A thorough search of the sky exposed at the Pierre Auger Cosmic Ray Observatory reveals no statistically significant excess of events in any small solid angle that would be indicative of a flux of ...neutral particles from a discrete source. The search covers from -90degrees to +15degrees in declination using four different energy ranges above 1 EeV (10 super(18) eV). The method used in this search is more sensitive to neutrons than to photons. The upper limit on a neutron flux is derived for a dense grid of directions for each of the four energy ranges. These results constrain scenarios for the production of ultrahigh energy cosmic rays in the Galaxy.
A 45 deg{sup 2} radio continuum imaging campaign of the nearest radio galaxy, Centaurus A, is reported. Using the Australia Telescope Compact Array and the Parkes 64 m radio telescope at 1.4 GHz, the ...spatial resolution of the resultant image is {approx}600 pc ({approx}50''), resolving the {approx}>500 kpc giant radio lobes with approximately five times better physical resolution compared to any previous image, and making this the most detailed radio continuum image of any radio galaxy to date. In this paper, we present these new data and discuss briefly some of the most interesting morphological features that we have discovered in the images. The two giant outer lobes are highly structured and considerably distinct. The southern part of the giant northern lobe naturally extends out from the northern middle lobe with uniformly north-streaming emission. The well known northern loop is resolved into a series of semi-regular shells with a spacing of approximately 25 kpc. The northern part of the giant northern lobe also contains identifiable filaments and partial ring structures. As seen in previous single-dish images at lower angular resolution, the giant southern lobe is not physically connected to the core at radio wavelengths. Almost the entirety of the giant southern lobe is resolved into a largely chaotic and mottled structure which appears considerably different (morphologically) to the diffuse regularity of the northern lobe. We report the discovery of a vertex and a vortex near the western boundary of the southern lobe, two striking, high surface brightness features that are named based on their morphology and not their dynamics (which are presently unknown). The vortex and vertex are modeled as reaccelerated lobe emission due to shocks from the active galactic nucleus itself or from the passage of a dwarf elliptical galaxy through the lobe. Preliminary polarimetric and spectral index studies support a plasma reacceleration model and could explain the origin of the Faraday rotation structure detected in the southern lobe. In addition, there are a series of low surface brightness wisps detected around the edges of both the giant lobes.
Atmospheric parameters, such as pressure (
P), temperature (
T) and density
(
ρ
∝
P
/
T
)
, affect the development of extensive air showers initiated by energetic cosmic rays. We have studied the ...impact of atmospheric variations on extensive air showers by means of the surface detector of the Pierre Auger Observatory. The rate of events shows a
∼
10
%
seasonal modulation and
∼
2
%
diurnal one. We find that the observed behaviour is explained by a model including the effects associated with the variations of
P and
ρ
. The former affects the longitudinal development of air showers while the latter influences the Molière radius and hence the lateral distribution of the shower particles. The model is validated with full simulations of extensive air showers using atmospheric profiles measured at the site of the Pierre Auger Observatory.
We describe a new method of identifying night-time clouds over the Pierre Auger Observatory using infrared data from the Imager instruments on the GOES-12 and GOES-13 satellites. We compare cloud ...identifications resulting from our method to those obtained by the Central Laser Facility of the Auger Observatory. Using our new method we can now develop cloud probability maps for the 3000km2 of the Pierre Auger Observatory twice per hour with a spatial resolution of ∼2.4km by ∼5.5km. Our method could also be applied to monitor cloud cover for other ground-based observatories and for space-based observatories.
The energy spectrum of ultra-high energy cosmic rays above 10
18
eV is measured using the hybrid events collected by the Pierre Auger Observatory between November 2005 and September 2010. The large ...exposure of the Observatory allows the measurement of the main features of the energy spectrum with high statistics. Full Monte Carlo simulations of the extensive air showers (based on the CORSIKA code) and of the hybrid detector response are adopted here as an independent cross check of the standard analysis (Phys. Lett. B
685
, 239 (2010)). The dependence on mass composition and other systematic uncertainties are discussed in detail and, in the full Monte Carlo approach, a region of confidence for flux measurements is defined when all the uncertainties are taken into account. An update is also reported of the energy spectrum obtained by combining the hybrid spectrum and that measured using the surface detector array.
ABSTRACT
A thorough search for large-scale anisotropies in the distribution of arrival
directions of cosmic rays detected above 10
18
eV at the Pierre Auger
Observatory is presented. This search is ...performed as a function of both
declination and right ascension in several energy ranges above 10
18
eV, and reported in terms of dipolar and quadrupolar coefficients. Within the
systematic uncertainties, no significant deviation from isotropy is revealed.
Assuming that any cosmic-ray anisotropy is dominated by dipole and quadrupole
moments in this energy range, upper limits on their amplitudes are derived.
These upper limits allow us to test the origin of cosmic rays above
10
18
eV from stationary Galactic sources densely distributed in
the Galactic disk and predominantly emitting light particles in all
directions.
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