513 research outputs found
ASTErIsM - Application of topometric clustering algorithms in automatic galaxy detection and classification
We present a study on galaxy detection and shape classification using
topometric clustering algorithms. We first use the DBSCAN algorithm to extract,
from CCD frames, groups of adjacent pixels with significant fluxes and we then
apply the DENCLUE algorithm to separate the contributions of overlapping
sources. The DENCLUE separation is based on the localization of pattern of
local maxima, through an iterative algorithm which associates each pixel to the
closest local maximum. Our main classification goal is to take apart elliptical
from spiral galaxies. We introduce new sets of features derived from the
computation of geometrical invariant moments of the pixel group shape and from
the statistics of the spatial distribution of the DENCLUE local maxima
patterns. Ellipticals are characterized by a single group of local maxima,
related to the galaxy core, while spiral galaxies have additional ones related
to segments of spiral arms. We use two different supervised ensemble
classification algorithms, Random Forest, and Gradient Boosting. Using a sample
of ~ 24000 galaxies taken from the Galaxy Zoo 2 main sample with spectroscopic
redshifts, and we test our classification against the Galaxy Zoo 2 catalog. We
find that features extracted from our pipeline give on average an accuracy of ~
93%, when testing on a test set with a size of 20% of our full data set, with
features deriving from the angular distribution of density attractor ranking at
the top of the discrimination power.Comment: 20 pages, 13 Figures, 8 Tables, Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
The 26 year-long X-ray light curve and the X-ray spectrum of the BL Lac Object 1E 1207.9+3945 in its brightest state
We studied the temporal and spectral evolution of the synchrotron emission
from the high energy peaked BL Lac object 1E 1207.9+3945. Two recent
observations have been performed by the XMM-Newton and Swift satellites; we
carried out X-ray spectral analysis for both of them, and photometry in
optical-ultraviolet filters for the Swift one. Combining the results thus
obtained with archival data we built the long-term X-ray light curve, spanning
a time interval of 26 years, and the Spectral Energy Distribution (SED) of this
source. The light curve shows a large flux increasing, about a factor of six,
in a time interval of a few years. After reaching its maximum in coincidence
with the XMM-Newton pointing in December 2000 the flux decreased in later
years, as revealed by Swift. The very good statistics available in the 0.5-10
keV XMM-Newton X-ray spectrum points out a highly significant deviation from a
single power law. A log-parabolic model with a best fit curvature parameter of
0.25 and a peak energy at ~1 keV describes well the spectral shape of the
synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides
a milder curvature (b~0.1) and a peak at higher energies (~15 keV), suggesting
a different state of source activity. In both cases UVOT data support the
scenario of a single synchrotron emission component extending from the
optical/UV to the X-ray band. New X-ray observations are important to monitor
the temporal and spectral evolution of the source; new generation gamma-ray
telescopes like AGILE and GLAST could for the first time detect its inverse
Compton emission.Comment: 7 pages, 6 figures, accepted for publication in A&
On the connection between turbulent motions and particle acceleration in galaxy clusters
Giant radio halos are Mpc-scale diffuse radio sources associated with the
central regions of galaxy clusters. The most promising scenario to explain the
origin of these sources is that of turbulent re-acceleration, in which MeV
electrons injected throughout the formation history of galaxy clusters are
accelerated to higher energies by turbulent motions mostly induced by cluster
mergers. In this Letter, we use the amplitude of density fluctuations in the
intracluster medium as a proxy for the turbulent velocity and apply this
technique to a sample of 51 clusters with available radio data. Our results
indicate a segregation in the turbulent velocity of radio halo and radio quiet
clusters, with the turbulent velocity of the former being on average higher by
about a factor of two. The velocity dispersion recovered with this technique
correlates with the measured radio power through the relation , which implies that the radio power is
nearly proportional to the turbulent energy rate. Our results provide an
observational confirmation of a key prediction of the turbulent re-acceleration
model and possibly shed light on the origin of radio halos.Comment: Submitted to ApJ Letter
Functional biases in GRB's spectral parameter correlations
Gamma Ray Bursts (GRBs) show evidence of different spectral shapes, light
curves, duration, host galaxies and they explode within a wide redshift range.
However, the most of them seems to follow very tight correlations among some
observed quantities relating to their energetic. If true, these correlations
have significant implications on burst physics, giving constraints on
theoretical models. Moreover, several suggestions have been made to use these
correlations in order to calibrate GRBs as standard candles and to constrain
the cosmological parameters. We investigate the cosmological relation between
low energy index in GRBs prompt spectra and the redshift . We
present a statistical analysis of the relation between the total isotropic
energy and the peak energy (also known as Amati relation) in
GRBs spectra searching for possible functional biases. Possible implications on
the vs relation of the vs correlation are
evaluated. We used MonteCarlo simulations and the boostrap method to evaluate
how large are the effects of functional biases on the vs . We
show that high values of the linear correlation coefficent, up to about 0.8, in
the vs relation are obtained for random generated samples of
GRBs, confirming the relevance of functional biases. Astrophysical consequences
from vs relation are then to be revised after a more accurate
and possibly bias free analysis.Comment: 6 pages, 6 figures, conference poster session: "070228: The Next
Decade of Gamma-Ray Burst Afterglows", Amsterdam, March 2007, MNRAS submitte
The role of hairs in the adhesion of octopus suckers: a hierarchical peeling approach.
Organisms like the octopus or the clingfish are a precious source of inspiration for the design of innovative adhesive systems based on suction cups, but a complete mechanical description of their attachment process is still lacking. In this paper, we exploit the recent discovery of the presence of hairs in the acetabulum roof of octopus suction cups to revise the current model for its adhesion to the acetabulum wall. We show how this additional feature, which can be considered an example of a hierarchical structure, can lead to an increase of adhesive strength, based on the analysis of the cases of a simple tape and an axisymmetrical membrane adhering to a substrate. Using peeling theory, we discuss in both cases the influence of hierarchical structure and the resulting variation of geometry on the adhesive energy, highlighting how an increase in number of hierarchical levels contributes to its increment, with a corresponding improvement in functionality for the octopus suckers
X-ray spectral evolution of TeV BL Lac objects: eleven years of observations with BeppoSAX, XMM-Newton and SWIFT satellites
Many of the extragalactic sources detected in rays at TeV energies
are BL Lac objects. In particular, they belong to the subclass of ``high
frequency peaked BL Lacs" (HBLs), as their spectral energy distributions
exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra
appear to be generally curved into a log-parabolic shape. In a previous
investigation of Mrk 421, two correlations were found between the spectral
parameters. One involves the height increasing with the position of
the first peak; this was interpreted as a signature of synchrotron emission
from relativistic electrons. The other involves the curvature parameter
decreasing as increases; this points toward statistical/stochastic
acceleration processes for the emitting electrons. We analyse X-ray spectra of
several TeV HBLs to pinpoint their behaviours in the and
planes and to compare them with Mrk 421. We perfom X-ray spectral analyses of a
sample of 15 BL Lacs. We report the whole set of observations obtained with the
\sax, \xmm and \swf satellites between 29/06/96 and 07/04/07. We focus on five
sources (PKS 0548-322, 1H 1426+418, Mrk 501, 1ES 1959+650, PKS2155-304) whose
X-ray observations warrant detailed searching of correlations or trends. Within
our database, we find that four out of five sources, namely PKS 0548-322, 1H
1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 in the
plane, while PKS 2155-304 differs. As for the plane, all TeV
HBLs follow a similar behaviour. The trends exhibited by Mrk 421 appear to be
shared by several TeV HBLs, such as to warrant discussing predictions from the
X-ray spectral evolution to that of TeV emissions.Comment: 14 pages, 9 figures, A&A accepte
Swift observations of the very intense flaring activity of Mrk 421 during 2006: I. Phenomenological picture of electron acceleration and predictions for the MeV/GeV emission
We present results from a deep spectral analysis of all the Swift
observations of Mrk 421 from April 2006 to July 2006, when it reached its
largest X-ray flux recorded until 2006. The peak flux was about 85 milli-Crab
in the 2.0-10.0 keV band, with the peak energy (Ep) of the spectral energy
distribution (SED) laying often at energies larger than 10 keV. We performed
spectral analysis of the Swift observations investigating the trends of the
spectral parameters in terms of acceleration and energetic features
phenomenologically linked to the SSC model parameters, predicting their effects
in the gamma-ray band, in particular the spectral shape expected in the Fermi
Gamma-ray Space Telescope-LAT band. We confirm that the X-ray spectrum is well
described by a log-parabolic distribution close to Ep, with the peak flux of
the SED (Sp) being correlated with Ep, and Ep anti-correlated with the
curvature parameter (b). During the most energetic flares the UV-to-soft-X-ray
spectral shape requires an electron distribution spectral index s about 2.3.
Present analysis shows that the UV-to-X-ray emission from Mrk 421 is likely to
be originated by a population of electrons that is actually curved, with a low
energy power-law tail. The observed spectral curvature is consistent both with
stochastic acceleration or energy dependent acceleration probability
mechanisms, whereas the power-law slope form XRT-UVOT data is very close to
that inferred from the GRBs X-ray afterglow and in agreement with the universal
first-order relativistic shock acceleration models. This scenario hints that
the magnetic turbulence may play a twofold role: spatial diffusion relevant to
the first order process and momentum diffusion relevant to the second order
process.Comment: Accepted, Astronomy and Astrophysic
The PCA Lens-Finder: application to CFHTLS
We present the results of a new search for galaxy-scale strong lensing
systems in CFHTLS Wide. Our lens-finding technique involves a preselection of
potential lens galaxies, applying simple cuts in size and magnitude. We then
perform a Principal Component Analysis of the galaxy images, ensuring a clean
removal of the light profile. Lensed features are searched for in the residual
images using the clustering topometric algorithm DBSCAN. We find 1098 lens
candidates that we inspect visually, leading to a cleaned sample of 109 new
lens candidates. Using realistic image simulations we estimate the completeness
of our sample and show that it is independent of source surface brightness,
Einstein ring size (image separation) or lens redshift. We compare the
properties of our sample to previous lens searches in CFHTLS. Including the
present search, the total number of lenses found in CFHTLS amounts to 678,
which corresponds to ~4 lenses per square degree down to i=24.8. This is
equivalent to ~ 60.000 lenses in total in a survey as wide as Euclid, but at
the CFHTLS resolution and depth.Comment: 21 pages, 12 figures, accepted for publication on A&
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