Some galaxy clusters show diffuse radio emission in the form of peripheral relics (so far attributed to primary, shock-(re)accelerated electrons) or central halos. Analysing radio and X-ray data from the literature, we find new connections between halos and relics, such as a universal linear relation between their peak radio brightness and the gas column density. Our results indicate that halos, relics, and halo-relic bridges in a cluster, all arise from the same, homogeneous cosmic ray (CR) ion (CRI) distribution. We analytically derive the signature of synchrotron emission from secondary electrons and positrons (CREs) produced in hadronic CRI collisions, for an arbitrary magnetic field evolution. In our model, flat spectrum halos (both giant and minihalos) arise from steady-state magnetic fields, whereas relics and steep halos reflect recent or irregular magnetic growth. This naturally explains the properties of halos, relics, and the connections between them, without invoking particle (re)acceleration in weak shocks or turbulence. We find CRI energy densities in the range u_p~10^-[12.4,13.3]erg cm^-3, with a spectral index s_p=-2.20+-0.05, and identify a ~0.1 magnetic energy fraction in some halos and behind relics, as far as 2Mpc from the cluster's centre. The CRI homogeneity suggests strong CR diffusion, D(100GeV)>~10^32cm^2s^-1. The strong magnetisation imposes strict upper limits on >10GeV CRE (re)acceleration in weak shocks (efficiency <10^-4) and turbulence; indeed, each weak shock slightly lowers the energy fraction of flat CRs.
As the largest gravitationally bound structures in the Universe, galaxy clusters are the focus of intense cosmological and astrophysical research. Nonthermal radiation from clusters was observed in the radio band (for review, see Feretti 2005) and in hard X-rays (for review, see Rephaeli et al. 2008), and is expected to be observed in γ-rays in the near future (by the 5-year Fermi mission; see Keshet et al. 2003).
Such nonthermal signals trace the cosmic rays (CRs) and magnetic fields permeating the intracluster medium (ICM). These nonthermal components play an important role in the evolution of clusters on multiple scales, affecting their dynamical and thermal structure, for example by modifying the transport and dissipation processes. The distributions of CRs and magnetic fields in the ICM hold a unique record of past dynamical processes, such as the history of merger-induced shocks and turbulence in the cluster. Modeling these component also constrains the poorly understood processes of particle acceleration and plasma magnetisation.
A fair fraction of the hot galaxy clusters (∼ 35% of the clusters with X-ray luminosty LX > 10 45 erg s -1 ; Giovannini et al. 2002) show extended, nonthermal radio emission with low surface brightness, which is not associated with any particular member galaxy. This is believed to be synchrotron radiation emitted by CR electrons or positrons (CREs), injected locally into the ICM and gyrating in its pervasive magnetic fields. Arguably, such radio observations hold more information regarding the nonthermal components of the ICM than presently available in any other band.
ICM radio sources are broadly classified, according to their location, morphology, and polarisation, as giant halos (GHs; also known as a cluster-wide halos), minihalos (MHs; or core halos), or relics (Feretti & Giovannini 1996). In general, halos (both GHs and MHs) are regular, unpolarised emission around the cluster’s centre, whereas relics are peripheral, polarised, typically elongated, and thought to be associated with shocks. For a recent review, see Ferrari et al. (2008). (We use the conventional term “relic”, although the recently suggested terms “flotsam” or “gischt” may be more appropriate; see Kempner et al. 2004).
GHs are found in the centres of merger, non-cool core clusters. They are typically unpolarised, and show a regular morphology which follows the thermal plasma. Their spectral indices lie in the range αν ≡ d log(Pν)/d log ν = -[1.0, 1.5] (flat halos) or αν = -[1.5, 2.0] (steep halos), where Pν is the specific radio power and ν is the frequency. GHs extend over large, ∼ Mpc scales, farther than the distance a CRE can cross before cooling. Therefore, CREs must be injected locally and continuously into the ICM. Two types of models have been proposed for CRE injection in GHs: (i) secondary production by hadronic collisions between CR ions (CRI) and the ambient plasma (Dennison 1980;Blasi & Colafrancesco 1999); and (ii) in-situ turbulent acceleration or reacceleration of primary CREs (Enßlin et al. 1999;Brunetti et al. 2001;Petrosian 2001). It was recently shown (Kushnir et al. 2009;Keshet & Loeb 2010) that the radio-X-ray correlations in GH luminosity (Brunetti et al. 2007) and in surface brightness (Govoni et al. 2001;Keshet & Loeb 2010) strongly support the first, secondary CRE model, and imply that the defining property of GHs is a strongly magnetised, B 3 µG ICM. (For a different view, see Brunetti et al. (2009).) This model reproduces the spectral, morphological and energetic properties of flat GHs (Keshet & Loeb 2010, henceforth KL10). Independent measurements of B within halos are presently not sufficiently precise to test this connection; low-significance evidence for higher magnetisation in halo clusters was reviewed in KL10.
MHs are found in the centres of more relaxed, cool-core clusters (CCs). They extend roughly over the cooling region (Gitti et al. 2002), encompassing up to a few percent of the typical GH volume, and often overlap the radio emission from an active galactic nucleus (AGN). They resemble miniature versions of flat GHs, typically being unpolarised, regular, and spectrally flat with α = -[1.0, 1.5]. They show radio-X-ray correlations consistent with those of GHs, and a similar ratio η ≡ νIν/FX between the radio and X-ray surface brightness (KL10). This indicates that they arise from the same mechanism as GHs: secondary CREs losing most of their energy to synchrotron radiation in highly magnetised cores (KL10). This conclusion is supported by the morphological association between MH edges and cold fronts (CFs), reported by Mazzotta & Giacintucci (2008). Such CFs, present in most CCs (Markevitch & Vikhlinin 2007), were identified as tangential discontinuities lying above (i.e. at larger distances r from the cluster’s centre) regions magnetised by bulk shear flow (Keshet et al. 2010). We do not focus on MHs here; for a discussion of their properties, see KL10.
As op
This content is AI-processed based on open access ArXiv data.
