Pasquale Blasi

Clusters of Galaxies as Storage Room for Cosmic Rays

It is demonstrated that clusters of galaxies are able to keep cosmic rays for a time exceeding the age of the universe. This phenomenon reveals itself by the production of the diffuse flux of high-energy gamma and neutrino radiation due to the interaction of the cosmic rays with the intracluster gas. The produced flux is determined by the cosmological density of baryons, Ωb, if a large part of this density is provided by the intracluster gas. The signal from relic cosmic rays has to be compared with the flux produced by the late sources, which can be considered as a background in the search for cosmic-ray production in the past. We calculate this flux considering the normal galaxies and active galactic nuclei (AGNs) in the clusters as the sources of cosmic rays. Another potential cosmic-ray source is the shock in the gas accreting to a cluster. We found that this background is relatively high: the diffuse fluxes produced by relic cosmic rays are of the same order of magnitude that can be expected from AGNs in the clusters. In all cases the predicted diffuse gamma-ray flux is smaller than the observed one, and the diffuse neutrino flux can be seen as the small bump at E ~ 106 GeV over the atmospheric neutrino flux. A bright phase in the galaxy evolution can be a source of the relic cosmic rays in clusters, revealing itself by diffuse gamma and neutrino radiation. We found that the observation of a signal from the bright phase is better for an individual cluster.

Cosmic rays, radio halos and nonthermal X-ray emission in clusters of galaxies

We calculate the flux of radio, hard X-ray and UV radiation from clusters of galaxies as produced by synchrotron emission and Inverse Compton Scattering of electrons generated as secondaries in cosmic ray interactions in the intracluster medium. Both the spatial distribution of cosmic rays due to their diffusion and the spatial distribution of the intracluster gas are taken into account. Our calculations are specifically applied to the case of the Coma cluster. The fluxes and spectra of the radio halo emission and of the hard X-ray excess from Coma can be explained in this model if an average magnetic field B ∼ 0.1µG is assumed. However, such a low value for the intracluster magnetic field implies a large cosmic ray energy density which in turn is responsible, through neutral pion decay, for a gamma ray flux above 100 MeV which exceeds the EGRET upper limit. This gamma ray bound can be relaxed if the hard X-ray excess and the radio halo emission from Coma are not due to the same population of electrons. We finally stress the unique role that the new generation gamma ray satellites will play to discriminate among different models for the non thermal emission in clusters of galaxies.

Cosmological Magnetic Field Limits in an Inhomogeneous Universe

We study the effect of inhomogeneities in the matter distribution of the universe on the Faraday rotation of light from distant QSOs and derive new limits on the cosmological magnetic field. The matter distribution in the universe is far from homogeneous, and for the redshifts of interest in relation to rotation measures (RMs), it is well described by the observed Lyα forest. We use a lognormal distribution to model the Lyα forest, and we assume that a cosmological magnetic field is frozen into the plasma and therefore is a function of the density inhomogeneities. The Lyα forest results are much less sensitive to the cosmological magnetic field coherence length than the results for a homogeneous universe, and they show an increase in the magnitude of the expected RM for a given field by over an order of magnitude. The forest also introduces a large scatter in RMs for different lines of sight, with a highly non-Gaussian tail that renders the variance and the mean RM impractical for setting limits. The median |RM| is a better statistical indicator that we use to derive the following limits using the observed RMs for QSOs between z=0 and z=2.5. We set Ωbh2=0.02 and get, for cosmological fields that are coherent across the present horizon, BH−10 10−9 G in the case of a Lyα forest that is stronger than the limit for a homogeneous universe, BhH−10 2×10−8 G; for a 50 Mpc coherence length, the inhomogeneous case gives B50 Mpc 6×10−9 G, while the homogeneous limit is Bh50 Mpc 10−7 G; and for a coherence length equal to the Jeans length, BλJ 10−8 G for the Lyα case, while BhλJ 10−6 G.

Clusters of galaxies and the diffuse gamma-ray background

Abstract We discuss the diffuse emission of gamma rays and neutrinos from galaxy clusters in the viable models for structure formation in the universe. We use a self-consistent picture for cluster formation and evolution starting from a primordial density perturbation spectrum, and a realistic modelling for the distribution of the intergalactic medium which is abundantly present within galaxy clusters. We find that an evolving population of clusters can produce a fraction ∼ 0.5 ÷ 2% of the diffuse gamma-ray background (DGRB) observed by EGRET. This result is robust and is weakly dependent on the cosmological scenario and on the degree of evolution of the intergalactic medium (IGM) in distant clusters, because the bulk of the sources contributing to the DGRB is located at redshifts z ≲ 0.2. We also found a correlation between the non-thermal, gamma-ray and the thermal X-ray emissions from these structures. Using this result, we derived a list of gamma-ray clusters observable with the next generation γ-ray detectors. Finally, we briefly discuss the possible relevance of galaxy clusters for neutrino astronomy and for very high energy particle astronomy.

On the Equipartition of Thermal and Nonthermal Energy in Clusters of Galaxies

Clusters of galaxies are revealing themselves as powerful sources of nonthermal radiation in a wide range of wavelengths. In order to account for these multifrequency observations, equipartition of cosmic rays (CRs) with the thermal gas in clusters of galaxies is often invoked. This condition might suggest a dynamical role played by cosmic rays in the virialization of these large-scale structures and is now testable through gamma-ray observations. We show here, in the specific case of the Coma and Virgo clusters, for which upper limits on the gamma-ray emission exist, that equipartition implies gamma-ray fluxes that are close to or even in excess of the EGRET limit, depending on the adopted model of CR injection. We use this limit to constrain the validity of the equipartition condition. We also show that, contrary to what was claimed in previous calculations, the equipartition assumption implies gamma-ray fluxes in the TeV range that can be detected even by currently operating gamma-ray observatories if the injection cosmic-ray spectrum is flatter than E-2.4.

Gamma-Ray Constraints on Neutralino Dark Matter Clumps in the Galactic Halo

According to high-resolution cold dark matter (CDM) simulations, large virialized halos are formed through the constant merging of smaller halos formed at earlier times. In particular, the halo of our Galaxy may have hundreds of dark matter clumps. The annihilation of dark matter particles such as the neutralino in these clumps generates gamma-ray fluxes that can potentially be detected by future experiments such as GLAST. We find that, depending on the parameters of the clump density profile and on the distribution of clumps in the Galactic halo, the contribution to the diffuse gamma-ray background from clumps can constrain the properties of neutralinos such as the mass and annihilation cross section. We model the density profile of clumps by three representative dark matter profiles, singular isothermal spheres (SISs), Moore profiles, and Navarro, Frenk, and White (NFW) density profiles, and calculate the spectrum and angular distribution in the sky of the gamma-ray flux due to neutralino annihilation in the clumpy halo of the Galaxy. The calculations are carried out in the context of two different scenarios for the distribution of clumps in the Galaxy and their concentrations, which result in very different conclusions.

Gamma rays from superheavy relic particles in the halo

Superheavy (SH) quasistable particles generated in the Early Universe could be responsable for Ultra High Energy Cosmic Rays (UHECR) and be a component of Cold Dark Matter (CDM) in the universe. These particles are likely to cluster in the galactic halo, so that the main part of UHECR are gamma rays produced in the decay of neutral pions. Charged pions are also produced in the same process and result in high energy electrons. We consider here the production of gamma rays by synchrotron emission of these electrons in the galactic magnetic field. The gamma ray fluxes are in the region of interest for some current and proposed experiments (e.g. EGRET, GLAST, MILAGRO) in the energy range

Signatures of topological defects

0.1-10^4

Galactic magnetic field structure and ultrahigh-energy cosmic ray propagation

GeV. A comparison with the existing upper limits at

Cosmic Rays in Clusters of Galaxies and Radio Halos

10^5-10^8