Issue |
A&A
Volume 488, Number 3, September IV 2008
|
|
---|---|---|
Page(s) | 879 - 885 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:200810108 | |
Published online | 24 June 2008 |
Ultra-high energy cosmic rays detected by the Pierre Auger observatory
First direct evidence, and its implications, that a subset originate in nearby radiogalaxies
Astronomy Department, Universidad de Concepción, Concepción, Chile e-mail: nagar@astro-udec.cl;jmatulich@udec.cl
Received:
1
May
2008
Accepted:
16
June
2008
Context. The Pierre Auger Collaboration has reported 27 Ultra-High
Energy Cosmic Ray Events (UHECRs) with energies above
eV (56 EeV) and
well determined arrival directions as of 2007 August 31.
They find that the arrival directions of these UHECRs are not
isotropic, but instead appear correlated with the positions of
nearby Active Galactic Nuclei (AGNs)
from the catalog of Véron-Cetty & Véron.
Aims. Our aim was to determine the sources of these UHECRs by comparing their arrival directions with more complete and/or comprehensive astronomical source catalogs.
Methods. We have cross correlated the arrival directions of the UHECRs with the positions of supernovae, radio supernovae, galaxies, active galaxies, radiogalaxies, and clusters of galaxies, all at distances within ~200 Mpc.
Results. Four (eight) of the 27 UHECRs with energy greater than 56 EeV
detected by the Pierre Auger Observatory have arrival directions
within 1.5° (3.5°) of the extended (≥180 kpc)
radio structures of nearby radiogalaxies or the single nearby BL Lac
with extended radio structure.
Conversely the radio structures of three (six) of all ten nearest extended
radiogalaxies are
within 1.5° (3.5°) of a UHECR; three of the remaining four radiogalaxies
are in directions with lower exposure times. This correlation between
nearby extended radiogalaxies and a subset of UHECRs is significant at the
99.9% level.
For the remaining ~20 UHECRs, an isotropic distribution cannot be
ruled out at high significance.
The correlation found by the Auger Collaboration
between the 27 UHECRs and AGNs in the
Véron-Cetty & Véron catalog at Mpc
has a much lower significance when one considers only the ~20 UHECRs
not “matched” to nearby extended radiogalaxies.
No clear correlation is seen between UHECRs and
supernovae, supernova remnants, nearby galaxies,
or nearby groups and clusters of galaxies.
Conclusions. Nearby extended radiogalaxies are the most likely source of at least some UHECRs detected by the Pierre Auger Observatory. The remaining UHECRs are not inconsistent with an isotropic distribution; their correlation to nearby AGNs is much less significant than earlier estimated. This is the first direct observational proof that radio galaxies are a significant source of UHECRs. The primary difference between the UHECR detections at the Pierre Auger Observatory and previous experiments, e.g. AGASA, may thus primarily be that the Southern Hemisphere is more privileged with respect to nearby radiogalaxies with highly extended radio jets and lobes.
Key words: ISM: cosmic rays / galaxies: active / galaxies: jets
© ESO, 2008
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