Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies

The inverse Compton (IC) interpretation of the excess EUV emission, that was recently reported from several clusters of galaxies, suggests that the amount of relativistic electrons in the intracluster medium is highly significant, W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal radiation, we discuss implications of the inverse Compton origin of the excess EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as required for the IC interpretation of the observed excess hard X-ray flux, and in the case of high fields of order 1 muG as suggested by Faraday rotation measurements. Although for such high intracluster fields the excess hard X-rays will require an explanation other than by the IC effect, we show that the excess EUV flux can be explained by the IC emission of a `relic' population of electrons driven into the incipient intracluster medium at the epoch of starburst activity by galactic winds, and later on reenergized by adiabatic compression and/or large-scale shocks transmitted through the cluster as the consequence of more recent merger events. For high magnetic fields B > 1 muG the interpretation of the radio fluxes of Coma requires a second population of electrons injected recently. They can be explained as secondaries produced by a population of relativistic protons. We calculate the fluxes of gamma-rays to be expected in both the low and high magnetic field scenarios, and discuss possibilities to distinguish between these two principal options by future gamma-ray observations.Comment: LaTeX, 6 figures; accepted for publication in Ap

Oat growth under different nitrogen doses in an eucalyptus alley cropping system in subtropical Brazil.

Foi realizada uma análise de crescimento para verificar como a aveia (Avena sativa L. cv. IPR 126) cultivada para grãos responde a um sistema agroflorestal (SAF) com eucaliptos (Eucalyptus dunnii Maiden) no subtrópico brasileiro. A hipótese deste trabalho é que a resposta de crescimento da aveia não é modificada pelo nitrogênio em distâncias relativas a faixas de eucaliptos. O objetivo deste estudo foi determinar como o crescimento da aveia é influenciado por níveis de nitrogênio (12 e 80 kg ha-1 de N) em cinco posições equidistantes entre faixas de linhas duplas de eucaliptos [20 m (4 m x 3 m)] em SAF e em agricultura tradicional de plantio direto. O experimento foi em faixas no delineamento de blocos ao acaso com quatro repetições. Foram avaliadas as taxas de crescimento relativo e de assimilação líquida, fração de massa foliar e taxa de enchimento relativo da panícula. O nitrogênio alterou a resposta do crescimento diferentemente em posições relativas às faixas de árvores, portanto diferentes doses de nitrogênio devem ser utilizadas nestas posições para aumentar o crescimento da aveia

Hard X-ray emission from the galaxy cluster A3667

We report the results of a long BeppoSAX observation of Abell 3667, one of the most spectacular galaxy cluster in the southern sky. A clear detection of hard X-ray radiation up to ~ 35 keV is reported, while a hard excess above the thermal gas emission is present at a marginal level that should be considered as an upper limit to the presence of nonthermal radiation. The strong hard excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection of nonthermal emission in A3667 can be explained in the framework of the inverse Compton model. We argue that the nonthermal X-ray detections in the PDS energy range are related to the radio index structure of halos and relics present in the observed clusters of galaxie.Comment: 15 pages, 1 figure, ApJL in pres

Environmental drivers of soil phosphorus composition in natural ecosystems

Soil organic and inorganic phosphorus (P) compounds can be influenced by distinctive environmental properties. This study aims to analyze soil P composition in natural ecosystems, relating organic (inositol hexakisphosphate, DNA and phosphonates) and inorganic (orthophosphate, polyphosphate and pyrophosphate) compounds with major temporal (weathering), edaphic and climatic characteristics. A dataset including 88 sites was assembled from published papers that determined soil P composition using one-dimensional liquid state 31P nuclear magnetic resonance spectroscopy of NaOH-EDTA extracts of soils. Bivariate and multivariate regression models were used to better understand the environmental properties influencing soil P. In bivariate relationships, trends for soil P compounds were similar for mineral and organic layers but with different slopes. Independent and combined effects of weathering, edaphic and climatic properties of ecosystems explained up to 78% (inositol hexakisphosphates) and 89% (orthophosphate) of variations in organic and inorganic P compounds across the ecosystems, likely deriving from parent material differences. Soil properties, particularly pH, total carbon, and carbon-to-phosphorus ratios, over climate and weathering mainly explained the P variation. We conclude that edaphic and climatic drivers regulate key ecological processes that determine the soil P composition in natural ecosystems. These processes are related to the source of P inputs, primarily determined by the parent material and soil forming factors, plant and microbe P cycling, the bio-physico-chemical properties governing soil phosphatase activity, soil solid surface specific reactivity, and P losses through leaching, and finally the P persistence induced by the increasing complexity of organic and inorganic P compounds as the pedogenesis evolves. Soil organic and inorganic P compounds respond differently to combinations of environmental drivers, which likely indicates that each P compound has specific factors governing its presence in natural ecosystems. © Author(s) 2018

Neutrinos and Gamma Rays from Galaxy Clusters

The next generation of neutrino and gamma-ray detectors should provide new insights into the creation and propagation of high-energy protons within galaxy clusters, probing both the particle physics of cosmic rays interacting with the background medium and the mechanisms for high-energy particle production within the cluster. In this paper we examine the possible detection of gamma-rays (via the GLAST satellite) and neutrinos (via the ICECUBE and Auger experiments) from the Coma cluster of galaxies, as well as for the gamma-ray bright clusters Abell 85, 1758, and 1914. These three were selected from their possible association with unidentified EGRET sources, so it is not yet entirely certain that their gamma-rays are indeed produced diffusively within the intracluster medium, as opposed to AGNs. It is not obvious why these inconspicuous Abell-clusters should be the first to be seen in gamma-rays, but a possible reason is that all of them show direct evidence of recent or ongoing mergers. Their identification with the EGRET gamma-ray sources is also supported by the close correlation between their radio and (purported) gamma-ray fluxes. Under favorable conditions (including a proton spectral index of 2.5 in the case of Abell 85, and sim 2.3 for Coma, and Abell 1758 and 1914), we expect ICECUBE to make as many as 0.3 neutrino detections per year from the Coma cluster of galaxies, and as many as a few per year from the Abell clusters 85, 1758, and 1914. Also, Auger may detect as many as 2 events per decade at ~ EeV energies from these gamma-ray bright clusters.Comment: Accepted for publication in Ap

Diagnostic Signatures of Radio and HXR Emission on Particle Acceleration Processes in the Coma Cluster

We investigate theoretical models for the radio halo and hard X-ray (HXR) excess in the Coma galaxy cluster. Time-independent and time-dependent re-acceleration models for relativistic electrons have been carried out to study the formation of the radio halo and HXR excess. In these models, the relativistic electrons are injected by merger shocks and re-accelerated by ensuing violent turbulence. The effects of different Mach numbers of the merger shocks on the radio and HXR excess emission are also investigated. We adopt 6 $\mu$G as the central magnetic field and reproduce the observed radio spectra via the synchrotron emission. We also obtain a central "plateau" in the radio spectral-index distribution, which have been observed in radio emission distribution. Our models can also produce the observed HXR excess emission via the inverse Compton scattering of the cosmic microwave background photons. We find that only the merger shocks with the Mach numbers around 1.6--2 can produce results in agreement with both the radio and HXR emission in the Coma cluster.Comment: 29 pages, 10 figures, ApJ in pres

Cosmic Ray Electrons in Groups and Clusters of Galaxies: Primary and Secondary Populations from a Numerical Cosmological Simulation

We study the generation and distribution of high energy electrons in cosmic environment and their observational consequences by carrying out the first cosmological simulation that includes directly cosmic ray (CR) particles. Starting from cosmological initial conditions we follow the evolution of primary and secondary electrons (CRE), CR ions (CRI) and a passive magnetic field. CRIs and primary CREs are injected and accelerated at large scale structure shocks. Secondary CREs are continuously generated through inelastic p-p collisions. We include spatial transport, adiabatic expansion/compression, Coulomb collisions, bremsstrahlung, synchrotron (SE)and inverse Compton (IC) emission. We find that, from the perspective of cosmic shock energy and acceleration efficiency, the few detections of hard X-ray radiation excess could be explained in the framework of IC emission of primary CREs in clusters undergoing high accretion/merger phase. Instead, IC emission from both primary and secondary CREs accounts at most for a small fraction of the radiation excesses detected in the extreme-UV (except for the Coma cluster as reported by Bowyer et al.1999). Next, we calculate the SE after normalizing the magnetic field so that for a Coma-like cluster ^1/2~3 \muG. Our results indicate that the SE from secondary CREs reproduces several general properties of radio halos, including the recently found P_1.4GHz vs T relation, the morphology and polarization of the emitting region and, to some extent, the spectral index. Moreover, SE from primary CREs turns out sufficient to power extended regions resembling radio relics observed at the outskirts of clusters. Again we find striking resemblance between morphology, polarization and spectral index of our synthetic maps and those reported in the literature.Comment: emulateapj, 27 pages, 10 figures, 5 tables; ApJ in pres

Simulating cosmic rays in clusters of galaxies - II. A unified scheme for radio halos and relics with predictions of the gamma-ray emission

The thermal plasma of galaxy clusters lost most of its information on how structure formation proceeded as a result of dissipative processes. In contrast, non-equilibrium distributions of cosmic rays (CR) preserve the information about their injection and transport processes and provide thus a unique window of current and past structure formation processes. This information can be unveiled by observations of non-thermal radiative processes, including radio synchrotron, hard X-ray, and gamma-ray emission. To explore this, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of about two orders of magnitudes, and follow self-consistent CR physics on top of the radiative hydrodynamics. We model CR electrons that are accelerated at cosmological structure formation shocks and those that are produced in hadronic interactions of CRs with ambient gas protons. We find that CR protons trace the time integrated non-equilibrium activities of clusters while shock-accelerated CR electrons probe current accretion and merging shock waves. The resulting inhomogeneous synchrotron emission matches the properties of observed radio relics. We propose a unified model for the generation of radio halos. Giant radio halos are dominated in the centre by secondary synchrotron emission with a transition to the synchrotron radiation emitted from shock-accelerated electrons in the cluster periphery. This model is able to explain the observed correlation of mergers with radio halos, the larger peripheral variation of the spectral index, and the large scatter in the scaling relation between cluster mass and synchrotron emission. Future low-frequency radio telescopes (LOFAR, GMRT, MWA, LWA) are expected to probe the accretion shocks of clusters. [abridged]Comment: 32 pages, 19 figures, small changes to match the version to be published by MNRAS, full resolution version available at http://www.cita.utoronto.ca/~pfrommer/Publications/CRs_non-thermal.pd

Neutralinos and the Origin of Radio Halos in Clusters of Galaxies

We assume that the supersymmetric lightest neutralino is a good candidate for the CDM and explore the possibility to produce diffuse radio emission from high-energy electrons arising from the neutralino annihilation in galaxy clusters whose intracluster medium is filled with a large-scale magnetic field. We show that these electrons fit the population of seed relativistic electrons postulated in many models for the origin of cluster radio halos. For magnetic fields with central values $3 \div 30$ $\mu$G (depending on the DM profile), the population of seed relativistic electrons from neutralino annihilation can fit the radio halo spectra of Coma and 1E0657-56. The shape and the frequency extension of the radio halo spectra are connected with the mass and physical composition of the neutralino. A pure-gaugino neutralino with mass $M_{\chi} \geq 80$ GeV can reasonably fit the spectra of both Coma and 1E0657-56. This model provides a number of extra predictions that make it definitely testable. On the one hand, it agrees with the observations that {\it (i)} the radio halo is centered on the cluster dynamical center, usually coincident with the X-ray center, {\it (ii)} the radio halo surface brightness is similar to the X-ray one, and {\it (iii)} the monochromatic radio luminosity at 1.4 GHz correlates strongly with the IC gas temperature. On the other hand, the model predicts that radio halos should be present in every cluster, which is not actually observed, although the predicted radio halo luminosities can change by a large amount ($\sim 10^2 \div 10^6$), depending on the amplitude and the structure of the IC magnetic field. Also, neutral pions arising from neutralino annihilation should give rise to substantial gamma-ray emission that could be tested by the next generation gamma-ray experiments.Comment: 49 pages, 11 Figures, Latex (using epsfig), submitted to The Astrophysical Journal. submitted to The Astrophysical Journa