The Pierre Auger Observatory: status, results and perspective

While the completion of the Pierre Auger Observatory (or simply ``Auger'') is still underway, the 5165 km^2.sr.yr integrated acceptance accumulated since the January 1st, 2004 is now significantly larger than what was gathered by the previous experiments dedicated to the detection of ultra-high-energy cosmic rays (UHECRs). We report on the development status of Auger and present some results related to the cosmic-ray energy spectrum, composition and anisotropies, and the photon fraction at ultra-high energy. We briefly discuss the importance of the ankle region to understand the overall phenomenology of cosmic-rays, and mention future enhancements of Auger focusing on this energy range.Comment: 10 pages, 10 figures, Invited talk at the International Symposium on Astronomy and Astrophysics of the Extreme Universe, March 22-23, 2007, RIKEN, Tokyo (Japan

Light Element Abundance Patterns in the Orion Association: I) HST Observations of Boron in G-dwarfs

The boron abundances for two young solar-type members of the Orion association, BD -6 1250 and HD 294297, are derived from HST STIS spectra of the B I transition at 2496.771 A. The best-fit boron abundances for the target stars are 0.13 and 0.44 dex lower than the solar meteoritic value of log e(B)=2.78. An anticorrelation of boron and oxygen is found for Orion when these results are added to previous abundances obtained for 4 B-type stars and the G-type star BD -5 1317. An analysis of the uncertainties in the abundance calculations indicates that the observed anticorrelation is probably real. The B versus O relation observed in the Orion association does not follow the positive correlation of boron versus oxygen which is observed for the field stars with roughly solar metallicity. The observed anticorrelation can be accounted for by a simple model in which two poorly mixed components of gas (supernova ejecta and boron-enriched ambient medium) contribute to the new stars that form within the lifetime of the association. This model predicts an anticorrelation for Be as well, at least as strong as for boron.Comment: 16 pages + 1 table + 7 figures, accepted for publication in Ap

GRBs and the 511 keV emission of the Galactic bulge

We consider the phenomenology of the 511 keV emission in the Galactic bulge, as recently observed by INTEGRAL, and propose a model is which the positrons are produced by gamma-ray bursts (GRB) associated with mini starbursts in the central molecular zone (CMZ). We show that the positrons can easily diffuse across the bulge on timescales of about 10^7 yr, and that their injection rate by GRBs is compatible with the observed fluxes if the mean time between two GRBs in the bulge is about 8 10^4 yr x E_GRB_51. We also explain the low disk-to-bulge emission ratio by noting that positrons from GRBs in the Galactic disk should annihilate on timescales of < 10^4 yr in the dense shell of the underlying supernova remnant, after the radiative transition, while the remnants of GRBs occurring in the hot, low-density medium produced by recurrent starbursts in the CMZ become subsonic before they can form a radiative shell, allowing the positrons to escape and fill the whole Galactic bulge. If the mean time between GRBs is smaller than 10^4 E_51 yr, INTEGRAL should be able to detect the (localized) 511 keV emission associated with one or a few GRB explosions in the disk.Comment: 6 pages, accepted for publication in A&

On the viability of holistic cosmic-ray source models

We consider the energy spectrum of cosmic-rays (CRs) from a purely phenomenological point of view and investigate the possibility that they all be produced by the same type of sources with a single power-law spectrum, in E^{-x}, from thermal to ultra-high energies. We show that the relative fluxes of the Galactic (GCR) and extra-galactic (EGCR) components are compatible with such a holistic model, provided that the index of the source spectrum be x \simeq 2.23\pm 0.07. This is compatible with the best-fit indices for both GCRs and EGCRs, assuming that their source composition is the same, which is indeed the case in a holistic model. It is also compatible with theoretical expectations for particle acceleration at relativistic shocks.Comment: 5 pages, 1 figure, Accepted for publication in Astronomy and Astrophysic

The Compton trail of gamma-ray bursts: A long-after glow

AUGERAs they travel through the gas of the host galaxy, some of the gamma-rays emitted in a Gamma-Ray Burst (GRB) may experience Compton scattering and reach an observer even if he is not located in the direction of the primary photon beam. Such a process will last until the GRB photons have left their host galaxy, and the ambient electron density becomes negligible. We investigate the observability of this indirect GRB light, which would be seen as a faint trail along the path of the GRB photons, long after the initial event. We find that the so-called Compton trail of a 1051 erg GRB can easily be observed from Earth, wherever the explosion occurred in our Galaxy in the past few thousand years. Gamma-ray surveys of the Galaxy can therefore provide constraints on the true GRB rate (or number of GRBs per supernova), independently of the GRB beaming angle. We also calculate the expected light curve and shape of the emitting region as a function of time