Title: Probing Cosmic-Ray Transport with Radio Synchrotron Harps in the Galactic Center
Authors: Timon Thomas, Christoph Pfrommer , and Torsten Enßlin
First Author’s Institution: Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
Status: Published in ApJL [open access]
Radio observations of the Galactic center reveal fascinating structures: many extended, isolated filaments stretching across space. Some of these filaments are grouped together, and arranged in parallel, just like the strings of a giant cosmic harp (Fig. 1). These filaments are illuminated by synchrotron emissions radiated by cosmic ray (CR) electrons, and are usually accompanied by aligned magnetic fields. Scientists still do not fully understand what powers these CR electrons, or how they travel from their sources to form such orderly structures. The authors of today’s paper suggest that CR electrons that illuminate these filaments all come from the same sources. Therefore, these filaments offer an exciting opportunity to study how cosmic rays move through space.
Fig. 1: Two radio harps in the MeerKAT observation. Left: G359.85+0.39 discovered by LaRosa et al. (2001). Right: G359.4+0.12 first imaged by Heywood et al. (2019). The authors of today’s paper use the right image for the analysis in this paper.
To explain the formation of these filament structures, we need to take a closer look at the mechanism behind synchrotron emission. This type of radiation occurs when CR electrons spiral around magnetic fields. The stronger the magnetic field or the larger the number of CR electrons, the brighter the synchrotron emission. So, if these CR electrons originated from the same source, how do we get these “string-like” structures instead of a smooth radio glow? One possibility is that the source continuously releases CR electrons into regions with uneven magnetic fields. Areas with stronger magnetic fields act like “magnetic flux tubes” and emit more radio emissions. Alternatively, the source might periodically release CR electrons into uniform magnetic fields, creating distinct filaments.
The authors propose that the first scenario (see left panel of Fig. 2) could occur in shockwaves produced by stellar winds of massive stars. These winds create not only shocks that accelerate CRs but also layers of compressed magnetic fields, which form localized magnetic peaks. Alternatively, the second scenario (see right panel of Fig. 2) could occur when filaments form around pulsar wind nebulae. Here, CRs can escape the nebulae when the magnetic field inside the wind nebulae reconnects with the magnetic field in the surrounding ISM. This reconnection intermittently releases bursts of CR electrons, sending them into interstellar space in pulses.
Fig. 2: Illustrations of possible ways CRs could enter the filaments. Left: A massive star at the center sends out powerful stellar winds, generating shockwaves that accelerate the CR particles and intensify the magnetic field. Right: A pulsar nebula, powered by a dense, fast-rotating star remnant, accelerates CRs within its strong electromagnetic field, periodically releasing them when magnetic reconnection occurs.
Once these CR electrons enter the interstellar medium (ISM), they travel along magnetic field lines. Their movement combines three main processes: advection, diffusion, and streaming (see Fig. 3): On a larger scale, CRs advect in the same way as the magnetic field, which is frozen into the gas in magnetohydrodynamic (MHD) simulations. However, the authors think advection cannot form these narrow, straight filaments, since gas motions are too turbulent to create these streamlined shapes. On a microphysical scale, how CRs move along magnetic field lines depends more on their scattering frequency. When scattering is weak, CRs tend to diffuse outward from their source, spreading out in a wider area. But if they scatter often, they stream together at a shared speed, forming sharper boundaries (see right panel of Fig. 3). Because diffusion and streaming create different shapes and movement speeds, researchers can study the brightness patterns of these CR-produced filaments to differentiate these two transport modes.
Fig 3. Typical ways CRs travel through space, with different colors showing how CRs spread over time. Left: In advection, CRs move steadily along with the magnetic field and gas. Middle: When CRs are weakly scattered, they diffuse outward from their source, creating a smooth, spread-out shape. Right: In streaming, where CRs are effectively scattered, they move in a more structured way along the magnetic field, forming sharp, well-defined edges.
The authors conduct simulations to explore how CRs might move in two different scenarios: one with only diffusion and another with both diffusion and streaming. They then compared the patterns that these simulations produced to real observations of a radio filament G359.47+0.12. Fig. 4 shows the results of these comparisons, the simulations (orange line) are arranged from an earlier time (top row) to a later time (bottom row), while the observation (black line) is ordered from the shortest filament (top row) to the longest (bottom row). The results reveal that the simulation that considers both streaming and diffusion (left panel) closely matches the observed brightness profile. In contrast, the diffusion-only model (right panel) does not agree well with the observation; it keeps the brightest spot at the center and falls short in brightness farther out. This suggests that including streaming better represents how CRs might actually spread through space.
Fig 4: Comparison of radio emission data with simulated patterns of synchrotron emission produced by CRs. The black line shows the radio brightness at different stages—16, 26, 37, and 72 thousand years after the CRs were first released (from top to bottom), while the orange line presents the observational signal. The model that includes both streaming and diffusion (left panel) closely matches the observed radio data, while the diffusion-only model (right panel) does not capture the patterns as accurately.
We now have a clear picture of how these ordered CR filaments form: they are illuminated by synchrotron emission emitted by CR electrons, which originate from the same source either within bundles of magnetic flux tubes or through bursts of CR ejections over time. When CR electrons are released, they move along magnetic field lines, stretching out over time. As a result, the newest CR electrons create shorter, brighter filaments, while the older ones form longer, fainter trails. These filament structures capture a sort of time-lapse of the CR journey and serve as a natural test of CR transport models. This research sheds new light on how cosmic rays travel and reveals hidden details about the magnetic landscape near the center of our galaxy, showing that both streaming and diffusion are key to capturing the full picture in galaxy simulations.
