Magnetic fields in radio relics and in the outskirts of galaxy clusters

The observation of diffuse synchrotron radio emission in galaxy clusters reveals the existence of relativistic electrons and large-scale magnetic fields permeating the intra-cluster volume. Shocks developed during mergers of galaxy clusters are thought to originate the elongated, arc-shaped, and polarized sources found in cluster outskirts known as radio relics. The properties of the magnetic fields in radio relics, the possible magnetic field amplification caused by the shock passage, and its connection to the relics formation mechanism, as well as the value of large-scale magnetic fields in cluster outskirts, are the main topics addressed in this Thesis. Using polarimetric radio observations performed with the Jansky Very Large Array (JVLA) and with the Low Frequency Array (LOFAR), I have investigated the properties of magnetic fields in galaxy clusters outskirts via both statistical Faraday rotation measures and diffuse source polarimetry. The comparison of measured quantities with advanced numerical simulations of, both, radio relic emission and intra-cluster magnetic fields has proved to be a fundamental tool to constrain magnetic field physical parameters. Among the results, the Faraday rotation studies of two double relic galaxy clusters presented in this Thesis show that magnetic-fields are not significantly amplified in the shock region. Despite this, low Mach numbers shocks can stretch and align magnetic field lines, which appear well ordered on the emission scales of radio relics from polarimetric observations. The magnetic field structure inside radio relics is complex and likely filamentary. These results will be corroborated by an ongoing systematic study of clusters with double radio relics. The results achieved in this Thesis proved the extraordinary potential of low-frequency polarization observations for the study of large-scale magnetic fields in the outskirts of galaxy clusters and beyond

Discovering the most elusive radio relic in the sky: Diffuse Shock Acceleration caught in the act?

The origin of radio relics is usually explained via diffusive shock acceleration (DSA) or re-acceleration of electrons at/from merger shocks in galaxy clusters. The case of acceleration is challenged by the low predicted efficiency of low-Mach number merger shocks, unable to explain the power observed in most radio relics. In this Letter we present the discovery of a new giant radio relic around the galaxy cluster Abell 2249 ($z=0.0838$) using LOFAR. It is special since it has the lowest surface brightness of all known radio relics. We study its radio and X-ray properties combinig LOFAR data with uGMRT, JVLA and XMM. This object has a total power of $L_{1.4\rm GHz}=4.1\pm 0.8 \times 10^{23}$ W Hz$^{-1}$ and integrated spectral index $\alpha = 1.15\pm 0.23$. We infer for this radio relic a lower bound on the magnetisation of $B\geq 0.4\, \mu$G, a shock Mach number of $\mathcal{M}\approx 3.79$, and a low acceleration efficiency consistent with DSA. This result suggests that a missing population of relics may become visible thanks to the unprecedented sensitivity of the new generation of radio telescopes.Comment: Letter, 5 pages, 4 figures, accepted for publication on MNRAS Letter

Powerful Yet Lonely: Is 3C 297 a High-redshift Fossil Group?

The environment of the high-redshift (z = 1.408), powerful radio-loud galaxy 3C 297 has several distinctive features of a galaxy cluster. Among them, a characteristic halo of hot gas revealed by Chandra X-ray observations. In addition, a radio map obtained with the Very Large Array shows a bright hotspot in the northwestern direction, created by the interaction of the active galactic nucleus (AGN) jet arising from 3C 297 with its environment. In the X-ray images, emission cospatial with the northwestern radio lobe is detected, and peaks at the position of the radio hotspot. The extended, complex X-ray emission observed with our new Chandra data is largely unrelated to its radio structure. Despite having attributes of a galaxy cluster, no companion galaxies have been identified from 39 new spectra of neighboring targets of 3C 297 obtained with the Gemini Multi-Object Spectrograph. None of the 19 galaxies for which a redshift was determined lies at the same distance as 3C 297. The optical spectral analysis of the new Gemini spectrum of 3C 297 reveals an isolated Type II radio-loud AGN. We also detected line broadening in [O ii] λ3728 with a FWHM about 1700 km s−1 and possible line shifts of up to 500–600 km s−1. We postulate that the host galaxy of 3C 297 is a fossil group, in which most of the stellar mass has merged into a single object, leaving behind an X-ray halo

The NuSTAR and Chandra View of CL 0217+70 and Its Tell-tale Radio Halo

Mergers of galaxy clusters are the most energetic events in the universe, driving shock and cold fronts, generating turbulence, and accelerating particles that create radio halos and relics. The galaxy cluster CL 0217+70 is a remarkable late-stage merger, with a double peripheral radio relic and a giant radio halo. Chandra detects surface brightness (SB) edges that correspond to radio features within the halo. In this work, we present a study of this cluster with Nuclear Spectroscopic Telescope Array and Chandra data using spectro-imaging methods. The global temperature is found to be kT = 9.1 keV. We set an upper limit for the inverse Compton (IC) flux of ∼2.7 × 10 ^−12 erg s ^−1 cm ^−2 , and a lower limit to the magnetic field of 0.08 μ G. Our local IC search revealed a possibility that IC emission may have a significant contribution at the outskirts of the radio halo emission and on/near shock regions within ∼0.6 r _500 of clusters. We detected a “hot spot” feature in our temperature map coincident with an SB edge, but our investigation on its origin is inconclusive. If the “hot spot” is the downstream of a shock, we set a lower limit of kT > 21 keV to the plasma that corresponds to ${ \mathcal M }$ ∼2. We found three shock fronts within 0.5 r _500 . Multiple weak shocks within the cluster center hint at an ongoing merger activity and continued feeding of the giant radio halo. CL 0217+70 is the only example hosting these secondary shocks in multiple form

Extended X-ray emission around FR II radio galaxies: hotspots, lobes and galaxy clusters

We present a systematic analysis of the extended X-ray emission discovered around 35 FR II radio galaxies from the revised Third Cambridge catalog (3CR) Chandra Snapshot Survey with redshifts between 0.05 to 0.9. We aimed to (i) test for the presence of extended X-ray emission around FR II radio galaxies, (ii) investigate if the extended emission origin is due to Inverse Compton scattering of seed photons arising from the Cosmic Microwave Background (IC/CMB) or to thermal emission from an intracluster medium (ICM) and (iii) test the impact of this extended emission on hotspot detection. We investigated the nature of the extended X-ray emission by studying its morphology and compared our results with low-frequency radio observations (i.e., ∼150 MHz), in the TGSS and LOFAR archives, as well as with optical images from Pan-STARRS. In addition, we optimized a search for X-ray counterparts of hotspots in 3CR FR II radio galaxies. We found statistically significant extended emission (>3σ confidence level) along the radio axis for ∼90%, and in the perpendicular direction for ∼60% of our sample. We confirmed the detection of 7 hotspots in the 0.5 - 3 keV. In the cases where the emission in the direction perpendicular to the radio axis is comparable to that along the radio axis, we suggest that the underlying radiative process is thermal emission from ICM. Otherwise, the dominant radiative process is likely non-thermal IC/CMB emission from lobes. We found that non-thermal IC/CMB is the dominant process in ∼70% of the sources in our sample, while thermal emission from the ICM dominates in ∼15% of them