Very high energy gamma-rays and the Hubble parameter

A new method, based on the absorption of very high-energy gamma-rays by the cosmic infrared background, is proposed to constrain the value of the Hubble constant. As this value is both fundamental for cosmology and still not very well measured, it is worth developing such alternative methods. Our lower limit at the 68% confidence level is H0 > 74 km/s/Mpc, leading, when combined with the HST results, to H0 ~ 76 km/s/Mpc. Interestingly, this value, which is significantly higher than the usually considered one, is in exact agreement with other independent approaches based on baryonic acoustic oscillations and X-ray measurements. Forthcoming data from the experiments HESS-2 and CTA should help improving those results. Finally, we briefly mention a plausible correlation between absorption by the extragalactic background light and the absence of observation of gamma-ray bursts (GRBs) at very high energies.Comment: Proc. of the 12th Marcel Grossmann meeting on general relativity. 3 pages, 1 figur

Proton Modulation in the Heliosphere for Different Solar Conditions and Prediction for AMS-02

Spectra of Galactic Cosmic Rays (GCRs) measured at the Earth are the combination of several processes: sources production and acceleration, propagation in the interstellar medium and propagation in the heliosphere. Inside the solar cavity the flux of GCRs is reduced due to the solar modulation, the interaction which they have with the interplanetary medium. We realized a 2D stochastic simulation of solar modulation to reproduce CR spectra at the Earth, and evaluated the importance in our results of the Local Interstellar Spectrum (LIS) model and its agreement with data at high energy. We show a good agreement between our model and the data taken by AMS-01 and BESS experiments during periods with different solar activity conditions. Furthermore we made a prediction for the flux which will be measured by AMS-02 experiment.Comment: Accepted for publication in the Proceedings of the ICATPP Conference on Cosmic Rays for Particle and Astroparticle Physics, Villa Olmo (Como, Italy), 7-8 October, 2010, to be published by World Scientific (Singapore

Dark matter halos around isolated ellipticals

We investigate the distribution of the luminous and the dark matter components in the isolated ellipticals NGC 7052 and NGC 7785, embedded in an emitting hot gas halo, by means of relevant X-ray and photometric data. In order to calculate the dark matter distribution in these rare objects, we performed an improved X-ray analysis of the XMM-Newton data of NGC 7785, and we used former results based on Chandra data of NGC 7052. For each object we also derived the stellar spheroid length scale from the surface photometry and the spheroid stellar mass from an analysis of the galaxy spectral energy distribution. We find that a dark matter component is present in these objects. It is subdominant and mixed with the luminous matter inside the optical region half-light radius wide, while it dominates the gravitational potential at outer radii. On the whole, the dark halo structure is very similar to that found around spirals of comparable luminosity and it is well reproduced by a Burkert halo, while a Sersic spheroid accounts well for the baryonic component.Comment: 9 pages, 5 figures, matching the version published by Astronomy & Astrophysic

The Gamma-Ray Sky Under a "new" Light

During the last year the Gamma-Ray sky has glowed of new light: The PAMELA experiment, the Pierre Auger Observatory, the AGILE satellite, and the Fermi Gamma-ray Space Telescope are contributing in an unprecedented way to the unveiling of the cosmic distribution of gamma-ray sources and their viable relation with cosmic rays. The Alpha Magnetic Spectrometer, to be launched next July 2010, will add, for the first time, a precious energetic window, up to a few TeV, to this exciting investigation. We analyze the perspectives of AMS results and the possibilities for the detection of Galactic and Extragalactic gammaray astrophysical sources like Pulsars, Microquasars, Gamma-Ray Bursts, and Active Galactic Nuclei. © 2010 by World Scientific Publishing Co. Pte. Ltd

Collimating, relativistic, magnetic jets from rotating disks

The magnetic flux distribution is determined by the solution of the Grad-Shafranov equation. With differential rotation, i.e. the variation of the iso-rotation parameter, the shape of the light surface must be calculated in an iterative way. For the first time, we have calculated the force-free magnetic structure of truly two-dimensional, relativistic jets, anchored in a differentially rotating disk. Such an approach allows for a direct connection between parameters of the central source (mass, rotation) and the extension of the radio jet. We present an analytical estimate for the jet opening angle along the asymptotic branches of the light surface. In general, differential rotation of the iso-rotation parameter leads to an increase of the jet opening angle. Comparison to the M87 jet shows agreement in the collimation distance. We derive a light cylinder radius of the M87 jet of 50 Schwarzschild radii.Comment: 11 pags, 10 figs, Latex, accepted for Astron.Astroph., [email protected], [email protected]

AMS-02 photon data reduction approach

The Alpha Magnetic Spectrometer (AMS-02) is going to be launched next February 2011 from the Kennedy Space Center. It will be located by the Space Shuttle on the International Space Station and will have its same lifetime: 10 years or even more. The experiment will observe high energy gamma-ray photons from several Astrophysical sources. We consider here the AMS-02 single-photon-mode, i.e. gamma ray photons revealed by the ECAL through electromagnetic shower production. By means of Monte Carlo simulations and Test Beam data, we present a preliminary approach to the gamma-ray photon selection procedure and discuss the potentiality of the detector

Energy loss for electrons in the Heliosphere and local interstellar spectrum for solar modulation

Galactic Cosmic Rays (GCR) entering the Heliosphere are affected by the solar modulation, which is a combination of diffusion, convection, magnetic drift, and adiabatic energy losses usually seen as a decrease of the flux at low energies (less than 10 GeV). We improved a quasi time-dependent 2D Stochastic Simulation code describing such effects. We focused our attention on the electron modulation, adding energy losses mechanisms in the Heliosphere that can be neglected for protons and ions: inverse Compton, ionization, synchrotron, and bremsstrahlung. These effects have been evaluated in the region affected by the solar magnetic field, up to 100 AU, where the environment conditions are not constant, especially the magnetic field intensity, and the photon density. In our calculation the inverse compton energy losses are dominant, but they contribute only a few percent in comparison with the adiabatic losses. We also compared the Local Interstellar Spectrum (LIS) of primary electrons with experimental data collected in the past years at energies 20 GeV. We found that, inside one standard deviation, LIS fits the data and can be used in a Monte carlo code reproducing CR propagation in the Heliosphere

Hot gas flows on global and nuclear galactic scales

Since its discovery as an X-ray source with the Einstein Observatory, the hot X-ray emitting interstellar medium of early-type galaxies has been studied intensively, with observations of improving quality, and with extensive modeling by means of numerical simulations. The main features of the hot gas evolution are outlined here, focussing on the mass and energy input rates, the relationship between the hot gas flow and the main properties characterizing its host galaxy, the flow behavior on the nuclear and global galactic scales, and the sensitivity of the flow to the shape of the stellar mass distribution and the mean rotation velocity of the stars.Comment: 22 pages. Abbreviated version of chapter 2 of the book "Hot Interstellar Matter in Elliptical Galaxies", Springer 201