emission, which may be the origin of the observed signal.
5. We studied (i) the transition between the collisionless and collisional regimes, which affect the shock ability to accelerate particles to high energy. (ii) We further discovered a new type of shock using particle-in-cell numerical code. This article was selected for the cover page of Physics of Plasmas journal
6. We studied the properties of the magnetic field in the quiescent state of several X-ray binaries. We found that the magnetic field in the disk is several orders of magnitude below the commonly assumed equipartition value.
So far, we have made several breakthroughs. To my view, some of them may lead to a paradigm shift, and others represent a major technical and scientific progress, which establish the basis for additional achievements and breakthroughs in the nearby future.

1. A new version of the GR-MHD code HARM, whose basic version is publicly available. In my group, we have re-written major parts of this code in cuda programming language, which enables it to run on a GPU-based machine. Due to the significantly cheaper cost, a small-size GPU- based machine is capable of improving the calculation speed relative to standard CPU-based parallel computer of similar size by a factor of >~100.
We expect to continue to develop this method, which we believe will turn out crucial when adding further complexities to the calculations involved, in particular to the radiative transfer.

2. A new analysis method and new interpretation we have for understanding the origin of the GRB plateau phase. By analyzing the data and confronting it with known dynamical models, we realized that the solution may in fact be much simpler and straight-forward than what many people believe. We show that both the x-ray plateau as well as the weakly decaying optical radiation seen during this early afterglow phase are fully consistent with an explosion into a density gradient, provided that (i) the initial Lorentz factor of the blast wave is “only” a few tens and not few hundreds as previously thought; and (b) the stellar wind is much diluted relative to the expectation from a Wolf-Rayet star.
This interpretation, if correct, marks a ‘paradigm shift’ in this field, as it proves that the range of Lorentz factor of the plasma ejected in the GRB event is much wider than thought, and bridges the ‘enthalpy gap’, by showing that relativistic ejecta extends over the entire range, from trans-relativistic in XRBs, ~few in active galactic nuclei to few tens and then to few hundreds in GRBs.

3. The study we carry of the particle-wave interaction, and the conditions for population inversion that can result in synchrotron maser emission. When interaction with a population of thermal electrons, Alfveic waves modify the distribution of the electron population, which can lead to a population inversion. This, in turn, will result in maser emission. Despite many years of research, solutions so far existed only in the non-relativistic regime. By extending the solution to the relativistic regime, we open a new horizon towards understanding coherent emission in astronomical objects. An immediate application is the physics of FRBs, which is the subject of an intense study. Understanding the basis of the radiative processes involved will lead to a thorough understanding of the environments which produce these phenomena.
A snapshot of a 3-d GR-MHD simulation which was made using GPUs