Constraining the population of cosmic ray protons in cooling flow clusters with γ-ray and radio observations: Are radio mini-halos of hadronic origin?

By C. Pfrommer, Torsten A. Ensslin. 2003

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We wish to constrain the cosmic-ray proton (CRp) population in galaxy clusters. By hadronic interactions with the thermal gas of the intra-cluster medium (ICM), the CRp produce γ -rays for which we develop an analytic formalism to deduce their spectral distribution. Assuming the CRp-to-thermal energy density ratio $X_\mathrm{CRp}$ and the CRp spectral index to be spatially constant, we derive an analytic relation between the γ -ray and bolometric X-ray fluxes, $\mathcal{F}_{\gamma}$ and $F_\mathrm{X}$. Based on our relation, we compile a sample of suitable clusters which are promising candidates for future detection of γ -rays resulting from hadronic CRp interactions. Comparing to EGRET upper limits, we constrain the CRp population in the cooling flow clusters Perseus and Virgo to $X_\mathrm{CRp} κ , we find the central CRp injection luminosity of M 87 to be limited to $ 10^{43}\mbox{ erg s}^{-1} \, \kappa/(10^{29} \mbox{ cm}^{2}\mbox{ s}^{-1})$. The synchrotron emission from secondary electrons generated in CRp hadronic interactions allows even tighter limits to be placed on the CRp population using radio observations. We obtain excellent agreement between the observed and theoretical radio brightness profiles for Perseus, but not for Coma without a radially increasing CRp-to-thermal energy density profile. Since the CRp and magnetic energy densities necessary to reproduce the observed radio flux are very plausible, we propose synchrotron emission from secondary electrons as an attractive explanation of the radio mini-halos found in cooling flow clusters. This model can be tested with future sensitive γ -ray observations of the accompanying $\pi^0$-decays. We identify Perseus (A 426), Virgo, Ophiuchus, and Coma (A 1656) as the most promising candidate clusters for such observations.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/ce279454373ff9932be32efea9f07b4059971ddd

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Citations in the corpus

Simulating cosmic rays in clusters of galaxies – II. A unified scheme for radio haloes and relics with predictions of the γ-ray emission (2007, methodology): “The weak energy dependencies of the pion multiplicity and the inelastic cross section can be absorbed in a semianalytical parametrisation of the cross section,σπpp(αp) (for details see Pfrommer & Enßlin 2004a).”
Detecting shock waves in cosmological smoothed particle hydrodynamics simulations (2006, methodology): “For that, we assume a stationary CR electron spectrumwhich balances hadronic injection of secondaries and synchrotron and inverse Compton cooling processes (Brunetti 2002; Pfrommer & Enßlin 2004a,b).”
THE ATACAMA COSMOLOGY TELESCOPE: COSMOLOGY FROM GALAXY CLUSTERS DETECTED VIA THE SUNYAEV–ZEL'DOVICH EFFECT (2010, background): “Simulations and observations suggest contributions to the total pressure from cosmic rays to be about 5%–10% (Jubelgas et al. 2008; Pfrommer & Enßlin 2004) and from turbulent pressure to be between 5% and 20% (Lau et al. 2009; Meneghetti et al. 2010; Burns et al. 2010), with only the latter work…”
SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS (2013, methodology): “Complementary limits on CRs are also derived from radio (synchrotron) observations (Pfrommer & Enßlin 2004a; Brunetti et al. 2007).”