On the origin and acceleration of cosmic rays: Cooling flow clusters and AGN hosts

We are looking for radio relics' and halos’ in an X-ray selected sample of clusters of galaxies. These radio features are not a product of the Active Galactic Nuclei (AGN)-mechanism, but more likely are associated with past cluster merger events. AGN hosts of cooling flow clusters contain particle bubbles that show non-thermal radio emission. These bubbles could explain the presence of radio relics and halos if they can restrict cosmic rays efficiently. Intracluster magnetic fields and cluster environments can reveal the acceleration mechanisms of cosmic rays. Using radio/X-ray data and analytical methods we examine three AGN hosts out of our 70 clusters, namely Hercules A, 3C310 and 3C388. We found that none of these clusters contain relics and/or halos.

💡 Research Summary

The paper investigates the origin and acceleration of ultra‑high‑energy cosmic rays (UHECRs) in galaxy clusters that host powerful active galactic nuclei (AGN) and exhibit cooling‑flow (CF) phenomena. The authors begin by selecting an X‑ray‑based sample of 70 Abell clusters, motivated by the known correlation between radio and X‑ray properties of clusters. Within this sample they focus on three particularly luminous radio galaxies—Hercules A (3C 348), 3C 388, and 3C 310—each residing at the centre of a dense, cooling‑flow cluster and classified as FR I/II hybrid sources. The central aim is to search for diffuse, low‑surface‑brightness radio structures known as “relics” and “halos,” which are generally interpreted as signatures of past merger‑driven turbulence or shock acceleration in the intracluster medium (ICM).

Observations and Data Reduction
Radio data were obtained with the VLA in multiple configurations and frequencies, delivering total intensity and polarization maps at ~1.4 arcsec resolution. Very Long Baseline Interferometry (EVN) observations targeted the weak cores at milliarcsecond scales. X‑ray imaging and spectroscopy were drawn from ROSAT PSPC/HRI, BeppoSAX, ASCA, and Chandra archives, allowing the authors to model the thermal gas distribution with a β‑model, derive central electron densities (ne0 ≈ 10⁴–10⁵ m⁻³), core radii (rc ≈ 15–30 kpc), and temperatures (kT ≈ 0.5–3.5 keV).

Radio Morphology and Spectral Properties
Hercules A displays a strikingly asymmetric jet–ring system: the eastern jet shows a helical morphology, while the western jet terminates in a ring‑like feature. Spectral index mapping reveals flatter spectra (α ≈ ‑0.7) in the jets and rings, steepening to α ≈ ‑1.2 in the surrounding lobes, indicating a recent outburst superimposed on older plasma. The core is extremely weak (≈15 mJy) and unresolved even at 18 mas resolution. 3C 388 is a classic FR I double with a faint, steep‑spectrum halo in the eastern lobe, but this structure appears tied to the host galaxy rather than the cluster as a whole. 3C 310 exhibits two prominent radio rings whose magnetic field vectors trace the ring edges; the overall spectral index is very steep (α ≈ ‑1.4), suggesting an aged electron population with possible intermittent re‑acceleration. VLBI imaging at 18 cm resolves two compact components within ~5 pc of the core, accounting for ~7 % of the total VLA flux.

X‑ray Environment and Cooling Flows
All three clusters show evidence for cooling flows: the X‑ray surface‑brightness profiles are well described by β‑models with β ≈ 0.44–0.74, and the central cooling times are a few gigayears (≈2–6 Gyr for the hot phase, ≈0.5–3 Gyr for the cooler phase). ROSAT and Chandra images reveal cavities (X‑ray depressions) coincident with the radio lobes in Hercules A and 3C 388, confirming that the jets displace the thermal gas. However, in Hercules A the cavities are not perfectly aligned with the radio lobes, suggesting a more complex interaction, possibly involving buoyant bubbles or previous outbursts.

Magnetic Field Estimates
The authors combine Faraday rotation measurements with inverse‑Compton constraints to infer central magnetic field strengths of B ≈ 3–30 µG, with a characteristic tangling scale of 4–35 kpc. The field appears amplified near the edges of the radio rings and lobes, consistent with compression of the ICM magnetic field by expanding bubbles. Equipartition arguments suggest that the magnetic pressure is an order of magnitude lower than the thermal pressure of the ICM, implying that particle pressure dominates within the lobes while magnetic pressure is significant in the surrounding shell.

Cosmic‑Ray Acceleration Scenarios
Three mechanisms are discussed: (i) shock acceleration at jet termination points and at the leading edges of expanding bubbles; (ii) turbulent re‑acceleration driven by merger‑induced ICM turbulence (although no clear merger signatures are observed in the X‑ray data); and (iii) stochastic (second‑order Fermi) acceleration within the magnetized bubbles themselves. Electron cooling times due to synchrotron and inverse‑Compton losses are short (∼ Myr), which, together with the observed large radio extents, demands ongoing re‑acceleration. Protons, by contrast, have cooling times comparable to or exceeding the Hubble time, allowing them to accumulate and potentially escape the cluster as UHECRs.

Search for Relics and Halos
Despite the presence of powerful AGN and cooling flows, the authors find no diffuse relic or halo emission in any of the three clusters, even after careful low‑surface‑brightness analysis. This negative result challenges the conventional view that relics/halos are ubiquitous in massive clusters, especially those undergoing mergers. The authors argue that the combination of a strong, centrally located AGN and a relaxed, cooling‑flow ICM may suppress the formation of large‑scale turbulent structures required for relic/halo generation.

Conclusions and Outlook
The study concludes that while AGN‑driven bubbles can inject magnetic fields and relativistic particles into the ICM, the absence of relics/halos in these cooling‑flow clusters suggests that merger‑driven turbulence is a more critical ingredient for generating diffuse cluster‑scale radio emission. Nevertheless, the observed magnetic field configurations and the energetics of the bubbles indicate that particle acceleration is ongoing, and that these environments could still contribute to the population of UHECRs, especially via long‑lived proton components. The authors recommend deeper low‑frequency radio observations (e.g., with LOFAR or the upcoming SKA‑Low) and high‑resolution X‑ray spectroscopy (e.g., Athena) to detect faint relic/halo remnants and to better quantify the turbulence and magnetic field structure in such clusters.