3,724 research outputs found
Atmospheric Calorimetry above 10 eV: Shooting Lasers at the Pierre Auger Cosmic-Ray Observatory
The Pierre Auger Cosmic-Ray Observatory uses the earth's atmosphere as a
calorimeter to measure extensive air-showers created by particles of
astrophysical origin. Some of these particles carry joules of energy. At these
extreme energies, test beams are not available in the conventional sense. Yet
understanding the energy response of the observatory is important. For example,
the propagation distance of the highest energy cosmic-rays through the cosmic
microwave background radiation (CMBR) is predicted to be strong function of
energy. This paper will discuss recently reported results from the observatory
and the use of calibrated pulsed UV laser "test-beams" that simulate the
optical signatures of ultra-high energy cosmic rays. The status of the much
larger 200,000 km companion detector planned for the northern hemisphere
will also be outlined.Comment: 6 pages, 11 figures XIII International Conference on Calorimetry in
High Energy Physic
Light Nuclei solving Auger puzzles. The Cen-A imprint
Ultra High Energy Cosmic Rays (UHECR) map at 60 EeV have been found recently
by AUGER group spreading anisotropy signatures in the sky. The result have been
interpreted as a manifestation of AGN sources ejecting protons at GZK edges
mostly from Super-galactic Plane. The result is surprising due to the absence
of much nearer Virgo cluster. Moreover, early GZK cut off in the spectra may be
better reconcile with light nuclei (than with protons). In addition a large
group (nearly a dozen) of events cluster suspiciously along Cen-A. Finally,
proton UHECR composition nature is in sharp disagreement with earlier AUGER
claim of a heavy nuclei dominance at 40 EeV. Therefore we interpret here the
signals as mostly UHECR light nuclei (He, Be, B, C, O), very possibly mostly
the lightest (He,Be) ones, ejected from nearest AGN Cen-A, UHECR smeared by
galactic magnetic fields, whose random vertical bending is overlapping with
super-galactic arm. The eventual AUGER misunderstanding took place because of
such a rare coincidence between the Super Galactic Plane (arm) and the smeared
(randomized) signals from Cen-A, bent orthogonally to the Galactic fields. Our
derivation verify the consistence of the random smearing angles for He, Be and
B, C, O, in reasonable agreement with the AUGER main group events around Cen-A.
Only few other rare events are spread elsewhere. The most collimated from Cen-A
are the lightest. The most spread the heavier. Consequently Cen-A is the best
candidate UHE neutrino tau observable by HEAT and AMIGA as enhanced AUGER array
at tens-hundred PeV energy. This model maybe soon tested by new events
clustering around the Cen-A and by composition imprint study.Comment: 4 pages, 5 figures
The Right Mutation Strength for Multi-Valued Decision Variables
The most common representation in evolutionary computation are bit strings.
This is ideal to model binary decision variables, but less useful for variables
taking more values. With very little theoretical work existing on how to use
evolutionary algorithms for such optimization problems, we study the run time
of simple evolutionary algorithms on some OneMax-like functions defined over
. More precisely, we regard a variety of
problem classes requesting the component-wise minimization of the distance to
an unknown target vector . For such problems we see a crucial
difference in how we extend the standard-bit mutation operator to these
multi-valued domains. While it is natural to select each position of the
solution vector to be changed independently with probability , there are
various ways to then change such a position. If we change each selected
position to a random value different from the original one, we obtain an
expected run time of . If we change each selected position
by either or (random choice), the optimization time reduces to
. If we use a random mutation strength with probability inversely proportional to and change
the selected position by either or (random choice), then the
optimization time becomes , bringing down
the dependence on from linear to polylogarithmic. One of our results
depends on a new variant of the lower bounding multiplicative drift theorem.Comment: an extended abstract of this work is to appear at GECCO 201
Studying the nuclear mass composition of Ultra-High Energy Cosmic Rays with the Pierre Auger Observatory
The Fluorescence Detector of the Pierre Auger Observatory measures the
atmospheric depth, , where the longitudinal profile of the high energy
air showers reaches its maximum. This is sensitive to the nuclear mass
composition of the cosmic rays. Due to its hybrid design, the Pierre Auger
Observatory also provides independent experimental observables obtained from
the Surface Detector for the study of the nuclear mass composition. We present
-distributions and an update of the average and RMS values in
different energy bins and compare them to the predictions for different nuclear
masses of the primary particles and hadronic interaction models. We also
present the results of the composition-sensitive parameters derived from the
ground level component.Comment: Proceedings of the 12th International Conference on Topics in
Astroparticle and Underground Physics, TAUP 2011, Munich, German
The Environments of Low and High Luminosity Radio Galaxies at Moderate Redshifts
In the local Universe, high-power radio galaxies live in lower density
environments than low-luminosity radio galaxies. If this trend continues to
higher redshifts, powerful radio galaxies would serve as efficient probes of
moderate redshift groups and poor clusters. Photometric studies of radio
galaxies at 0.3 < z < 0.5 suggest that the radio luminosity-environment
correlation disappears at moderate redshifts, though this could be the result
of foreground/background contamination affecting the photometric measures of
environment. We have obtained multi-object spectroscopy of in the fields of 14
lower luminosity (L_1.4GHz
1.2x10^25 W/Hz) radio galaxies at z ~ 0.3 to spectroscopically investigate the
link between the environment and the radio luminosity of radio galaxies at
moderate redshifts. Our results support the photometric analyses; there does
not appear to be a correlation between the luminosity of a radio galaxy and its
environment at moderate redshifts. Hence, radio galaxies are not efficient
signposts for group environments at moderate redshifts.Comment: 7 pages, 9 figures, Accepted for publication in A
Radio-mode feedback in local AGNs: dependence on the central black hole parameters
Radio mode feedback, in which most of the energy of an active galactic
nucleus (AGN) is released in a kinetic form via radio-emitting jets, is thought
to play an important role in the maintenance of massive galaxies in the
present-day Universe. We study the link between radio emission and the
properties of the central black hole in a large sample of local radio galaxies
drawn from the Sloan Digital Sky Survey (SDSS), based on the catalogue of Best
and Heckman (2012). Our sample is mainly dominated by massive black holes
(mostly in the range ) accreting at very low Eddington
ratios (typically ). In broad agreement with previously
reported trends, we find that radio galaxies are preferentially associated with
the more massive black holes, and that the radio loudness parameter seems to
increase with decreasing Eddington ratio. We compare our results with previous
studies in the literature, noting potential biases. The majority of the local
radio galaxies in our sample are currently in a radiatively inefficient
accretion regime, where kinetic feedback dominates over radiative feedback. We
discuss possible physical interpretations of the observed trends in the context
of a two-stage feedback process involving a transition in the underlying
accretion modes.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Lens Galaxy Properties of SBS1520+530: Insights from Keck Spectroscopy and AO Imaging
We report on an investigation of the SBS 1520+530 gravitational lens system
and its environment using archival HST imaging, Keck spectroscopic data, and
Keck adaptive-optics imaging. The AO imaging has allowed us to fix the lens
galaxy properties with a high degree of precision when performing the lens
modeling, and the data indicate that the lens has an elliptical morphology and
perhaps a disk. The new spectroscopic data suggest that previous determinations
of the lens redshift may be incorrect, and we report an updated, though
inconclusive, value z_lens = 0.761. We have also spectroscopically confirmed
the existence of several galaxy groups at approximately the redshift of the
lens system. We create new models of the lens system that explicitly account
for the environment of the lens, and we also include improved constraints on
the lensing galaxy from our adaptive-optics imaging. Lens models created with
these new data can be well-fit with a steeper than isothermal mass slope (alpha
= 2.29, with the density proportional to r^-alpha) if H_0 is fixed at 72
km/s/Mpc; isothermal models require H_0 ~ 50 km/s/Mpc. The steepened profile
may indicate that the lens is in a transient perturbed state caused by
interactions with a nearby galaxy.Comment: 12 pages, 10 figures, submitted to Ap
Recent results from the Pierre Auger Observatory
The main results from the Auger Observatory are described. A steepening of
the spectrum is observed at the highest energies, supporting the expectation
that above eV the cosmic ray energies are significantly
degraded by interactions with the CMB photons (the GZK effect). This is further
supported by the correlations observed above eV with the
distribution of nearby active galaxies, which also show the potential of Auger
to start the era of charged particle astronomy. The lack of observation of
photons or neutrinos strongly disfavors top-down models, and these searches may
approach in the long term the sensitivity required to test the fluxes expected
from the secondaries of the very same GZK process. Bounds on the anisotropies
at EeV energies contradict hints from previous experiments that suggested a
large excess from regions near the Galactic centre or the presence of a dipolar
type modulation of the cosmic ray flux.Comment: 6 p., 8 figs., proceedings of the XXIII International Conference on
Neutrino Physics and Astrophysics (Neutrino 08
Measurement of the energy spectrum of cosmic rays using the Pierre Auger Observatory
The flux of cosmic rays above 1018 eV has been measured with
unprecedented precision at the Pierre Auger Observatory. Two analysis techniques have been used to extend the spectrum downwards from 3×1018 eV, with the lower energies being explored using a unique technique that exploits the hybrid strengths of the instrument. The spectral features are also presented
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