Chandra observations of AGN-candidates correlated with Auger UHECRs
  The Auger observatory has observed a possible correlation between Ultrahigh
Energy Cosmic Rays (UHECRs) above 57 EeV and nearby candidate Active Galactic
Nuclei (AGN) from the Veron-Cetty Veron catalog (VCV). In this paper we report
on Chandra X-ray observations of 10 unconfirmed VCV AGN-candidates and luminous
IR galaxies correlating with the first set of Auger UHECRs, to determine
whether or not they have active nuclei. The X-ray data, when combined with
optical luminosities, show that in fact none of the 10 galaxies have a
significant AGN component; if there is any nuclear activity at all, it is weak
rather than obscured. This reduces the number of UHECRs in the original Auger
dataset possibly correlating with AGNs from 20 of 27 down to 14 of 27. We also
used Chandra to measure the X-ray luminosity of ESO 139-G12, an AGN which
correlates with 2 of the Auger UHECRs, to obtain the first estimate of its
bolometric luminosity; this completes the determination of the bolometric
luminosities of all correlating AGNs. Taking our results into account, only one
of the 27 UHECRs in the original Auger data-release is correlated on a
few-degree angular scale with an identified AGN that is powerful enough in its
steady-state to accelerate protons to the observed energies, according to
conventional acceleration mechanisms. Intriguingly, approximately 60% of the
UHECRs with $|b|>10^\circ$ do correlate with genuine AGNs, but these are too
weak to meet the acceleration criterion for protons; this may be an indication
that AGNs experience transient high-luminosity states which can accelerate
UHECRs. To determine the source(s) and composition of UHECRs through
statistical correlation studies requires reliable, complete and uniform
catalogs of identified AGNs; our a posteriori inspection of ambiguous source
candidates underscores the inadequacies of the VCV catalog in this respect.
