140 research outputs found
On the low detection efficiency of disk water megamasers in Seyfert 2 AGN
Disk megamasers are a unique tool to study active galactic nuclei (AGN)
sub-pc environment, and precisely measure some of their fundamental parameters.
While the majority of disk megamasers are hosted in heavily obscured (i.e.,
Seyfert 2, Sy2) AGN, the converse is not true, and disk megamasers are very
rarely found even in obscured AGN. The very low detection rate of such systems
in Sy2 AGN could be due to the geometry of the maser beaming, which requires a
strict edge-on condition. We explore some other fundamental factors which could
play a role in a volume-limited survey of disk megamasers in Sy2 galaxies, most
importantly the radio luminosity.Comment: 2 pages, 2 figures. To appear in the Proceedings IAU Symposium No.
336, 2017 "Astrophysical Masers: Unlocking the Mysteries of the Universe
Metodologie innovative per il supporto alla progettazione e all’analisi delle prestazioni di impianti ad energia solare a concentrazione ed eolici off-shore utilizzando immagini satellitari ottiche e SAR
In questo lavoro presentiamo delle nuove metodologie, sviluppate nell’ambito del progetto SATENERG
(Servizi sATellitari per le ENergie Rinnovabili di nuova Generazione) finanziato dall’Agenzia
Spaziale Italiana, sia per il supporto alla progettazione/pianificazione che per il monitoraggio
quasi in tempo reale e l’analisi delle prestazioni degli impianti ad energia rinnovabile di nuova
generazione (CSP, CPV ed eolici off-shore) utilizzando immagini satellitari.
In particolare per quanto riguarda gli impianti solari a concentrazione (CSP e CPV), abbiamo
sviluppato un metodo per ricavare l’irradianza solare incidente al suolo (in particolare la sua
componente diretta normale rispetto ai raggi solari, fondamentale per questo tipo di impianti) da
immagini ottiche satellitari. Ciò, unito ad un modello di funzionamento di tali impianti e degli
inverter, ci ha resi in grado di poter sviluppare un servizio di supporto nella progettazione e pianificazione
di nuove costruzioni di impianti CSP e CPV (analizzando serie storiche di dati satellitari)
ed anche un servizio di monitoraggio e analisi delle prestazioni per quelli già esistenti (usando
invece immagini satellitari quasi in tempo reale).
In maniera simile, usando immagini SAR (Synthetic Aperture Radar), abbiamo sviluppato un metodo
per ricavare l’intensità e la direzione del vento in aree marine da remoto che ci ha permesso,
utilizzando anche un modello di impianto eolico off-shore (turbina e inverter), di sviluppare sia
un servizio di supporto alla progettazione/pianificazione che un servizio di monitoraggio quasi in
tempo reale della produzione di un impianto eolico off-shore.
Le prime applicazioni di queste nuove metodologie hanno già portato ad avere ottimi risultati in
vari casi di prova sia per quanto concerne il monitoraggio dell’irradianza diretta su piano normale,
in cui l’irradianza misurata e quella ricavata da dato satellitare non si sono discostate più del 10%, sia per quanto riguarda il calcolo dell’intensità e direzione del vento da immagini SAR, in cui l’errore
rispetto al dato misurato è rimasto al di sotto del 15%, fornendo quindi una buona base per il
monitoraggio della energia AC prodotta dagli impianti.In this work we present new methodologies aimed to support both planning and near-real-time
monitoring of new generation solar and wind energy plants (CSP, CPV and wind off-shore) using
satellite imagery. Such methodologies are currently being developed in the scope of SATENERG, a
project funded by ASI (Italian Space Agency).
In particular, for what concerns the concentrating solar energy plants (CSP and CPV) we developed
a method to calculate solar irradiance at ground (and its direct normal component, that has
primary importance in this type of plants) starting from satellite optical images. Then, using also
detailed opto-electronic models of the plants and inverters, we are able to calculate the producible
energy, which can be used to support either the design of potential plants (using historical series
of satellite images) or the monitoring and performance analysis of existing plants (using near-real-
time satellite imagery). Producible energy and other interesting parameters, like production
efficiency, return on investment etc., are delivered through dedicated web services.
In a similar way, we developed also a method to calculate the intensity and the direction of off-shore
wind from satellite SAR (Synthetic Aperture Radar) images that permitted us, together with
detailed models of wind turbine and inverters, to develop a new service in support to both planning
and near-real-time monitoring activities of off-shore wind plants.
The first applications of these methods gave successful results in several test cases: we obtained
a maximum error of 10% for satellite retrieved direct normal solar irradiance and a maximum
error of 15% for wind direction and intensity calculated from SAR images (with respect to in-situ
measured data)
Obscured and Compton-thick AGN in NuSTAR hard X-ray surveys
This Thesis focuses on NuSTAR hard X-ray surveys of Active Galactic Nuclei (AGN). One of the major goals of the NuSTAR mission is to study with unprecedented detail the obscured and heavily obscured (Compton-thick, CT) populations of AGN, significantly contributing to the diffuse cosmic X-ray background (CXB),and major actors within the evolutionary cycle of galaxies.
In the first part, a survey of a small sample (∼ a dozen) of local (z < 0.03) heavily obscured AGN, selected by their water megamaser emission at ∼22.23 GHz, is presented. Thanks to the NuSTAR hard X-ray spectral coverage (extending between ∼3−80 keV), a robust estimate of the CT fraction is obtained. Building up on this result and combining X-ray and radio measurements, a toy model of the dusty megamaser disk within the classical dusty torus is proposed.
A spin-off of the megamaser project is related to a well-known local obscured AGN (Mrk1210) and devoted to studying its long-term (∼17 yrs) X-ray variability with NuSTAR, from which some constraints on the AGN environment can be drawn.
Finally, the search and study of CT AGN is pushed to higher redshifts, fully exploiting the NuSTAR X-ray capabilities in terms of sensitivity. The hunt for faint and distant heavily obscured AGN is performed exploiting the combined NuSTAR, XMM-Newton and Chandra coverages of the UKIDSS-UDS field. The deep NuSTAR survey strategy is presented, along with the modeling of the NuSTAR background in order to optimize the detection of faint sources. A broadband X-ray spectroscopic analysis of all the detected sources is performed, and combined with the standard hardness ratio (HR) diagnostic, in order to select all the possible CT candidates. Such CT candidates are then analyzed again with appropriate X-ray spectral models specifically developed to deal with CT absorbers
A comprehensive X-ray view of the active nucleus in NGC 4258
(Abridged) We present a detailed broadband X-ray spectrum of NGC 4258, with
the goal of precisely measuring the coronal luminosity and accretion flow
properties of the AGN, and track any possible variation across two decades of
observations. We collect archival XMM-Newton, Chandra, Swift/BAT and NuSTAR
spectroscopic observations spanning 15 years, and fit them with a suite of
state of the art models, including a warped disk model which is suspected to
provide the well known obscuration observed in the X-rays. We complement this
information with archival results from the literature. Clear spectral
variability is observed among the different epochs. The obscuring column
density shows possibly periodic fluctuations on a timescale of 10 years, while
the intrinsic luminosity displays a long term decrease of a factor of three in
a time span of 15 years (from erg s
in the early 2000s, to erg s
in 2016). The average absorption-corrected X-ray luminosity
, combined with archival determinations of the bolometric
luminosity, implies a bolometric correction , intriguingly
typical for Seyferts powered by accretion through geometrically thin,
radiatively efficient disks. Moreover, the X-ray photon index is
consistent with the typical value of the broader AGN population. However, the
accretion rate in Eddington units is very low, well within the expected RIAF
regime. Our results suggest that NGC 4258 is a genuinely low-luminosity Seyfert
II, with no strong indications in its X-ray emission for a hot, RIAF-like
accretion flow.Comment: Accepted for publication in A&A. 14 pages, 6 figures, 3 table
The NuSTAR Extragalactic Surveys: unveiling rare, buried AGNs and detecting the contributors to the peak of the Cosmic X-ray Background
We report on the results of active galactic nuclei (AGNs) detection by NuSTAR
performed in three extragalactic survey fields (COSMOS, UDS, ECDFS) in three
hard bands, namely H1 (8-16 keV), H2 (16-24 keV) and VH (35-55 keV). The
aggregated area of the surveys is deg. While a large number of
sources is detected in the H1 band (72 at the level of reliability), the
H2 band directly probing close to the peak of the Cosmic X-ray Background (CXB)
returns four significant detections, and two tentative, although not
significant, detections are found in the VH band. All the sources detected
above 16 keV are also detected at lower energies. We compute the integral
number counts for sources in such bands, which show broad consistency with
population synthesis models of the CXB. We furthermore identify two
Compton-thick AGNs, one in the COSMOS field, associated with a hard and faint
Chandra source, and one in the UDS field, never detected in the X-ray band
before. Both sources are at the same redshift , which shifts their
Compton-hump into the H1 band, and were previously missed in the usually
employed NuSTAR bands, confirming the potential of using the H1 band to
discover obscured AGNs at in deep surveys.Comment: 12 pages, 5 figures, accepted for publication in The Astrophysical
Journa
Biological application of Compressed Sensing Tomography in the Scanning Electron Microscope
The three-dimensional tomographic reconstruction of a biological sample, namely collagen fibrils in human dermal tissue, was obtained from a set of projection-images acquired in the Scanning Electron Microscope. A tailored strategy for the transmission imaging mode was implemented in the microscope and proved effective in acquiring the projections needed for the tomographic reconstruction. Suitable projection alignment and Compressed Sensing formulation were used to overcome the limitations arising from the experimental acquisition strategy and to improve the reconstruction of the sample. The undetermined problem of structure reconstruction from a set of projections, limited in number and angular range, was indeed supported by exploiting the sparsity of the object projected in the electron microscopy images. In particular, the proposed system was able to preserve the reconstruction accuracy even in presence of a significant reduction of experimental projections
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
A Hardware-in-the-Loop Evaluation of the Impact of the V2X Channel on the Traffic-Safety Versus Efficiency Trade-offs
Vehicles are increasingly becoming connected and short-range wireless
communications promise to introduce a radical change in the drivers' behaviors.
Among the main use cases, the intersection management is surely one of those
that could mostly impact on both traffic safety and efficiency. In this work,
we consider an intersection collision warning application and exploit an
hardware-in-the-loop (HIL) platform to verify the impact on the risk of
accidents as well as the average time to travel a given distance. Besides
including real ITS-G5 compliant message exchanges, the platform also includes a
channel emulator with real signals. Results show that the risk of collisions
can be drastically reduced, with an overall trade-off between safety and
traffic efficiency. At the same time, it is shown that the presence of real
channel conditions cannot guarantee the same condition of zero-risk as with
ideal channel propagation, remarking the importance of channel conditions and
signal processing
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