510 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 Fermi blazars' divide based on the diagnostic of the SEDs peak frequencies
We have studied the quasi-simultaneous Spectral Energy Distributions (SED) of
48 LBAS blazars, detected within the three months of the LAT Bright AGN Sample
(LBAS) data taking period, combining Fermi and Swift data with radio
NIR-Optical and hard-X/gamma-ray data. Using these quasi-simultaneous SEDs,
sampling both the low and the high energy peak of the blazars broad band
emission, we were able to apply a diagnostic tool based on the estimate of the
peak frequencies of the synchrotron (S) and Inverse Compton (IC) components.
Our analysis shows a Fermi blazars' divide based on the peak frequencies of the
SED. The robust result is that the Synchrotron Self Compton (SSC) region
divides in two the plane were we plot the peak frequency of the synchrotron SED
vs the typical Lorentz factor of the electrons most contributing to the
synchrotron emission and to the inverse Compton process. Objects within or
below this region, radiating likely via the SSC process, are
high-frequency-peaked BL Lac object (HBL), or low/intermediate-frequency peaked
BL Lac object (LBL/IBL). All of the IBLs/LBLs within or below the SSC region
are not Compton dominated. The objects lying above the SSC region, radiating
likely via the External radiation Compton (ERC) process, are Flat Spectrum
Radio Quasars and IBLs/LBLs. All of the IBLs/LBLs in the ERC region show a
significant Compton dominance.Comment: Contribution to the Workshop SciNeGHe 2009/Gamma-ray Physics in the
LHC era (Assisi - Italy, Oct. 7-9 2009
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&
HESS J1632-478: an energetic relic
HESS J1632-478 is an extended and still unidentified TeV source in the
galactic plane. In order to identify the source of the very high energy
emission and to constrain its spectral energy distribution, we used a deep
observation of the field obtained with XMM-Newton together with data from
Molonglo, Spitzer and Fermi to detect counterparts at other wavelengths. The
flux density emitted by HESS J1632-478 peaks at very high energies and is more
than 20 times weaker at all other wavelengths probed. The source spectrum
features two large prominent bumps with the synchrotron emission peaking in the
ultraviolet and the external inverse Compton emission peaking in the TeV. HESS
J1632-478 is an energetic pulsar wind nebula with an age of the order of 10^4
years. Its bolometric (mostly GeV-TeV) luminosity reaches 10% of the current
pulsar spin down power. The synchrotron nebula has a size of 1 pc and contains
an unresolved point-like X-ray source, probably the pulsar with its wind
termination shock.Comment: A&A accepted, 9 pages, 5 figures, 4 table
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
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
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&
Log-parabolic spectra and particle acceleration in blazars. III: SSC emission in the TeV band from Mkn 501
Curved broad-band spectral distributions of non-thermal sources like blazars
are described well by a log-parabolic (LP) law where the second degree term
measures the curvature. LP energy spectra can be obtained for relativistic
electrons by means of a statistical acceleration mechanism whose probability of
acceleration depends on energy. In this paper we compute the spectra radiated
by an electron population via synchrotron (S) and Synchro-Self Compton(SSC)
processes to derive the relations between the LP parameters. These spectra were
obtained by means of an accurate numerical code. We found that the ratio
between the curvature parameters of the S spectrum to that of the electrons is
equal to about 0.2 instead of 0.25, the value foreseen in the delta
approximation. Inverse Compton spectra are also intrinsically curved and can be
approximated by a log-parabola only in limited ranges. The curvature parameter,
estimated around the SED peak, may vary from a lower value than that of the S
spectrum up to that of emitting electrons depending on whether the scattering
is in the Thomson or in the Klein-Nishina regime. We applied this analysis to
computing the SSC emission from the BL Lac object Mkn 501 during the large
flare of April 1997. We fit simultaneous BeppoSAX and CAT data and reproduced
intensities and spectral curvatures of both components with good accuracy. The
large curvature observed in the TeV range was found to be mainly intrinsic, and
therefore did not require a large pair production absorption against the
extragalactic background. We regard this finding as an indication that the
Universe is more transparent at these energies than previously assumed by
several models found in the literature. This conclusion is supported by recent
detection of two relatively high redshift blazars with H.E.S.S.Comment: Comments: 12 pages, 11 figures. Accepted for publication in the
Astronomy and Astrophysic
Organic farming systems for adaptation to and mitigation of climate change: Effects on soil fertility and resource use efficiency
Organic farming is pointed as one of the most sustainable farming practices in terms of environmental sustainability and climate change mitigation potential. At the core of organic farming practices there are practices aimed at improving soil fertility, increasing soil C content and enhancing system biodiversity. A long-term field experiment (LTE) (MASCOT) was started on 2001 in San Piero a Grado, Pisa (Italy) with the aim to compare two different cropping systems, one managed organically and one conventionally, in terms of agronomical, economic and environmental sustainability. In 2016, the MASCOT was redesigned as a full system trial and the organic system was reshaped according to up to date agroecological standards. Climate change adaptation capacity of the two systems is being assessed through agronomic and economic parameters, whilst greenhouse gas emission mitigation potential is mainly expressed in terms of soil C sequestration
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