Cosmic‐Ray Protons Accelerated at Cosmological Shocks and Their Impact on Groups and Clusters of Galaxies

Abstract

We investigate the production of cosmic ray (CR) protons at cosmological shocks by performing, for the first time, numerical simulations of large scale structure formation that include directly the acceleration, transport and energy losses of the high energy particles. CRs are injected at shocks according to the thermal leakage model and, thereafter, accelerated to a power-law distribution as indicated by the test particle limit of the diffusive shock acceleration theory. The evolution of the CR protons accounts for losses due to adiabatic expansion/compression, Coulomb collisions and inelastic p-p scattering. Our results suggest that CR protons produced at shocks formed in association with the process of large scale structure formation could amount to a substantial fraction of the total pressure in the intra-cluster medium. Their presence should be easily revealed by GLAST through detection of γ-ray flux from the decay of π 0 produced in inelastic p-p collisions of such CR protons with nuclei of the intra-cluster gas. This measurement will allow a direct determination of the CR pressure contribution in the intra-cluster medium. We also find that the spatial distribution of CR is typically more irregular than that of the thermal gas because it is more influenced by the underlying distribution of shocks. This feature is reflected in the appearance of our γ-ray synthetic images. Finally, the average CR pressure distribution appears statistically slightly more extended than the thermal pressure.

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

“…in the Bullet cluster, Liang et al 2000) emerge naturally, since they reflect the power-law spectra of CRp acceleration. -The observed smooth, regular morphology of halos is also expected in hadronic models, since the long lived CRp had enough time to become distributed within the cluster volume (Miniati et al 2001b;Miniati 2003;Pfrommer et al 2008). -The observed correlation of radio halo luminosity with X-ray luminosity (L ν − L X relation, see Fig.…”

“…Here α(r, t) ≈ α ≈ 2.1−2.5 is the dominating CRp spectral index around 100 GeV expected in the ICM from structure formation shock waves (Miniati et al 2001b;Pinzke & Pfrommer 2010) and C(r, t) the spectral normalisation constant, which will capture the spatial variations and temporal evolution. The full CR transport equation reads…”

Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwCl 2341.1+0000

“…At the same time, although particle acceleration at cosmic shocks is expected on theoretical grounds (Bell 1978, Blandford & Ostriker 1978, theoreticians have been "anxiously" waiting for direct or indirect evidence that it really occurs and at what level it could be linked to the origin of radio relics (e.g., Miniati et al (2001b), Enßlin et al (1998)). On the other hand, if diffusive shock acceleration takes place with some efficiency during the non-linear stage of large-scale structure formation, cosmic-ray ions accumulating in the forming structure could become dynamically important with interesting cosmological consequences (Miniati et al 2001a). …”

Evidence for a pressure discontinuity at the position of the Coma relic from Planck Sunyaev–Zel'dovich effect data

“…The shock waves can amplify particle energies through the Fermi-I process as well as amplify the magnetic fields, resulting in the strong synchrotron emissions seen as radio relics (e.g. Enßlin et al 1998;Miniati et al 2001;Hong et al 2014; see also Guo, Sironi & Narayan 2014). However, there are several shortcomings to this accepted picture and the relevant details are not well understood.…”