Discovering the most elusive radio relic in the sky: diffuse shock acceleration caught in the act?

Locatelli, Nicola; Rajpurohit, K.; Vazza, Franco; Gastaldello, F.; Dallacasa, D.; Bonafede, A.; Rossetti, M.; Stuardi, C.; Bonassieux, Etienne; Brunetti, G.; Brüggen, M.; Shimwell, T. W.

doi:10.1093/mnrasl/slaa074

Cited by 40 publications

(35 citation statements)

References 46 publications

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“…Observations of some cluster relics suggest that they can be explained by diffusive shock acceleration of electrons from the thermal pool (standard DSA), as in e.g. Abell 2249 (Locatelli et al 2020), El Gordo and Abell 521 (Giacintucci et al 2008;Botteon et al 2016Botteon et al , 2020. However, in general this is not the case.…”

Section: Introductionmentioning

confidence: 99%

Radio relics in PSZ2 G096.88+24.18: a connection with pre-existing plasma

Jones

Gasperin

Cuciti

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Giant radio relics are arc-like structures of diffuse, non-thermal synchrotron radiation that trace shock waves induced by galaxy cluster mergers. The particle (re-)acceleration mechanism producing such radio relics is unclear. One major open question is whether relics can be formed directly from a population of thermal seed electrons, or if pre-existing relativistic seed electrons are required. In some cases AGN can provide such a population of sub-GeV electrons. However, it is unclear how common this connection is. In this paper we present LOFAR 140 MHz and VLA L-band radio observations, as well as Chandra data of PSZ2 G096.88+24.18, a merging galaxy cluster system hosting a pair of radio relics. A large patch of diffuse emission connects a bright radio galaxy with one of the relics, likely affecting the properties of the relic. We find that the most plausible explanation for the connection is that the merger shock wave has passed over an AGN lobe. The shock passing over this seed population of electrons has led to an increased brightness in the relic only in the region filled with seed electrons.

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Section: Introductionmentioning

confidence: 99%

Radio relics in PSZ2 G096.88+24.18: a connection with pre-existing plasma

Jones

Gasperin

Cuciti

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…For such weak shocks, the DSA mechanism is not efficient enough to accelerate particles up to GeV energies from the thermal pool (see also Botteon et al 2020). Only recently, a new class of radio relics with low surface brightness, and emissivity compatible with the standard DSA scenario might have been discovered at low frequency using the Low Frequency Array (LOFAR, see Locatelli et al 2020). In some cases, the Mach number inferred from X-ray observations and the one obtained from radio spectra are not in agreement (see e.g.…”

Section: Introductionmentioning

confidence: 99%

Spectropolarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening

Loi

Murgia

Vacca

et al. 2020

Monthly Notices of the Royal Astronomical Society

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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 ∼1.8 Mpc and a flux density equal to $\rm S_{14.25\, GHz}=(9.5\pm 3.9)\, mJy$ and $\rm S_{18.6\, GHz}=(7.67\pm 0.90)\, mJy$ 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 α = 1.12 ± 0.03: 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 $\sim 40\%$ and a magnetic field aligned with the ’filaments’ or ’sheets’ of the relic.

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“…Up until now, only a few radio relics can readily be explained by the DSA model (e.g. Locatelli et al 2020).…”

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confidence: 99%

Morphology of radio relics – I. What causes the substructure of synchrotron emission?

Domínguez-Fernández

Brüggen

Vazza

et al. 2020

Monthly Notices of the Royal Astronomical Society

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High-resolution radio observations of cluster radio relics often show complex spatial and spectral features. However, it is not clear what these features reveal about the underlying magnetic field properties. We performed three-dimensional magneto-hydrodynamical simulations of merger shock waves propagating through a magnetised, turbulent intracluster medium. Our model includes the diffusive shock acceleration of cosmic-ray electrons, their spatial advection and energy losses at run-time. With this set-up we can investigate the relation between radio substructure and pre-shock plasma conditions in the host cluster. We find that upstream turbulence plays a major role in shaping the properties of radio relics produced downstream. Within the assumption of diffusive shock acceleration, we can reproduce the observed discrepancy between the X-ray derived Mach number of shocks, and the Mach number inferred from radio spectra. Our simulated spectral index maps and profiles across the radio relic also suggest that the standard deviation of the upstream magnetic field must be relatively small (σB ≤ 1 μG) in order to reproduce observations and therefore, radio relics can potentially constrain the distribution of magnetic fields in galaxy clusters outskirts.

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Discovering the most elusive radio relic in the sky: diffuse shock acceleration caught in the act?

Cited by 40 publications

References 46 publications

Radio relics in PSZ2 G096.88+24.18: a connection with pre-existing plasma

Radio relics in PSZ2 G096.88+24.18: a connection with pre-existing plasma

Spectropolarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening

Morphology of radio relics – I. What causes the substructure of synchrotron emission?

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