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تسجيل مستخدم جديدSpectro-polarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening
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Observations of radio relics at very high frequency (>10 GHz) can help to understand how particles age and are (re-)accelerated in galaxy cluster outskirts and how magnetic fields are amplified in these environments. In this work, we present new single-dish 18.6 GHz Sardinia Radio Telescope and 14.25 GHz Effelsberg observations of the well known northern radio relic of CIZA J2242.8+5301. We detected the relic which shows a length of $sim$1.8 Mpc and a flux density equal to $rm S_{14.25,GHz}=(9.5pm3.9),mJy$ and $rm S_{18.6,GHz}=(7.67pm0.90),mJy$ at 14.25 GHz and 18.6 GHz respectively. The resulting best-fit model of the relic spectrum from 145 MHz to 18.6 GHz is a power-law spectrum with spectral index $alpha=1.12pm0.03$: no evidence of steepening has been found in the new data presented in this work. For the first time, polarisation properties have been derived at 18.6 GHz, revealing an averaged polarisation fraction of $sim40%$ and a magnetic field aligned with the filaments or sheets of the relic.
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Previous studies have shown that CIZA J2242.8+5301 (the Sausage cluster, $z=0.192$) is a massive merging galaxy cluster that hosts a radio halo and multiple relics. In this paper we present deep, high fidelity, low-frequency images made with the LOw-
Frequency Array (LOFAR) between 115.5 and 179 MHz. These images, with a noise of 140 mJy/beam and a resolution of $theta_{text{beam}}=7.3times5.3$, are an order of magnitude more sensitive and five times higher resolution than previous low-frequency images of this cluster. We combined the LOFAR data with the existing GMRT (153, 323, 608 MHz) and WSRT (1.2, 1.4, 1.7, 2.3 GHz) data to study the spectral properties of the radio emission from the cluster. Assuming diffusive shock acceleration (DSA), we found Mach numbers of $mathcal{M}_{n}=2.7{}_{-0.3}^{+0.6}$ and $mathcal{M}_{s}=1.9_{-0.2}^{+0.3}$ for the northern and southern shocks. The derived Mach number for the northern shock requires an acceleration efficiency of several percent to accelerate electrons from the thermal pool, which is challenging for DSA. Using the radio data, we characterised the eastern relic as a shock wave propagating outwards with a Mach number of $mathcal{M}_{e}=2.4_{-0.3}^{+0.5}$, which is in agreement with $mathcal{M}_{e}^{X}=2.5{}_{-0.2}^{+0.6}$ that we derived from Suzaku data. The eastern shock is likely to be associated with the major cluster merger. The radio halo was measured with a flux of $346pm64,text{mJy}$ at $145,text{MHz}$. Across the halo, we observed a spectral index that remains approximately constant ($alpha^{text{145 MHz-2.3 GHz}}_{text{across (sim)1 Mpc}^2}=-1.01pm0.10$) after the steepening in the post-shock region of the northern relic. This suggests a generation of post-shock turbulence that re-energies aged electrons.
We observed the galaxy cluster CIZA J2242.8+5301 with the Sardinia Radio Telescope to provide new constraints on its spectral properties at high frequency. We conducted observations in three frequency bands centred at 1.4 GHz, 6.6 GHz and 19 GHz, res
ulting in beam resolutions of 14$^{prime}$, 2.9$^{prime}$ and 1$^{prime}$ respectively. These single-dish data were also combined with archival interferometric observations at 1.4 and 1.7 GHz. From the combined images, we measured a flux density of ${rm S_{1.4GHz}=(158.3pm9.6),mJy}$ for the central radio halo and ${rm S_{1.4GHz}=(126pm8),mJy}$ and ${rm S_{1.4GHz}=(11.7pm0.7),mJy}$ for the northern and the southern relic respectively. After the spectral modelling of the discrete sources, we measured at 6.6 GHz ${rm S_{6.6GHz}=(17.1pm1.2),mJy}$ and ${rm S_{6.6GHz}=(0.6pm0.3),mJy}$ for the northern and southern relic respectively. Assuming simple diffusive shock acceleration, we interpret measurements of the northern relic with a continuous injection model represented by a broken power-law. This yields an injection spectral index ${rm alpha_{inj}=0.7pm0.1}$ and a Mach number ${rm M=3.3pm0.9}$, consistent with recent X-ray estimates. Unlike other studies of the same object, no significant steepening of the relic radio emission is seen in data up to 8.35 GHz. By fitting the southern relic spectrum with a simple power-law (${rm S_{ u}propto u^{-alpha}}$) we obtained a spectral index ${rm alphaapprox1.9}$ corresponding to a Mach number (${rm Mapprox1.8}$) in agreement with X-ray estimates. Finally, we evaluated the rotation measure of the northern relic at 6.6 GHz. These results provide new insights on the magnetic structure of the relic, but further observations are needed to clarify the nature of the observed Faraday rotation.
Content: We present the results from $Suzaku$ observations of the merging cluster of galaxies CIZA J2242.8+5301 at $z$=0.192. Aims. To study the physics of gas heating and particle acceleration in cluster mergers, we investigated the X-ray emission f
rom CIZA J2242.8+5301, which hosts two giant radio relics in the northern/southern part of the cluster. Methods. We analyzed data from three-pointed Suzaku observations of CIZA J2242.8+5301 to derive the temperature distribution in four different directions. Results: The Intra-Cluster Medium (ICM) temperature shows a remarkable drop from 8.5$_{-0.6}^{+0.8}$ keV to 2.7$_{-0.4}^{+0.7}$ keV across the northern radio relic. The temperature drop is consistent with a Mach number ${cal M}_n=2.7^{+0.7}_{-0.4}$ and a shock velocity $v_{shock:n}=2300_{-400}^{+700}rm,km,s^{-1}$. We also confirm the temperature drop across the southern radio relic. However, the ICM temperature beyond this relic is much higher than beyond the northern one, which gives a Mach number ${cal M}_s=1.7^{+0.4}_{-0.3}$ and shock velocity $v_{shock:s}=2040_{-410}^{+550}rm ,km,s^{-1}$. These results agree with other systems showing a relationship between the radio relics and shock fronts which are induced by merging activity. We compare the X-ray derived Mach numbers with the radio derived Mach numbers from the radio spectral index under the assumption of diffusive shock acceleration in the linear test particle regime. For the northern radio relic, the Mach numbers derived from X-ray and radio observations agree with each other. Based on the shock velocities, we estimate that CIZA J2242.8+5301 is observed approximately 0.6 Gyr after core passage. The magnetic field pressure at the northern relic is estimated to be 9% of the thermal pressure.
CIZA J2242.8+5301, a merging galaxy cluster at z=0.19, hosts a double-relic system and a faint radio halo. Radio observations at frequencies ranging from a few MHz to several GHz have shown that the radio spectral index at the outer edge of the N rel
ic corresponds to a shock of Mach number 4.6+/-1.1, under the assumptions of diffusive shock acceleration of thermal particles in the test particle regime. Here, we present results from new Chandra observations of the cluster. The Chandra surface brightness profile across the N relic only hints to a surface brightness discontinuity (<2-sigma detection). Nevertheless, our reanalysis of archival Suzaku data indicates a temperature discontinuity across the relic that is consistent with a Mach number of 2.5+/-0.5, in agreement with previously published results. This confirms that the Mach number at the shock traced by the N relic is much weaker than predicted from the radio. Puzzlingly, in the Chandra data we also identify additional inner small density discontinuities both on and off the merger axis. Temperature measurements on both sides of the discontinuities do not allow us to undoubtedly determine their nature, although a shock front interpretation seems more likely. We speculate that if the inner density discontinuities are indeed shock fronts, then they are the consequence of violent relaxation of the dark matter cores of the clusters involved in the merger.
Despite progress in understanding radio relics, there are still open questions regarding the underlying particle acceleration mechanisms. In this paper we present deep 1--4 GHz VLA observations of CIZA,J2242.8+5301 ($z=0.1921$), a double radio relic
cluster characterized by small projection on the plane of the sky. Our VLA observations reveal, for the first time, the complex morphology of the diffuse sources and the filamentary structure of the northern relic. We discover new faint diffuse radio emission extending north of the main northern relic. Our Mach number estimates for the northern and southern relics, based on the radio spectral index map obtained using the VLA observations and existing LOFAR and GMRT data, are consistent with previous radio and X-ray studies ($mathcal{M}_{rm RN}=2.58pm0.17$ and $mathcal{M}_{rm RS}=2.10pm0.08$). However, color-color diagrams and modelings suggest a flatter injection spectral index than the one obtained from the spectral index map, indicating that projection effects might be not entirely negligible. The southern relic consists of five arms. Embedded in it, we find a tailed radio galaxy which seems to be connected to the relic. A spectral index flattening, where the radio tail connects to the relic, is also measured. We propose that the southern relic may trace AGN fossil electrons that are re-accelerated at a shock, with an estimated strength of $mathcal{M}=2.4$. High-resolution mapping of other tailed radio galaxies also supports a scenario where AGN fossil electrons are revived by the merger event and could be related to the formation of some diffuse cluster radio emission.
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