Jonathan Biteau (IPNO): Anisotropies at ultra-high energies: an indication and a discovery
The Earth's atmosphere is constantly hit by charged particles from
outer-space that carry individual energies of more than a Joule
(~6x10^18 eV or 6 EeV): ultra-high energy cosmic rays. The constancy of
their flux, their scarcity - a few per kilometer square every year -,
and the uniformity - or isotropy - of their arrival directions over the
sky could have made the quest for their origin illusory...
Because of their tremendous energies, ultra-high energy cosmic rays were
often believed to be accelerated in extragalactic astrophysical sources.
We report in this talk on the first significant observational evidence
confirming this hypothesis: the discovery by the Pierre Auger
Collaboration of a few-percent-amplitude deviation from cosmic-ray
isotropy on large angular scales.
At higher energies, presumably higher rigidities (energy/charge), the
deflections of cosmic rays within Galactic and extragalactic magnetic
fields are expected to be reduced, possibly leaving an imprint of the
sources at intermediate angular scales. We report on an indication of
cosmic-ray anisotropy above ~40 EeV obtained through a comparison with
the flux pattern of extragalactic gamma-ray sources, in particular that
of starburst galaxies.
After a discussion of these exciting results, and necessary caveats in
their interpretation, we depict some of the new opportunities
observations bring to the revived quest for the origin of the most
extreme particles known in the Universe.