On the significance of the observed clustering of ultra-high energy cosmic rays

Three pairs of possibly correlated ultra-high energy cosmic ray events were reported by Hayashida et al (1996). Here we calculate the propagation of the corresponding particles through both the intergalactic and galactic magnetic fields. The large scale disc and halo magnetic components are approximated by the models of Stanev (1997). The intergalactic magnetic field intensity is modulated by the actual density of luminous matter along the corresponding lines of sight, calculated from the CfA redshift catalogue (Huchra et al, 1995). The results indicate that, if the events of each pair had a common source and were simultaneously produced, they either originated inside the galactic halo or otherwise very unlikely events were observed. On the other hand, an estimate of the arrival probability of ultra-high energy cosmic rays, under the assumption that the distribution of luminous matter in the nearby universe traces the distribution of the sources of the particles and intensity of the intergalactic magnetic field, suggests that the pairs are chance clusterings.Comment: Ap. J. Letters Accepted - 13 pages + 4 figure

Star clusters and the structure of the ISM. Tunnels and wakes in giant extragalactic HII regions

Several structures have been discovered embedded in regions of recent or ongoing star formation, which point to the importance of the interaction between fast moving wind-blowing stars and their environment. Using hydrodynamic simulations, we investigate the passage through the interstellar medium of a supersonic stellar wind source, and show how it can naturally lead to the formation of tubes, channels and swamps of globules as interfaces are crossed. The results are in excellent agreement with observation of 30 Doradus.Comment: 12 pages + 5 figures (GIF format) - Accepted for pub. in Astrophys. J. Letter

The energy spectrum observed by the AGASA experiment and the spatial distribution of the sources of ultra-high energy cosmic rays

Seven and a half years of continuous monitoring of giant air showers triggered by ultra high-energy cosmic rays have been recently summarized by the AGASA collaboration. The resulting energy spectrum indicates clearly that the cosmic ray spectrum extends well beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off at $\sim 5 \times 10^{19}$ eV. Furthermore, despite the small number statistics involved, some structure in the spectrum may be emerging. Using numerical simulations, it is demonstrated in the present work that these features are consistent with a spatial distribution of sources that follows the distribution of luminous matter in the local Universe. Therefore, from this point of view, there is no need for a second high-energy component of cosmic rays dominating the spectrum beyond the GZK cut-off.Comment: 14 pages, 4 figures, Astrophys. J. Letters (submitted

The photodisintegration of cosmic ray nuclei by solar photons: the Gerasimova-Zatsepin effect revisted

The interesting possibility of measuring the masses of high energy cosmic ray particles by observing pairs of extensive air showers arriving at the earth nearly simultaneously was proposed some years ago by Gerasimova and Zatsepin (1960). Such showers would be created by the nuclear fragments originating as a result of the photodisintegration of massive nuclei interacting with the solar radiation field. In this paper we re-visit this possibility in the context of existing and proposed detectors of high and ultra-high energy cosmic rays considering a simple, yet realistic, model of the interplanetary magnetic field. The possibilty of observing the mass fragmentation of cosmic rays directly, however, remains challenging.Comment: 14 pages, 7 figures (3 ps + 4 jpg), Astroparticle Physics (in press) - typos corrected and other minor modification