On the escape of cosmic rays from radio galaxy cocoons

Miniati

et al. 2011

A&A

202

294

We investigate the interplay of cosmic ray (CR) propagation and advection in galaxy clusters. Propagation in form of CR diffusion and streaming tends to drive the CR radial profiles towards being flat, with equal CR number density everywhere. Advection of CR by the turbulent gas motions tends to produce centrally enhanced profiles. We assume that the CR streaming velocity is of the order of the sound velocity. This is motivated by plasma physical arguments. The CR streaming is then usually larger than typical advection velocities and becomes comparable or lower than this only for periods with trans-and super-sonic cluster turbulence. As a consequence a bimodality of the CR spatial distribution results. Strongly turbulent, merging clusters should have a more centrally concentrated CR energy density profile with respect to relaxed ones with very subsonic turbulence. This translates into a bimodality of the expected diffuse radio and gamma-ray emission of clusters, since more centrally concentrated CR will find higher target densities for hadronic CR proton interactions, higher plasma wave energy densities for CR electron and proton re-acceleration, and stronger magnetic fields. Thus, the observed bimodality of cluster radio halos appears to be a natural consequence of the interplay of CR transport processes, independent of the model of radio halo formation, be it hadronic interactions of CR protons or re-acceleration of low-energy CR electrons. Energy dependence of the CR propagation should lead to spectral steepening of dying radio halos. Furthermore, we show that the interplay of CR diffusion with advection implies first order CR re-acceleration in the pressure-stratified atmospheres of galaxy clusters. Finally, we argue that CR streaming could be important in turbulent cool cores of galaxy clusters since it heats preferentially the central gas with highest cooling rate.

Section: Cr Transportmentioning

confidence: 99%

Cosmic ray transport in galaxy clusters: implications for radio halos, gamma-ray signatures, and cool core heating

Miniati

et al. 2011

A&A

202

294

“…with typically δ ⊥ ∼ 10 −4 −10 −2 (Giacalone & Jokipii 1999;Enßlin 2003), but see Narayan & Medvedev (2001) for arguments of a larger δ ⊥ ∼ 10 −1 . In order to be flexible about the underlying MHD turbulence which fixes the momentum dependency of κ , we assumē…”

Section: Cr Diffusionmentioning

confidence: 99%

Cosmic ray physics in calculations of cosmological structure formation

Springel

et al. 2007

A&A

Self Cite

134

214

Cosmic rays (CRs) play a decisive role within our own Galaxy. They provide partial pressure support against gravity, they trace past energetic events such as supernovae, and they reveal the underlying structure of the baryonic matter distribution through their interactions. To study the impact of CRs on galaxy and cosmic structure formation and evolution, we develop an approximative framework for treating dynamical and radiative effects of CRs in cosmological simulations. Our guiding principle is to try to find a balance between capturing as many physical properties of CR populations as possible while at the same time requiring as little extra computational resources as possible. We approximate the CR spectrum of each fluid element by a single power-law, with spatially and temporally varying normalisation, low-energy cut-off, and spectral index. Principles of conservation of particle number, energy, and pressure are then used to derive evolution equations for the basic variables describing the CR spectrum, both due to adiabatic and non-adiabatic processes. The processes considered include compression and rarefaction, CR injection via shocks in supernova remnants, injection in structure formation shock waves, in-situ re-acceleration of CRs, CR spatial diffusion, CR energy losses due to Coulomb interactions, ionisation losses, Bremsstrahlung losses, and, finally, hadronic interactions with the background gas, including the associated γ-ray and radio emission due to subsequent pion decay. We show that the formalism reproduces CR energy densities, pressure, and cooling rates with an accuracy of ∼10% in steady state conditions where CR injection balances cooling. It is therefore a promising formulation to allow simulations where CR physics is included. Finally, we briefly discuss how the formalism can be included in Lagrangian simulation methods such as the smoothed particle hydrodynamics technique. Our framework is therefore well suited to be included into numerical simulation schemes of galaxy and structure formation.

“…If the radio plasma releases all its CRp, a moderately flat injection spectrum can be expected (say α inj ≈ 2.5) since radio emission from radio galaxies indicates flat CRe spectra. If, however, only a small fraction of the CRp is able to leave the radio plasma diffusively, an even flatter spectrum (say α inj ≈ 2.2) can be expected due to increasing escape probability with momentum (Enßlin 2003). Supernova Remnants (SNR) are known to be able to produce flat (α inj ≈ 2.4) CR populations and they are believed to be the main CRp source of our galaxy (Schlickeiser 2002, and references therein).…”

Section: The Expected Spectral Index α Pmentioning

confidence: 99%

“…Most of the CRp that have been injected into the cluster center are either diffusively transported into the surrounding ICM (as assumed by Colafrancesco & Blasi 1998;Blasi 1999) or form relativistic bubbles which rise in the gravitational potential of the cluster due to buoyant forces (Churazov et al 2001, and references therein). An argument in favor of a significant central CRp injection into the ICM is the much more efficient escape of CRp from the magnetic confinement of the radio plasma bubble during the very early stages due to the bubbles higher geometrical compactness and and expected stronger turbulence level (Enßlin 2003). In addition to this, any galactic wind from a central galaxy will also inject CRp into the cluster center.…”

Section: Diffusion Of Crp Away From a Central Agnmentioning

confidence: 99%

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

2003

A&A

Self Cite

271

363

Abstract. 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 CRp and the CRp spectral index to be spatially constant, we derive an analytic relation between the γ-ray and bolometric X-ray fluxes, F γ and F 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 CRp < 20%. Assuming a plausible value for the CRp diffusion coefficient κ, we find the central CRp injection luminosity of M 87 to be limited to 10 43 erg s −1 κ/(10 29 cm 2 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 π 0 -decays. We identify Perseus (A 426), Virgo, Ophiuchus, and Coma (A 1656) as the most promising candidate clusters for such observations.