224,073 research outputs found
Shannon Information and Kolmogorov Complexity
We compare the elementary theories of Shannon information and Kolmogorov
complexity, the extent to which they have a common purpose, and where they are
fundamentally different. We discuss and relate the basic notions of both
theories: Shannon entropy versus Kolmogorov complexity, the relation of both to
universal coding, Shannon mutual information versus Kolmogorov (`algorithmic')
mutual information, probabilistic sufficient statistic versus algorithmic
sufficient statistic (related to lossy compression in the Shannon theory versus
meaningful information in the Kolmogorov theory), and rate distortion theory
versus Kolmogorov's structure function. Part of the material has appeared in
print before, scattered through various publications, but this is the first
comprehensive systematic comparison. The last mentioned relations are new.Comment: Survey, LaTeX 54 pages, 3 figures, Submitted to IEEE Trans
Information Theor
Propagation of Ultra-High-Energy Cosmic Ray Nuclei in Cosmic Magnetic Fields and Implications for Anisotropy Measurements
(Abridged) Recent results from the Pierre Auger Observatory (PAO) indicate
that the composition of ultra-high-energy cosmic rays (UHECRs) with energies
above eV may be dominated by heavy nuclei. An important question is
whether the distribution of arrival directions for such UHECR nuclei can
exhibit observable anisotropy or positional correlations with their
astrophysical source objects despite the expected strong deflections by
intervening magnetic fields. For this purpose, we have simulated the
propagation of UHECR nuclei including models for both the extragalactic
magnetic field and the Galactic magnetic field. Assuming that only iron nuclei
are injected steadily from sources with equal luminosity and spatially
distributed according to the observed large scale structure in the local
Universe, at the number of events published by the PAO so far, the arrival
distribution of UHECRs would be consistent with no auto-correlation at 95%
confidence if the mean number density of UHECR sources
Mpc, and consistent with no cross-correlation with sources within 95%
errors for Mpc. On the other hand, with 1000 events
above eV in the whole sky, next generation experiments can
reveal auto-correlation with more than 99% probability even for Mpc, and cross-correlation with sources with more than 99%
probability for Mpc. In addition, we find that the
contribution of Centaurus A is required to reproduce the currently observed
UHECR excess in the Centaurus region. Secondary protons generated by
photodisintegration of primary heavy nuclei during propagation play a crucial
role in all cases, and the resulting anisotropy at small angular scales should
provide a strong hint of the source location if the maximum energies of the
heavy nuclei are sufficiently high.Comment: 17 pages, 15 figure
Microlensing toward crowded fields: Theory and applications to M31
We present a comprehensive treatment of the pixel-lensing theory and apply it
to lensing experiments and their results toward M31. Using distribution
functions for the distances, velocities, masses, and luminosities of stars, we
derive lensing event rates as a function of the event observables. In contrast
to the microlensing regime, in the pixel-lensing regime (crowded or unresolved
sources) the observables are the maximum excess flux of the source above a
background and the full width at half-maximum (FWHM) time of the event. To
calculate lensing event distribution functions depending on these observables
for the specific case of M31, we use data from the literature to construct a
model of M31, reproducing consistently photometry, kinematics and stellar
population. We predict the halo- and self-lensing event rates for bulge and
disk stars in M31 and treat events with and without finite source signatures
separately. We use the M31 photon noise profile and obtain the event rates as a
function of position, field of view, and S/N threshold at maximum
magnification. We calculate the expected rates for WeCAPP and for a potential
Advanced Camera for Surveys (ACS) lensing campaign. The detection of two events
with a peak signal-to-noise ratio larger than 10 and a timescale larger than 1
day in the WeCAPP 2000/2001 data is in good agreement with our theoretical
calculations. We investigate the luminosity function of lensed stars for noise
characteristics of WeCAPP and ACS. For the pixel-lensing regime, we derive the
probability distribution for the lens masses in M31 as a function of the FWHM
timescale, flux excess and color, including the errors of these observables.Comment: 45 pages, 27 figures LaTeX; corrected typos; published in the
Astrophysical Journal Supplemen
Constraining halo occupation properties of X-ray AGNs using clustering of Chandra sources in the Bootes survey region
We present one of the most precise measurement to date of the spatial
clustering of X-ray selected AGNs using a sample derived from the Chandra X-ray
Observatory survey in the Bootes field. The real-space two-point correlation
function over a redshift interval from z=0.17 to z~3 is well described by the
power law, xi(r)=(r/r0)^-gamma, for comoving separations r<~20h^-1 Mpc. We find
gamma=1.84+-0.12 and r0 consistent with no redshift trend within the sample
(varying between r0=5.5+-0.6 h^-1 Mpc for =0.37 and r0=6.9+-1.0 h^-1 Mpc for
=1.28). Further, we are able to measure the projections of the two-point
correlation function both on the sky plane and in the line of sight. We use
these measurements to show that the Chandra/Bootes AGNs are predominantly
located at the centers of dark matter halos with the circular velocity Vmax>320
km/s or M_200 > 4.1e12 h^-1 Msun, and tend to avoid satellite galaxies in halos
of this or higher mass. The halo occupation properties inferred from the
clustering properties of Chandra/Bootes AGNs --- the mass scale of the parent
dark matter halos, the lack of significant redshift evolution of the clustering
length, and the low satellite fraction --- are broadly consistent with the
Hopkins et al. scenario of quasar activity triggered by mergers of
similarly-sized galaxies.Comment: Accepted to ApJ. The revision matches the accepted version. The most
significant changes include the recalculation of uncertainties using mock
catalogs and explicit comparison with the AGN HOD studies based on projected
correlation function, w(rp
Horizon of quantum black holes in various dimensions
We adapt the horizon wave-function formalism to describe massive static
spherically symmetric sources in a general -dimensional space-time, for
and including the case. We find that the probability
that such objects are (quantum) black holes behaves similarly to the
probability in the framework for . In fact, for , the
probability increases towards unity as the mass grows above the relevant
-dimensional Planck scale . At fixed mass, however,
decreases with increasing , so that a particle with mass has
just about probability to be a black hole in , and smaller for
larger . This result has a potentially strong impact on estimates of black
hole production in colliders. In contrast, for , we find the probability
is comparably larger for smaller masses, but , suggesting
that such lower dimensional black holes are purely quantum and not classical
objects. This result is consistent with recent observations that sub-Planckian
black holes are governed by an effective two-dimensional gravitation theory.
Lastly, we derive Generalised Uncertainty Principle relations for the black
holes under consideration, and find a minimum length corresponding to a
characteristic energy scale of the order of the fundamental gravitational mass
in . For we instead find the uncertainty due to the horizon
fluctuations has the same form as the usual Heisenberg contribution, and
therefore no fundamental scale exists.Comment: Latex, 16 pages, 8 figures. Final version to appear in PL
LABOCA mapping of the infrared dark cloud MSXDC G304.74+01.32
Infrared dark clouds (IRDCs) likely represent very early stages of high-mass
star/star cluster formation. In this study, we aim to determine the physical
properties and spatial distribution of dense clumps in the IRDC MSXDC
G304.74+01.32 (G304.74), and bring these characteristics into relation to
theories concerning the origin of IRDCs and their fragmentation into clumps and
star-forming cores. G304.74 was mapped in the 870 m dust continuum with
the LABOCA bolometer on APEX. Archival MSX and IRAS infrared data were used to
study the nature and properties of the submillimetre clumps within the cloud.
There are 8 clumps within G304.74 which are not associated with mid-infrared
(MIR) emission. Some of them are candidates for being/harbouring high-mass
starless cores (HMSCs). We compared the clump masses and their spatial
distribution in G304.74 with those in several other recently studied IRDCs.
There is a high likelihood that the clump mass distributions in G304.74 and in
several other IRDCs represent the samples of the same parent distribution. In
most cases the spatial distributions of clumps in IRDCs do not deviate
significantly from random distributions. This is consistent with the idea that
the origin of IRDCs, and their further sub-fragmentation down to scales of
clumps is caused by supersonic turbulence in accordance with results from giant
molecular clouds.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
The Bs oscillation amplitude analysis
The properties of the amplitude method for \Bs oscillation analyses are
studied in detail. The world combination of measured amplitudes is converted
into a likelihood profile as a function of oscillation frequency. A procedure
is proposed to estimate the probability that the minimum observed is due to a
statistical fluctuation. This method, applied to the data available at the time
of 1999 Winter Conferences, gives .Comment: 27 pages, 16 figur
Is the Number of Giant Arcs in LCDM Consistent With Observations?
We use high-resolution N-body simulations to study the galaxy-cluster
cross-sections and the abundance of giant arcs in the CDM model.
Clusters are selected from the simulations using the friends-of-friends method,
and their cross-sections for forming giant arcs are analyzed. The background
sources are assumed to follow a uniform ellipticity distribution from 0 to 0.5
and to have an area identical to a circular source with diameter 1\arcsec. We
find that the optical depth scales as the source redshift approximately as
\tau_{1''} = 2.25 \times 10^{-6}/[1+(\zs/3.14)^{-3.42}] (0.6<\zs<7). The
amplitude is about 50% higher for an effective source diameter of 0.5\arcsec.
The optimal lens redshift for giant arcs with the length-to-width ratio ()
larger than 10 increases from 0.3 for \zs=1, to 0.5 for \zs=2, and to
0.7-0.8 for \zs>3. The optical depth is sensitive to the source redshift, in
qualitative agreement with Wambsganss et al. (2004). However, our overall
optical depth appears to be only 10% to 70% of those from previous
studies. The differences can be mostly explained by different power spectrum
normalizations () used and different ways of determining the
ratio. Finite source size and ellipticity have modest effects on the optical
depth. We also found that the number of highly magnified (with magnification
) and ``undistorted'' images (with ) is comparable to the
number of giant arcs with and . We conclude that our
predicted rate of giant arcs may be lower than the observed rate, although the
precise `discrepancy' is still unclear due to uncertainties both in theory and
observations.Comment: Revised version after the referee's reports (32 pages,13figures). The
paper has been significantly revised with many additions. The new version
includes more detailed comparisons with previous studies, including the
effects of source size and ellipticity. New discussions about the redshift
distribution of lensing clusters and the width of giant arcs have been adde
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