226 research outputs found
Jamming of directed traffic on a square lattice
Phase transition from a free-flow phase to a jammed phase is an important
feature of traffic networks. We study this transition in the case of a simple
square lattice network for different values of data posting rate by
introducing a parameter which selects a neighbour for onward data transfer
depending on queued traffic. For every there is a critical value of
above which the system become jammed. The phase diagram shows some
interesting features. We also show that the average load diverges
logarithmically as approaches and the queue length distribution
exhibits exponential and algebraic nature in different regions of the phase
diagram.Comment: 12 pages, 9 figure
Self-generated Self-similar Traffic
Self-similarity in the network traffic has been studied from several aspects:
both at the user side and at the network side there are many sources of the
long range dependence. Recently some dynamical origins are also identified: the
TCP adaptive congestion avoidance algorithm itself can produce chaotic and long
range dependent throughput behavior, if the loss rate is very high. In this
paper we show that there is a close connection between the static and dynamic
origins of self-similarity: parallel TCPs can generate the self-similarity
themselves, they can introduce heavily fluctuations into the background traffic
and produce high effective loss rate causing a long range dependent TCP flow,
however, the dropped packet ratio is low.Comment: 8 pages, 12 Postscript figures, accepted in Nonlinear Phenomena in
Complex System
Reconstructing Galaxy Spectral Energy Distributions from Broadband Photometry
We present a novel approach to photometric redshifts, one that merges the
advantages of both the template fitting and empirical fitting algorithms,
without any of their disadvantages. This technique derives a set of templates,
describing the spectral energy distributions of galaxies, from a catalog with
both multicolor photometry and spectroscopic redshifts. The algorithm is
essentially using the shapes of the templates as the fitting parameters. From
simulated multicolor data we show that for a small training set of galaxies we
can reconstruct robustly the underlying spectral energy distributions even in
the presence of substantial errors in the photometric observations. We apply
these techniques to the multicolor and spectroscopic observations of the Hubble
Deep Field building a set of template spectra that reproduced the observed
galaxy colors to better than 10%. Finally we demonstrate that these improved
spectral energy distributions lead to a photometric-redshift relation for the
Hubble Deep Field that is more accurate than standard template-based
approaches.Comment: 23 pages, 8 figures, LaTeX AASTeX, accepted for publication in A
Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co-added images
More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys. We present a study, based on our co-added image stacking technique, in which our aim is to detect the optical emission from unresolved, isolated radio sources of the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82 co-added data set. From the FIRST catalogue, 2116 such radio point sources were selected, and cut-out images, centred on the FIRST coordinates, were generated from the Stripe 82 images. The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio, in the hope that optical emission from the radio sources would become detectable. Multiple stacks were generated, based on the radio luminosity of the point sources. The resulting stacked images show central peaks similar to point sources. The peaks have very red colours with steep optical spectral energy distributions. We have found that the optical spectral index α ν falls in the range -2.9 ≤ α ν ≤ -2.2 (Sν∝ναν), depending only weakly on the radio flux. The total integration times of the stacks are between 270 and 300h, and the corresponding 5σ detection limit is estimated to be about m r ≃ 26.6 mag. We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei. Dust-reddened quasars might represent an early phase of quasar evolution, and thus they can also give us an insight into the formation of massive galaxies. The data used in the paper are available on-line at http://www.vo.elte.hu/doublestacking. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
Searching for electromagnetic counterpart of LIGO gravitational waves in the Fermi GBM data with ADWO
The Fermi collaboration identified a possible electromagnetic counterpart of
the gravitational wave event of September 14, 2015. Our goal is to provide an
unsupervised data analysis algorithm to identify similar events in Fermi's
Gamma-ray Burst Monitor CTTE data stream. We are looking for signals that are
typically weak. Therefore, they can only be found by a careful analysis of
count rates of all detectors and energy channels simultaneously. Our
Automatized Detector Weight Optimization (ADWO) method consists of a search for
the signal, and a test of its significance. We developed ADWO, a virtual
detector analysis tool for multi-channel multi-detector signals, and performed
successful searches for short transients in the data-streams. We have
identified GRB150522B, as well as possible electromagnetic candidates of the
transients GW150914 and LVT151012. ADWO is an independently developed,
unsupervised data analysis tool that only relies on the raw data of the Fermi
satellite. It can therefore provide a strong, independent test to any
electromagnetic signal accompanying future gravitational wave observations.Comment: 4 pages and 4 figures, A&A Letters accepte
Evidence for a high-z ISW signal from supervoids in the distribution of eBOSS quasars
The late-time integrated Sachs-Wolfe (ISW) imprint of super-structures is sourced by evolving large-scale
potentials due to a dominant dark energy component in the CDM model.
The aspect that makes the ISW effect distinctly interesting is the repeated
observation of stronger-than-expected imprints from supervoids at
. Here we analyze the un-probed key redshift range
where the ISW signal is expected to fade in CDM, due to a weakening
dark energy component, and eventually become consistent with zero in the matter
dominated epoch. On the contrary, alternative cosmological models, proposed to
explain the excess low- ISW signals, predicted a sign-change in the ISW
effect at due to the possible growth of large-scale potentials
that is absent in the standard model. To discriminate, we estimated the
high- CDM ISW signal using the Millennium XXL mock catalogue, and
compared it to our measurements from about 800 supervoids identified in the
eBOSS DR16 quasar catalogue. At , we found an excess ISW signal with
amplitude. The signal is then consistent with
the CDM expectation () at where the
standard and alternative models predict similar amplitudes. Most interestingly,
we also detected an opposite-sign ISW signal at that is in
tension with the CDM prediction. Taken at face value,
these moderately significant detections of ISW anomalies suggest an alternative
growth rate of structure in low-density environments at scales.Comment: 12 pages, 9 figures, submitted to MNRA
Measuring the halo mass of MgII absorbers from their cross-correlation with Luminous Red Galaxies
We study the cross-correlation between 716 MgII quasar absorption systems and
about 100,000 Luminous Red Galaxies (LRGs) selected from the Sloan Digital Sky
Survey Data Release 3 in the redshift range 0.4<z<0.8. The MgII systems were
selected to have 2796 & 2803 rest-frame equivalent widths greater than 1.0 \AA
and identifications confirmed by the FeII 2600 or MgI 2852 lines. Over
co-moving scales 0.2--13/h Mpc, the MgII--LRG cross-correlation has an
amplitude 0.69+/-0.09 times that of the LRG--LRG auto-correlation. Since LRGs
have halo-masses of 10^{13} \msun, this strong cross-correlation implies that
the absorber host-galaxies have halo-masses 1--2 times 10^{12} \msun.Comment: 3 pages, 2 figures, to appear in IAU 199 conf. proc.: "Probing
Galaxies through Quasar Absorption Lines," eds. Williams, Shu, Menard; minor
changes to match the edited versio
rCUR: an R package for CUR matrix decomposition
BACKGROUND: Many methods for dimensionality reduction of large data sets such as those generated in microarray studies boil down to the Singular Value Decomposition (SVD). Although singular vectors associated with the largest singular values have strong optimality properties and can often be quite useful as a tool to summarize the data, they are linear combinations of up to all of the data points, and thus it is typically quite hard to interpret those vectors in terms of the application domain from which the data are drawn. Recently, an alternative dimensionality reduction paradigm, CUR matrix decompositions, has been proposed to address this problem and has been applied to genetic and internet data. CUR decompositions are low-rank matrix decompositions that are explicitly expressed in terms of a small number of actual columns and/or actual rows of the data matrix. Since they are constructed from actual data elements, CUR decompositions are interpretable by practitioners of the field from which the data are drawn. RESULTS: We present an implementation to perform CUR matrix decompositions, in the form of a freely available, open source R-package called rCUR. This package will help users to perform CUR-based analysis on large-scale data, such as those obtained from different high-throughput technologies, in an interactive and exploratory manner. We show two examples that illustrate how CUR-based techniques make it possible to reduce significantly the number of probes, while at the same time maintaining major trends in data and keeping the same classification accuracy. CONCLUSIONS: The package rCUR provides functions for the users to perform CUR-based matrix decompositions in the R environment. In gene expression studies, it gives an additional way of analysis of differential expression and discriminant gene selection based on the use of statistical leverage scores. These scores, which have been used historically in diagnostic regression analysis to identify outliers, can be used by rCUR to identify the most informative data points with respect to which to express the remaining data points
SONoMA: A Service Oriented Network Measurement Architecture
Distributed network measurements are essential to characterize
the structure, dynamics and operational state of the Internet.
Although in the last decades several such systems have been
created, the easy access of these infrastructures and the
orchestration of complex measurements are not solved. We propose
a system architecture that combines the flexibility of mature
network measurement facilities such as PlanetLab or ETOMIC with
the general accessibility and popularity of public services like
Web based bandwidth measurement or traceroute servers. To
realize these requirements we developed a network measurement
platform, called SONoMA, based on Web Services and the basic
principles of SOA, which is a well established paradigm in
distributed business application development. Our approach opens
the door to perform atomic and complex network measurements in
real time, handles heterogeneous measurement devices,
automatically stores the results in a public database and
protects against malicious users as well. Furthermore, SONoMA is
not only a tool for network researchers but it opens the door to
developing novel applications and services requiring real-time
and large scale network measurements
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