22,572 research outputs found
Description of Atmospheric Conditions at the Pierre Auger Observatory Using Meteorological Measurements and Models
Atmospheric conditions at the site of a cosmic ray observatory must be known
well for reconstructing observed extensive air showers, especially when
measured using the fluorescence technique. For the Pierre Auger Observatory, a
sophisticated network of atmospheric monitoring devices has been conceived.
Part of this monitoring was a weather balloon program to measure atmospheric
state variables above the Observatory. To use the data in reconstructions of
air showers, monthly models have been constructed. Scheduled balloon launches
were abandoned and replaced with launches triggered by high-energetic air
showers as part of a rapid monitoring system. Currently, the balloon launch
program is halted and atmospheric data from numerical weather prediction models
are used. A description of the balloon measurements, the monthly models as well
as the data from the numerical weather prediction are presented
The Pierre Auger Observatory: Results on Ultra-High Energy Cosmic Rays
The focus of this article is on recent results on ultra-high energy cosmic
rays obtained with the Pierre Auger Observatory. The world's largest instrument
of this type and its performance are described. The observations presented here
include the energy spectrum, the primary particle composition, limits on the
fluxes of photons and neutrinos and a discussion of the anisotropic
distribution of the arrival directions of the most energetic particles.
Finally, plans for the construction of a Northern Auger Observatory in
Colorado, USA, are discussed.Comment: Proceedings of the International Workshop on Advances in Cosmic Ray
Science, Waseda University, Shinjuku, Tokyo, Japan, March 2008; to be
published in the Journal of the Physical Society of Japan (JPSJ) supplemen
Addendum: Ultrahigh-energy cosmic-ray bounds on nonbirefringent modified-Maxwell theory
Nonbirefringent modified-Maxwell theory, coupled to standard Dirac particles,
involves nine dimensionless parameters, which can be bounded by the inferred
absence of vacuum Cherenkov radiation for ultrahigh-energy cosmic rays
(UHECRs). With selected UHECR events, two-sided bounds on the eight
nonisotropic parameters are obtained at the 10^{-18} level, together with an
improved one-sided bound on the single isotropic parameter at the 10^{-19}
level.Comment: 5 pages with revtex
Variability of GRB Afterglows Due to Interstellar Turbulence
Gamma-Ray Burst (GRB) afterglows are commonly interpreted as synchrotron
emission from a relativistic blast wave produced by a point explosion in an
ambient medium, plausibly the interstellar medium of galaxies. We calculate the
amplitude of flux fluctuations in the lightcurve of afterglows due to
inhomogeneities in the surrounding medium. Such inhomogeneities are an
inevitable consequence of interstellar turbulence, but could also be generated
by variability and anisotropy in a precursor wind from the GRB progenitor.
Detection of their properties could provide important clues about the
environments of GRB sources. We apply our calculations to GRB990510, where an
rms scatter of 2% was observed for the optical flux fluctuations on the 0.1--2
hour timescale during the first day of the afterglow, consistent with it being
entirely due to photometric noise (Stanek et al. 1999). The resulting upper
limits on the density fluctuations on scales of 20-200 AU around the source of
GRB990510, are lower than the inferred fluctuation amplitude on similar scales
in the Galactic interstellar medium. Hourly monitoring of future optical
afterglows might therefore reveal fractional flux fluctuations at the level of
a few percent.Comment: 18 pages, submitted to Ap
Constraints on Off-Axis X-Ray Emission from Beamed GRBs
We calculate the prompt x-ray emission as a function of viewing angle for
beamed Gamma-Ray Burst (GRB) sources. Prompt x-rays are inevitable due to the
less highly blueshifted photons emitted at angles greater than 1/gamma relative
to the beam symmetry axis, where gamma is the expansion Lorentz factor. The
observed flux depends on the combinations (gamma Delta theta) and (gamma
theta_v), where (Delta theta) is the beaming angle and theta_v is the viewing
angle. We use the observed source counts of gamma-ray-selected GRBs to predict
the minimum detection rate of prompt x-ray bursts as a function of limiting
sensitivity. We compare our predictions with the results from the Ariel V
catalog of fast x-ray transients, and find that Ariel's sensitivity is not
great enough to place significant constraints on gamma and (Delta theta). We
estimate that a detector with fluence limit ~10^{-7} erg/cm^2 in the 2-10 keV
channel will be necessary to distinguish between geometries. Because the x-ray
emission is simultaneous with the GRB emission, our predicted constraints do
not involve any model assumptions about the emission physics but simply follow
from special-relativistic considerations.Comment: Submitted to Ap
Lower Bounds in the Preprocessing and Query Phases of Routing Algorithms
In the last decade, there has been a substantial amount of research in
finding routing algorithms designed specifically to run on real-world graphs.
In 2010, Abraham et al. showed upper bounds on the query time in terms of a
graph's highway dimension and diameter for the current fastest routing
algorithms, including contraction hierarchies, transit node routing, and hub
labeling. In this paper, we show corresponding lower bounds for the same three
algorithms. We also show how to improve a result by Milosavljevic which lower
bounds the number of shortcuts added in the preprocessing stage for contraction
hierarchies. We relax the assumption of an optimal contraction order (which is
NP-hard to compute), allowing the result to be applicable to real-world
instances. Finally, we give a proof that optimal preprocessing for hub labeling
is NP-hard. Hardness of optimal preprocessing is known for most routing
algorithms, and was suspected to be true for hub labeling
Calibrating the Galaxy Halo - Black Hole Relation Based on the Clustering of Quasars
The observed number counts of quasars may be explained either by long-lived
activity within rare massive hosts, or by short-lived activity within smaller,
more common hosts. It has been argued that quasar lifetimes may therefore be
inferred from their clustering length, which determines the typical mass of the
quasar host. Here we point out that the relationship between the mass of the
black-hole and the circular velocity of its host dark-matter halo is more
fundamental to the determination of the clustering length. In particular, the
clustering length observed in the 2dF quasar redshift survey is consistent with
the galactic halo - black-hole relation observed in local galaxies, provided
that quasars shine at ~10-100% of their Eddington luminosity. The slow
evolution of the clustering length with redshift inferred in the 2dF quasar
survey favors a black-hole mass whose redshift-independent scaling is with halo
circular velocity, rather than halo mass. These results are independent from
observations of the number counts of bright quasars which may be used to
determine the quasar lifetime and its dependence on redshift. We show that if
quasar activity results from galaxy mergers, then the number counts of quasars
imply an episodic quasar lifetime that is set by the dynamical time of the host
galaxy rather than by the Salpeter time. Our results imply that as the redshift
increases, the central black-holes comprise a larger fraction of their host
galaxy mass and the quasar lifetime gets shorter.Comment: 10 pages, 5 figures. Submitted to Ap
Gravitational Lensing of the SDSS High-Redshift Quasars
We predict the effects of gravitational lensing on the color-selected
flux-limited samples of z~4.3 and z>5.8 quasars, recently published by the
Sloan Digital Sky Survey (SDSS). Our main findings are: (i) The lensing
probability should be 1-2 orders of magnitude higher than for conventional
surveys. The expected fraction of multiply-imaged quasars is highly sensitive
to redshift and the uncertain slope of the bright end of the luminosity
function, beta_h. For beta_h=2.58 (3.43) we find that at z~4.3 and i*<20.0 the
fraction is ~4% (13%) while at z~6 and z*<20.2 the fraction is ~7% (30%). (ii)
The distribution of magnifications is heavily skewed; sources having the
redshift and luminosity of the SDSS z>5.8 quasars acquire median magnifications
of med(mu_obs)~1.1-1.3 and mean magnifications of ~5-50. Estimates of
the quasar luminosity density at high redshift must therefore filter out
gravitationally-lensed sources. (iii) The flux in the Gunn-Peterson trough of
the highest redshift (z=6.28) quasar is known to be f_lambda<3 10^-19
erg/sec/cm^2/Angstrom. Should this quasar be multiply imaged, we estimate a 40%
chance that light from the lens galaxy would have contaminated the same part of
the quasar spectrum with a higher flux. Hence, spectroscopic studies of the
epoch of reionization need to account for the possibility that a lens galaxy,
which boosts the quasar flux, also contaminates the Gunn-Peterson trough. (iv)
Microlensing by stars should result in ~1/3 of multiply imaged quasars in the
z>5.8 catalog varying by more than 0.5 magnitudes over the next decade. The
median equivalent width would be lowered by ~20% with respect to the intrinsic
value due to differential magnification of the continuum and emission-line
regions.Comment: 27 pages, 10 figures. Expansion on the discussion in
astro-ph/0203116. Replaced with version accepted for publication in Ap
- …