Fast and "lossless" propagation of relativistic electrons along magnetized non-thermal filaments in galaxy clusters and the Galactic Center region

TL;DR

This paper proposes a model where relativistic electrons propagate losslessly along magnetized filaments in galaxy clusters, explaining large-scale radio structures without re-acceleration or shocks, and predicts polarization and pressure-dependent features.

Contribution

It introduces a new propagation mechanism along ordered magnetic filaments that allows electrons to travel large distances without energy loss, challenging previous limitations.

Findings

Electrons can propagate over hundreds of kpc without significant energy loss.

Synchrotron break frequency scales with ambient gas pressure as P^{1/2}.

Filaments should exhibit strong polarization.

Abstract

Relativistic leptons in galaxy clusters lose their energy via radiation (synchrotron and inverse Compton losses) and interactions with the ambient plasma. At z~0, pure radiative losses limit the lifetime of electrons emitting at ~GHz frequencies to t<100 Myr. Adiabatic losses can further lower Lorentz factors of electrons trapped in an expanding medium. If the propagation speed of electrons relative to the ambient weakly magnetized (plasma $\beta\sim10^2$) Intracluster Medium (ICM) is limited by the Alfv\'en speed, $v_{a,ICM}=c_{s,ICM}/\beta^{1/2}\sim 10^7\,{\rm cm\,s^{-1}}$, GHz-emitting electrons can travel only $l \sim v_{a,ICM}t_r\sim 10\,kpc$ relative to the underlying plasma. Yet, elongated structures spanning hundreds of kpc or even a few Mpc are observed, requiring either a re-acceleration mechanism or another form of synchronization, e.g., by a large-scale shock. We argue that filaments with ordered magnetic fields supported by non-thermal pressure have $v_{a}\gg v_{a,{\rm ICM}}$ and so can provide such a synchronization even without re-acceleration or shocks. In particular, along quasi-stationary filaments, electrons can propagate without experiencing adiabatic losses, and their velocity is not limited by the Alfv\'en or sound speeds of the ambient thermal plasma. This model predicts that along filaments that span significant pressure gradients, e.g., in the cores of galaxy clusters, the synchrotron break frequency $\nu_b\propto B$ should scale with the ambient gas pressure as $P^{1/2}$, and the emission from such filaments should be strongly polarized. While some of these structures can be observed as "filaments", i.e., long and narrow bright structures, others can be unresolved and have a collective appearance of a diffuse structure, or be too faint to be detected, while still providing channels for electrons' propagation.