Relativistic protons in the Coma galaxy cluster: first gamma-ray constraints ever on turbulent reacceleration

Abstract

The Fermi-LAT collaboration recently published deep upper limits to the gamma-ray emission of the Coma cluster, a cluster that exhibits non-thermal activity and that hosts the prototype of giant radio halos. In this paper we extend previous studies and use a general formalism that combines particle reacceleration by turbulence and the generation of secondary particles in the intracluster medium to constrain relativistic protons and their role for the origin of the radio halo. Our findings significantly strengthen the conclusions of previous studies and clearly disfavour pure secondary models for the halo. Indeed the new limits allow us to conclude that a pure hadronic origin of the radio halo would require magnetic fields that are too strong. For instance B 0 > 21µG is found in the cluster center assuming that the magnetic energy density scales with thermal density, to be compared with B 0 ∼ 4 − 5µG as inferred from Faraday Rotation Measures (RM) under the same assumption. Secondary particles can still generate the observed emission if they are reaccelerated. For the first time the deep gamma-ray limits allow us to derive meaningful constraints if the halo is generated during phases of reacceleration of relativistic protons and their secondaries by cluster-scale turbulence. In this paper we explore a limited, but relevant, range of parameter-space of reacceleration models. Within this parameter space a fraction of model configurations is already ruled out by current gamma-ray limits, including the cases that assume weak magnetic fields in the cluster core, B ≤ 2 − 3µG. Interestingly, we also find that the flux predicted by a large fraction of model configurations that assume a magnetic field consistent with Faraday RM is not far from the limits. This suggests that a detection of cosmic-ray-induced gamma rays from the cluster might be possible in the near future, provided that the electrons generating the radio halo are secondaries reaccelerated and the magnetic field in the cluster is consistent with that inferred from Faraday RM.

Gamma-Ray and Neutrino Emissions due to Cosmic-Ray Protons Accelerated at Intracluster Shocks in Galaxy Clusters

“…We here employ an illustrative model for the radial profile of the CRp density that scales with the shell-averaged number density of gas particles as n CR (r, p) ∝n gas (r) δ . We take δ = 0.5 − 1, which covers most of the range suggested in the previous simulation studies cited above and observations (e.g., Brunetti et al 2017). Considering that the ICM is roughly isothermal, δ < 1 results in the radial profile of P CR flatter than that of P gas .…”

Rotation measure synthesis applied to synthetic SKA images of galaxy clusters

“…In this latter scenario the energy density of the relativistic particles constitutes the 0.3% of the thermal energy density. This factor has been chosen in order to obtain radio power at 1.4 GHz similar to that of radio haloes in equipartition condition, and it is consistent with the upper limit set from γ-ray observations (ratio between the relativistic and thermal particle energy density less than 10%, see Brunetti, Zimmer & Zandanel 2017). To complete these radio simulations and make them suitable in the case of a pair of galaxy clusters, it is necessary to include a cluster population of radio sources.…”

Revived fossil plasma sources in galaxy clusters

“…In addition, continuous injection of secondary particles through proton-proton collisions (Dennison 1980, Enßlin et al 2011 can also be re-accelerated together with the primary electrons (Brunetti & Lazarian 2011, Pinzke et al 2017) in these regions and may play a role in the generation of radio halos. However, lack of gamma ray detection puts constraints on such a mechanism (e.g., Brunetti et al 2017 and references therein). Apart from radio halos, diffuse radio sources in clusters can broadly be divided into radio relics (e.g., Kempner et al 2004;van Weeren et al 2019) and radio phoenices (revived fossil plasma sources).…”