823 research outputs found
Characterizing the nature of Fossil Groups with XMM
We present an X-ray follow-up, based on XMM plus Chandra, of six Fossil Group
(FG) candidates identified in our previous work using SDSS and RASS data. Four
candidates (out of six) exhibit extended X-ray emission, confirming them as
true FGs. For the other two groups, the RASS emission has its origin as either
an optically dull/X-ray bright AGN, or the blending of distinct X-ray sources.
Using SDSS-DR7 data, we confirm, for all groups, the presence of an r-band
magnitude gap between the seed elliptical and the second-rank galaxy. However,
the gap value depends, up to 0.5mag, on how one estimates the seed galaxy total
flux, which is greatly underestimated when using SDSS (relative to Sersic)
magnitudes. This implies that many FGs may be actually missed when using SDSS
data, a fact that should be carefully taken into account when comparing the
observed number densities of FGs to the expectations from cosmological
simulations. The similarity in the properties of seed--FG and non-fossil
ellipticals, found in our previous study, extends to the sample of X-ray
confirmed FGs, indicating that bright ellipticals in FGs do not represent a
distinct population of galaxies. For one system, we also find that the velocity
distribution of faint galaxies is bimodal, possibly showing that the system
formed through the merging of two groups. This undermines the idea that all
selected FGs form a population of true fossils.Comment: 9 pages, 3 figures. Submitted 01/12/2011 to MNRAS, referee report
received 21/02/2012, accepted 22/02/201
A nearly zero-energy microgrid testbed laboratory: Centralized control strategy based on SCADA system
Currently, despite the use of renewable energy sources (RESs), distribution networks are facing problems, such as complexity and low productivity. Emerging microgrids (MGs) with RESs based on supervisory control and data acquisition (SCADA) are an effective solution to control, manage, and finally deal with these challenges. The development and success of MGs is highly dependent on the use of power electronic interfaces. The use of these interfaces is directly related to the progress of SCADA systems and communication infrastructures. The use of SCADA systems for the control and operation of MGs and active distribution networks promotes productivity and efficiency. This paper presents a real MG case study called the LAMBDA MG testbed laboratory, which has been implemented in the electrical department of the Sapienza University of Rome with a centralized energy management system (CEMS). The real-time results of the SCADA system show that a CEMS can create proper energy balance in a LAMBDA MG testbed and, consequently, minimize the exchange power of the LAMBDA MG and main grid
WATCAT: a tale of wide-angle tailed radio galaxies
We present a catalog of 47 wide-angle tailed radio galaxies (WATs), the
WATCAT; these galaxies were selected by combining observations from the
National Radio Astronomy Observatory/Very Large Array Sky Survey (NVSS), the
Faint Images of the Radio Sky at Twenty-Centimeters (FIRST), and the Sloan
Digital Sky Survey (SDSS), and mainly built including a radio morphological
classification. We included in the catalog only radio sources showing two-sided
jets with two clear "warmspots" (i.e., jet knots as bright as 20% of the
nucleus) lying on the opposite side of the radio core, and having classical
extended emission resembling a plume beyond them. The catalog is limited to
redshifts z 0.15, and lists only sources with radio emission extended
beyond 30 kpc from the host galaxy. We found that host galaxies of WATCAT
sources are all luminous (-20.5 Mr -23.7), red early-type
galaxies with black hole masses in the range M M. The spectroscopic classification indicates that they
are all low-excitation galaxies (LEGs). Comparing WAT multifrequency properties
with those of FRI and FRII radio galaxies at the same redshifts, we conclude
that WATs show multifrequency properties remarkably similar to FRI radio
galaxies, having radio power of typical FRIIs
The XMM deep survey in the CDF-S. X. X-ray variability of bright sources
We aim to study the variability properties of bright hard X-ray selected
Active Galactic Nuclei (AGN) with redshift between 0.3 and 1.6 detected in the
Chandra Deep Field South (XMM-CDFS) by a long XMM observation. Taking advantage
of the good count statistics in the XMM CDFS we search for flux and spectral
variability using the hardness ratio techniques. We also investigated spectral
variability of different spectral components. The spectra were merged in six
epochs (defined as adjacent observations) and in high and low flux states to
understand whether the flux transitions are accompanied by spectral changes.
The flux variability is significant in all the sources investigated. The
hardness ratios in general are not as variable as the fluxes. Only one source
displays a variable HR, anti-correlated with the flux (source 337). The
spectral analysis in the available epochs confirms the steeper when brighter
trend consistent with Comptonisation models only in this source. Finding this
trend in one out of seven unabsorbed sources is consistent, within the
statistical limits, with the 15 % of unabsorbed AGN in previous deep surveys.
No significant variability in the column densities, nor in the Compton
reflection component, has been detected across the epochs considered. The high
and low states display in general different normalisations but consistent
spectral properties. X-ray flux fluctuations are ubiquitous in AGN. In general,
the significant flux variations are not associated with a spectral variability:
photon index and column densities are not significantly variable in nine out of
the ten AGN over long timescales (from 3 to 6.5 years). The photon index
variability is found only in one source (which is steeper when brighter) out of
seven unabsorbed AGN. These results are consistent with previous deep samples.Comment: 14 pages, 11 figures. Accepted in A&
Individual optical variability of Active Galactic Nuclei from the MEXSAS2 sample
Most of the variability studies of active galactic nuclei (AGNs) are based on
ensemble analyses. Nevertheless, it is interesting to provide estimates of the
individual variability properties of each AGN, in order to relate them with
intrinsic physical quantities. A useful dataset is provided by the Catalina
Surveys Data Release 2 (CSDR2), which encompasses almost a decade of
photometric measurements of million objects repeatedly observed
hundreds of times. We aim to investigate the individual optical variability
properties of 795 AGNs originally included in the Multi-Epoch XMM Serendipitous
AGN Sample 2 (MEXSAS2). Our goals consist in: (i) searching for correlations
between variability and AGN physical quantities; (ii) extending our knowledge
of the variability features of MEXSAS2 from the X-ray to the optical. We use
the structure function (SF) to analyse AGN flux variations. We model the SF as
a power-law, , and we compute its
variability parameters. We introduce the V-correction as a simple tool to
correctly quantify the amount of variability in the rest frame of each source.
We find a significant decrease of variability amplitude with increasing
bolometric, optical and X-ray luminosity. We obtain the indication of an
intrinsically weak positive correlation between variability amplitude and
redshift, . Variability amplitude is also positively correlated with
. The slope of the SF, , is weakly correlated with
the bolometric luminosity and/or with the black hole mass
. When comparing optical to X-ray variability properties, we find
that X-ray variability amplitude is approximately the same for those AGNs with
larger or smaller variability amplitude in the optical. On the contrary, AGNs
with steeper SF in the optical do present steeper SF in the X-ray, and vice
versa.Comment: 13 pages, 14 figures, 3 tables, accepted for publication in MNRA
Diachronic and Synchronic Analysis for Knowledge Creation: Architectural Representation Geared to XR Building Archaeology (Claudius-Anio Novus Aqueduct in Tor Fiscale, the Appia Antica Archaeological Park)
This study summarises research progress to identify appropriate quality methodologies for representing, interpreting, and modelling complex contexts such as the Claudian Aqueduct in the Appian Way Archaeological Park. The goal is to intrinsically integrate (embed) geometric survey (Laser scanning and photogrammetric) with the materials and construction techniques (Stratigraphic Units-SU), semantic models in order to support the design with a better understanding of the artefact considered, and also to give indications that can be implemented in the future in a continuous cognitive process. Volume stratigraphic units in the form of architectural drawings, heritage building information modelling (HBIM) and extended reality (XR) environments have been oriented to comparative analyses based on the research case study's complex morphology. Analysis of geometries' intersection, construction techniques and materials open up new cognitive scenarios, self-feeding a progressive knowledge and making different studies correlatable, avoiding diaspora or incommunicability. Finally, an extended reality (XR) platform aims to enhance tangible and intangible values through new human-computer interaction and information sharing levels
Demography of obscured and unobscured AGN: prospects for a Wide Field X-ray Telescope
We discuss some of the main open issues in the evolution of Active Galactic
Nuclei which can be solved by the sensitive, wide area surveys to be performed
by the proposed Wide Field X-ray Telescope mission.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009. To appear in Memorie della Societa'
Astronomica Italiana 2010 (arXiv:1010.5889
Spinal motoneurons of the human newborn are highly synchronized during leg movements
Motoneurons of neonatal rodents show synchronous activity that modulates the development of the neuromuscular system. However, the characteristics of the activity of human neonatal motoneurons are largely unknown. Using a noninvasive neural interface, we identified the discharge timings of individual spinal motoneurons in human newborns. We found highly synchronized activities of motoneurons of the tibialis anterior muscle, which were associated with fast leg movements. Although neonates' motor units exhibited discharge rates similar to those of adults, their synchronization was significantly greater than in adults. Moreover, neonatal motor units showed coherent oscillations in the delta band, which is directly translated into force generation. These results suggest that motoneuron synchronization in human neonates might be an important mechanism for controlling fast limb movements, such as those of primitive reflexes. In addition to help revealing mechanisms of development, the proposed neural interface might monitor children at risk of developing motor disorders
The connection between star formation and supermassive Black Hole activity in the local Universe
We present a study of the active galactic nucleus (AGN) activity in the local
Universe (z < 0.33) and its correlation with the host galaxy properties,
derived from a Sloan Digital Sky Survey (SDSS DR8) sample with spectroscopic
star-formation rate (SFR) and stellar mass ()
determination. To quantify the level of AGN activity we used X-ray information
from the XMM-Newton Serendipitous Source Catalogue (3XMM DR8). Applying
multiwavelength AGN selection criteria (optical BPT-diagrams, X-ray/optical
ratio etc) we found that 24% of the detected sources are efficiently-accreting
AGN with moderate-to-high X-ray luminosity, which are twice as likely to be
hosted by star-forming galaxies than by quiescent ones. The distribution of the
specific Black Hole accretion rate (sBHAR, ) shows
that nuclear activity in local, non-AGN dominated galaxies peaks at very low
accretion rates () in all
stellar mass ranges. However, we observe systematically larger values of sBHAR
for galaxies with active star-formation than for quiescent ones, as well as an
increase of the mean with SFR for both star-forming
and quiescent galaxies. These findings confirm the decreased level of AGN
activity with cosmic time and are consistent with a scenario where both
star-formation and AGN activity are fuelled by a common gas reservoir.Comment: 20 pages, 20 figures, accepted for publication in MNRA
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