arXiv subjects

P. Blasi

Publications and source records attributed to P. Blasi.

82 records · Page 5Linked to original sources

arXiv1999

Galactic Ultra-High-Energy Cosmic Rays

The absence of the expected GZK cutoff strongly challenges the notion that the highest-energy cosmic rays are of distant extragalactic origin. We discuss the possibility that these ultra-high-energy events originate in our Galaxy and propose that they may be due to iron nuclei accelerated from young, strongly magnetic neutron stars. Newly formed pulsars accelerate ions from their surface through relativistic MHD winds. We find that pulsars whose initial spin periods are shorter than $\sim 4 (B_S/10^{13}{\rm G})$ ms, where $B_S$ is the surface magnetic field, can accelerate iron ions to greater than $10^{20} eV$. These ions can pass through the remnant of the supernova explosion that produced the pulsar without suffering significant spallation reactions. Depending on the structure of the galactic magnetic field, the trajectories of the iron ions from galactic sources can be consistent with the observed arrival directions of the highest energy events.

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arXiv1999

Cosmic Rays in Clusters of Galaxies and Radio Halos

We calculate the fluxes of radio, hard X-rays and gamma-ray emission from clusters of galaxies, in the context of a secondary electron model (SEM). In the SEM the radiating electrons are produced by the decay of charged pions in cosmic ray (CR) interactions with the intracluster medium, while gamma ray emission is mainly contributed by the decay of neutral pions. We specifically applied our calculations to the case of the Coma cluster, and found that the combined radio and hard X-ray fluxes can be explained in the SEM only if very small values of the intracluster magnetic field ($B\sim 0.1μG$) are assumed, which in turn imply a large energy density of the parent CRs. The consequent gamma-ray fluxes easily exceed the EGRET limit at 100 MeV. This conclusion can be avoided only if most of the hard X-ray emission from Coma is not produced by Inverse Compton Scattering (ICS).

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arXiv1999

Cosmic Rays, Radio Halos and Non-Thermal 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 $B \sim 0.1μG$ 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.

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arXiv1998

Signatures of Topological Defects

We argue that due to various restrictions cosmic strings and monopole-string networks are not likely to produce the observed flux of ultra-high energy cosmic rays (UHECR). Among the topological defects studied so far, the most promising UHECR sources are necklaces and monopolonia. Other viable sources which are similar to topological defects are relic superheavy particles. All these sources have an excess of pions (and thus photons) over nucleons at production. We demonstrate that in the case of necklaces the diffuse proton flux can be larger than photon flux, due to absorption of the latter on radiobackground, while monopolonia and relic particles are concentrated in the Galactic halo, and the photon flux dominates. Another signature of the latter sources is anisotropy imposed by asymmetric position of the sun in the Galactic halo. In all cases considered so far, including necklaces, photons must be present in ultra-high energy radiation observed from topological defects, and experimental discrimination between photon-induced and proton-induced extensive air showers can give a clue to the origin of ultra-high energy cosmic rays.

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arXiv1998

High Energy Phenomena in Clusters of Galaxies

Several phenomena in high energy astrophysics have been recently related to clusters of galaxies and to cosmic ray interactions occurring inside these structures. In many of these phenomena the observable effects depend on the energy density of cosmic rays confined in the Intra Cluster (IC) medium, which is a poorly known quantity. We propose here that useful indications about this quantity can be obtained from present and future observations of galaxy clusters in the radio and hard X-ray frequency ranges.

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arXiv1998

Clusters of Galaxies and the Diffuse Gamma Ray Background

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 $\sim 0.5 ÷2 %$ 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 inter galactic medium (IGM) in distant clusters, because the bulk of the sources contributing to the DGRB is located at redshifts $z \simlt 0.2$. 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.

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arXiv1997

Clusters of Galaxies as a 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, $Ω_b$, 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 AGN 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 which can be expected from AGN 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 $E\sim 10^6~GeV$ 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 radiations. We found that the observation of a signal from the bright phase is better for an individual cluster.

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arXiv1996

High energy gamma rays from old accreting neutron stars

We consider a magnetized neutron star with accretion from a companion star or a gas cloud around it, as a possible source of gamma rays with energy between $100$ $MeV$ and $10^{14}-10^{16}~eV$. The flow of the accreting plasma is terminated by a shock at the Alfvén surface. Such a shock is the site for the acceleration of particles up to energies of $\sim 10^{15}-10^{17}~eV$; gamma photons are produced in the inelastic $pp$ collisions between shock-accelerated particles and accreting matter. The model is applied to old neutron stars both isolated or in binary systems. The gamma ray flux above $100~MeV$ is not easily detectable, but we propose that gamma rays with very high energy could be used by Cherenkov experiments as a possible signature of isolated old neutron stars in dense clouds in our galaxy.

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arXiv1995

SNI: a new mechanism for gamma ray burst. I.Weak magnetic field

We propose a new mechanism for high energy gamma ray burst in Supernova type I (SNI) explosion. Presupernova is assumed to be a binary system comprised of a red giant and a white dwarf with a wind accretion. The accretion flow terminates by the accretion shock in the vicinity of the white dwarf at the distance of the order of the accretion radius. The gas inside the accretion radius constitutes the main body of a target for gamma ray production by $pp$ collisions.\par The supernova explosion and the shock propagation in the white dwarf result in the hydrodynamical acceleration of the outer layers of the star. The spectrum of accelerated particles is steep and the maximum energy does not exceed $1000$ $GeV$.\par The gamma ray burst is produced due to interaction of the accelerated particles with the gas in the binary system. Most of the photons have energies about $100$ $MeV$, the total number of the emitted photons is between $10^{46}$ and $10^{47}$. The gamma ray burst might have one or two precursors. The first one is produced during the shock breakout when the shock approaches the star surface and crosses it.\par The calculations are valid in the case of a weak magnetic field. The case of strong magnetic field will be considered in part II (in preparation).

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arXiv1993

Phenomenology of $B_s$ Decays

Using the QCD sum rules technique we study several aspects of the phenomenology of the b-flavoured strange meson $B_s^0$. In particular, we evaluate the mass of the particle, the leptonic constant and the form factors of the decays $\overline {B_s^0} \to D^{+}_s \ell^- \barν$, $\overline {B_s^0} \to D^{*+}_s \ell^- \barν$, $\overline {B_s^0} \to K^{*+} \ell^- \barν$. We also calculate, in the factorization approximation, a number of two-body non leptonic $\overline {B_s^0}$ decays.

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