Nonthermal emission from the radio relic of the galaxy cluster A2256 (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 1998
We report the first detection of an inverse Compton X-ray emission, spatially correlated with a very steep spectrum radio source (VSSRS), 0038-096, without any detected optical counterpart, in cluster Abell 85. The ROSAT PSPC data and its multiscale wavelet analysis reveal a large scale (linear diameter of the order of 500 h −1 50 kpc), diffuse X-ray component, in excess to the thermal bremsstrahlung, overlapping an equally large scale VSSRS. The primeval 3 K background photons, scattering off the relativistic electrons can produce the X-rays at the detected level. The inverse Compton flux is estimated to be (6.5 ± 0.5) × 10 −13 erg s −1 cm −2 in the 0.5-2.4 keV X-ray band. A new 327 MHz radio map is presented for the cluster field. The synchrotron emission flux is estimated to be (6.6 ± 0.90) × 10 −14 erg s −1 cm −2 in the 10-100 MHz radio band. The positive detection of both radio and X-ray emission from a common ensemble of relativistic electrons leads to an estimate of (0.95 ± 0.10) × 10 −6 G for the cluster-scale magnetic field strength. The estimated field is free of the 'equipartition' conjecture, the distance, and the emission volume. Further, the radiative fluxes and the estimated magnetic field imply the presence of 'relic' (radiative lifetime 10 9 yr) relativistic electrons with Lorentz factors γ ≈ 700-1700, that would be a significant source of radio emission in the hitherto unexplored frequency range ν ≈ 2-10 MHz.
Gamma-ray emission from galaxy cluster outskirts versus radio relics
Astronomy & Astrophysics, 2016
Context. Galaxy cluster peripheries provide important information on the nature of ICM/IGM linkage. In this paper we consider potential future observations in the gamma-ray domain at cluster edges involving the radio relic phenomenon. Aims. We focus on the spectral signature of gamma radiation that should be evident in the energy range of Fermi-LAT, i.e. 10 −1 GeV and the CTA energy range ∼10 2 GeV. The spectral signature results from a comparable gamma-ray flux due to the IC and π 0 decay on the edge of the cluster, and its spectral position is a function of the magnetic field and relative efficiency of the acceleration of protons and electrons. We aim to draw attention to the dependence of the gamma-ray structure on the magnetic field value. Methods. As an example, we carried out analyses of two types of non-thermal diffuse radio emission: the radio relic of A 2256 and the radio halo of Coma cluster. We suggest that in both cases the expected spatially correlated gamma-ray spectrum should have a characteristic structure that depends on the strength of the local magnetic field. In both of the clusters we calculated the combined flux of gamma radiation from the actual observational values of the used observables. Results. The revealed spectral dependence on the magnetic field would allow us to relate the future spectral observations, in particular the position of the gamma-ray signature, to the value of the magnetic field in the border area between galaxy clusters and their connecting filaments, possibly constraining the estimated relative efficiency of particle acceleration at the edge of the cluster.
Non-thermal processes in the cluster of galaxies Abell 3376
Monthly Notices of the Royal Astronomical Society, 2008
We model the high-energy emission that results from the interaction of relativistic particles with photons and matter in the cluster of galaxies Abell 3376. The presence of relativistic particles is inferred from the recently found radio relics in this cluster, being one of the most prominent examples of double opposite, giant ring-like radio structures. Assuming that diffusive shock acceleration takes place in the cluster regions where radio relics are observed, we calculate the spectral energy distribution resulting from the most relevant non-thermal processes, which are synchrotron radiation, inverse-Compton scattering, relativistic Bremsstrahlung and inelastic proton-proton collisions. In the context of our model, the major radiative component at high energies is inverse-Compton scattering, which could reach luminosities L ∼ 9 × 10 41 erg s −1 in the energy range between ∼1 MeV and 10 TeV. Hadronic interactions would yield a minor contribution to the overall non-thermal emission, but would dominate at ultrahigh energies. The cluster Abell 3376 might be detectable at gamma-rays by HESS, GLAST satellite and future planned Cherenkov arrays.
Radio relic and the diffuse emission trail discovered in low-mass galaxy cluster Abell 1697
Astronomy & Astrophysics
We report the discovery of a putative radio relic, 830 kpc in length, and found towards the outskirts of galaxy cluster Abell 1697 (z = 0.181), using the LOFAR Two Meter Sky Survey at 144 MHz. An X-ray-inferred mass of M500X-ray = 2.9−0.7+0.8 × 1014 M⊙ places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey and at 1.4 GHz in the NRAO VLA Sky Survey, with an average spectral index of α(144, 325, 1400 MHz) = − 0.98 ± 0.01, and magnetic field of Beq ∼ 0.6 μG. This relic, located in the north-east periphery of the cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the width ( α144 MHz1.4 GHz(inj) = −0.70 ± 0.11 to α144 MHz1.4 GHz(edge) = −1.19 ± 0.15 ), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P1.4 GHz = 8.5 ± 1.1 × 1023 W Hz−1, which is found to be in good agreement with the expected empirical correlation between...
Magnetic fields and cosmic rays in clusters of galaxies
Journal of Cosmology and Astroparticle Physics, 2009
We argue that the observed correlation between the radio luminosity and the thermal X-ray luminosity of radio emitting galaxy clusters implies that the radio emission is due to secondary electrons that are produced by p-p interactions and lose their energy by emitting synchrotron radiation in a strong magnetic field, B > (8πaT 4 CMB ) 1/2 ≃ 3 µG. We construct a simple model that naturally explains the correlation, and show that the observations provide stringent constraints on cluster magnetic fields and cosmic rays (CRs): Within the cores of clusters, the ratio β core between the CR energy (per logarithmic particle energy interval) and the thermal energy is β core ∼ 2 · 10 −4 ; The source of these CRs is most likely the cluster accretion shock, which is inferred to deposit in CRs ∼ 0.1 of the thermal energy it generates; The diffusion time of 100 GeV CRs over scales 100 kpc is not short compared to the Hubble time; Cluster magnetic fields are enhanced by mergers to 1% of equipartition, and decay (to < 1 µG) on 1 Gyr time scales. The inferred value of β core implies that high energy gammaray emission from secondaries at cluster cores will be difficult to detect with existing and planned instruments.
The current detectors of gamma-ray emission have too poor resolution to determine whether this emission is produced in the jet or in the core, specially of low luminous, non-blazar AGNs (as radio galaxies). In recent works it has been found that the power released by events of turbulent fast magnetic reconnection in the core region of these sources is more than sufficient to reproduce the observed gamma-ray luminosities. Besides, 3D MHD simulations with test particles have demonstrated that a first-order Fermi process within reconnection sites with embedded turbulence results very efficient particle acceleration rates. Employing this acceleration mechanism and the model above, and considering the relevant leptonic and hadronic loss processes in the core region, we computed the spectral energy distribution (SED) of radio galaxies for which very high energy (VHE) emission has been detected (namely, M87, Cen A, Per A, and IC 310). We found that these match very well specially with the ...
The origin of radio haloes and non-thermal emission in clusters of galaxies
Monthly Notices of the Royal Astronomical Society, 2002
We study the origin of the non-thermal emission from the intracluster medium, including the excess hard X-ray emission and cluster-wide radio haloes, through fitting two representative models to the Coma cluster. If the synchrotron emitting relativistic electrons are accelerated in situ from the vast pool of thermal electrons, then a quasi-stationary solution of the kinetic equation with particle acceleration through turbulence at high energies (> 200 keV) naturally produces a population of supra-thermal electrons responsible for the excess hard X-ray emission through bremsstrahlung. Inverse Compton scattering is negligible at hard X-ray energies in this case. The radio halo flux density constrains the magnetic field strength to a value close to that of equipartition ∼ 1µG. Alternatively, if the relativistic electrons are injected from numerous localised 'external' sources then the hard X-rays are best explained by inverse Compton scattering from GeV electrons, and little of the hard X-radiation has a bremsstrahlung origin. In this case, the magnetic field strength is constrained to ∼ 0.1 − 0.2 µG. Both models assume that the non-thermal emissions are generated by a single electron spectrum, so that only two free parameters, well constrained by the observed hard X-ray and radio halo spectra, are needed in either case. Measurements of the cluster magnetic field will distinguish between the models.
Acceleration of cosmic rays and gamma-ray emission from supernova remnants in the Galaxy
Monthly Notices of the Royal Astronomical Society, 2013
Galactic cosmic rays are believed to be accelerated at supernova remnant shocks. Though very popular and robust, this conjecture still needs a conclusive proof. The strongest support to this idea is probably the fact that supernova remnants are observed in gamma-rays, which are indeed expected as the result of the hadronic interactions between the cosmic rays accelerated at the shock and the ambient gas. However, also leptonic processes can, in most cases, explain the observed gamma-ray emission. This implies that the detections in gamma rays do not necessarily mean that supernova remnants accelerate cosmic ray protons. To overcome this degeneracy, the multi-wavelength emission (from radio to gamma rays) from individual supernova remnants has been studied and in a few cases it has been possible to ascribe the gamma-ray emission to one of the two processes (hadronic or leptonic). Here we adopt a different approach and, instead of a case-by-case study we aim for a population study and we compute the number of supernova remnants which are expected to be seen in TeV gamma rays above a given flux under the assumption that these objects indeed are the sources of cosmic rays. The predictions found here match well with current observational results, thus providing a novel consistency check for the supernova remnant paradigm for the origin of galactic cosmic rays. Moreover, hints are presented for the fact that particle spectra significantly steeper than E −2 are produced at supernova remnants. Finally, we expect that several of the supernova remnants detected by H.E.S.S. in the survey of the galactic plane should exhibit a gamma-ray emission dominated by hadronic processes (i.e. neutral pion decay). The fraction of the detected remnants for which the leptonic emission dominates over the hadronic one depends on the assumed values of the physical parameters (especially the magnetic field strength at the shock) and can be as high as roughly a half.
Probing the origin of giant radio haloes through radio and γ-ray data: the case of the Coma cluster
Monthly Notices of the Royal Astronomical Society, 2012
We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the Fermi-LAT and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons. First we investigate the case of pure secondary models that are merely based on the mechanism of continuous injection of secondary electrons via proton-proton collisions in the intra-cluster medium. We use the observed spatial distribution of the halo's radio brightness to constrain the amount of cosmic ray protons and their spatial distribution in the cluster that are required by the model. Under the canonical assumption that the spectrum of cosmic rays is a power-law in momentum and that the spectrum of secondaries is stationary, we find that the combination of the steep spectrum of cosmic ray protons necessary to explain the spectrum of the halo and the very broad spatial distribution (and large energy density) of cosmic rays result in a γ-ray emission in excess of present limits, unless the cluster magnetic field is relatively large. However this large magnetic field required to not violate present γ-ray limits appears inconsistent with that derived from recent Faraday rotation measures. Second we investigate more complex models in which the cosmic rays confined diffusively in the Coma cluster and their secondary electrons are all reaccelerated by MHD turbulence. We show that under these conditions it is possible to explain the radio spectrum and morphology of the radio halo and to predict γ-ray fluxes in agreement with the Fermi-LAT upper limits without tension with present constraints on the cluster magnetic field. Reacceleration of secondary particles also requires a very broad cosmic ray spatial profile, much flatter than that of the intracluster medium, at least provided that both the turbulent and magnetic field energy densities scale with that of the intracluster medium. However, this requirement can be easily alleviated if we assume that a small amount of (additional) seed primary electrons are reaccelerated in the cluster's external regions, or if we adopt flatter scalings of the turbulent and magnetic field energy densities with distance from the cluster center.