Probing Cosmic-Ray Transport with Radio Synchrotron Harps in the Galactic Center

Abstract

Recent observations with the MeerKAT radio telescope reveal a unique population of faint nonthermal filaments pervading the central molecular zone (CMZ). Some of those filaments are organized into groups of almost parallel filaments, seemingly sorted by their length, so that their morphology resembles a harp with radio emitting "strings". We argue that the synchrotron emitting GeV electrons of these radio harps have been consecutively injected by the same source (a massive star or pulsar) into spatially intermittent magnetic fiber bundles within a magnetic flux tube or via time-dependent injection events. After escaping from this source, the propagation of cosmic ray (CR) electrons inside a flux tube is governed by the theory of CR transport. We propose to use observations of radio harp filaments to gain insight into the specifics of CR propagation along magnetic fields of which there are two principle modes: CRs could either stream with self-excited magneto-hydrodynamical waves or diffuse along the magnetic field. To disentangle these possibilities, we conduct hydrodynamical simulations of either purely diffusing or streaming CR electrons and compare the resulting brightness distributions to the observed synchrotron profiles of the radio harps. We find compelling evidence that CR streaming is the dominant propagation mode for GeV CRs in one of the radio harps. Observations at higher angular resolution should detect more radio harps and may help to disentangle projection effects of the possibly three-dimensional flux-tube structure of the other radio harps.

One Source, Two Source(s): Ribs and Tethers

“…Filamentation may thus occur naturally in the ICM, with the magnetized filaments becoming populated, and thus illuminated by relativistic particles when reconnection takes place with radio galaxy lobes. Similar processes may play out in our own galaxy, where massive winds in the galactic center interact with strong fields in the interstellar medium, leading to equally spaced structures called "synchrotron harps" by Thomas et al [23]. Such structures could also be responsible for the "ribs" discussed above.…”

The Galactic center chimneys: the base of the multiphase outflow of the Milky Way

“…The nonthermal filaments would then appear anywhere there is a source of relativistic particles that illuminate the tube of field lines in which they are trapped (Morris & Serabyn 1996). The recent discovery of groups of nonthermal filaments spatially organized to resemble "harps," provides considerable credibility to such an interpretation (Thomas et al 2020). Several other interesting processes have also been invoked to explain the origin of the filaments (Lesch & Reich 1992;Rosner & Bodo 1996;Shore & LaRosa 1999;Bicknell & Li 2001;Yusef-Zadeh 2003;Bykov et al 2017;Sofue 2020a).…”

Connecting turbulent velocities and magnetic fields in galaxy cluster simulations with active galactic nuclei jets

“…In the ICM, CRs are expected to scatter on selfgenerated Alfvén waves (Kulsrud 2005). Due to inefficient Alfvén wave damping CRs are confined to stream down their pressure gra-dient ∇Pcr close to the Alfvén speed vA (Zweibel 2013;Thomas & Pfrommer 2019;Thomas et al 2020). CRs that stream faster than the local Alfvén speed excite the streaming instability that continuously generates Alfvén waves, which experience damping processes.…”