21,169 research outputs found
Deep Surveys of Massive Black Holes with LISA
Massive black hole binary systems are among the most interesting sources for
the Laser Interferometer Space Antenna (LISA); gravitational radiation emitted
during the last year of in-spiral could be detectable with a very large
signal-to-noise ratio for sources at cosmological distance. Here we discuss the
impact of LISA for astronomy and cosmology; we review our present understanding
of the relevant issues, and highlight open problems that deserve further
investigations.Comment: 10 pages, 2 figures, Third Amaldi Conference on Gravitational Wave
Analysis of the optical background sources in the ANTARES experiment and preliminary studies related to a larger scale detector
Il lavoro di tesi presentato si svolge nell'ambito dell'esperimento ANTARES, che prevede la costruzione nel Mediterraneo di un telescopio per
la rivelazione di neutrini da sorgenti astrofisiche e mantiene una collaborazione con altri settori della scienza che studiano le
profonditĂ del mare. Il telescopio sfrutterĂ l'acqua del mare come mezzo Cherenkov per rivelare le particelle cariche prodotte dall'interazione dei neutrini con il mare stesso (o il fondale), e rivelerĂ la luce emessa tramite un array di fotomoltiplicatori
distribuiti su dodici stringhe immerse a 2.4 km di profondità . L'energia di soglia per la ricostruzione di un evento è stimata
intorno a 50 GeV e si calcola che la massima superficie efficace del telescopio sia dell'ordine di 0.1 km^2. Sorgenti di fondo per l'esperimento sono i neutrini e i muoni atmosferici, ma è presente anche un " fondo ottico" costituito dalla bioluminescenza e
dalla luce Cherenkov risultante dai prodotti di decadimenti radioattivi dell'isotopo 40K presente nell'acqua.
In questo lavoro di tesi è affrontato il problema del fondo ottico dell'esperimento, in particolar modo è stata effettuata un'analisi delle caratteristiche del fondo da bioluminescenza a partire dai dati disponibili grazie all'ultima stringa di test dell'esperimento, la Prototype Sector Line. Una migliore comprensione del fondo può aiutare
ad ottimizzare gli algoritmi di trigger per ridurre il tempo morto del detector o aumentare l'efficacia della selezione. L'analisi presentata individua i parametri utili per simulare in maniera appropriata il fondo e valuta l'accuratezza delle simulazioni ora disponibili. La stessa analisi presenta inoltre interessanti risvolti nell'ambito della biologia, in quanto permette di estrarre informazioni sulla fauna
bioluminescente del Mediterraneo, fino ad ora poco studiata.
In vista dell'estensione dell'esperimento ad una fase ancora piÚ promettente, con un telescopio dal volume sensibile dell'ordine del km^3, è stato inoltre tentato uno studio preliminare sulle potenzialità di un nuovo
tipo di rivelatore di luce, l'Hybrid PhotoDiode (HPD), in sostituzione dei classici fotomoltiplicatori. Mediante un programma di simulazione dell'ottica elettronica è stato riprodotto un possibile modello di HPD in grado di
soddisfare i requisiti dell'esperimento. I risultati di tale simulazione sono stati poi introdotti nel Monte Carlo dell'esperimento, per
quantificarne (benchĂŠ grossolanamente) gli effetti sulla ricostruzione degli eventi
gamma-ray DBSCAN: a clustering algorithm applied to Fermi-LAT gamma-ray data. I. Detection performances with real and simulated data
The Density Based Spatial Clustering of Applications with Noise (DBSCAN) is a
topometric algorithm used to cluster spatial data that are affected by
background noise. For the first time, we propose the use of this method for the
detection of sources in gamma-ray astrophysical images obtained from the
Fermi-LAT data, where each point corresponds to the arrival direction of a
photon. We investigate the detection performance of the gamma-ray DBSCAN in
terms of detection efficiency and rejection of spurious clusters, using a
parametric approach, and exploring a large volume of the gamma-ray DBSCAN
parameter space. By means of simulated data we statistically characterize the
gamma-ray DBSCAN, finding signatures that differentiate purely random fields,
from fields with sources. We define a significance level for the detected
clusters, and we successfully test this significance with our simulated data.
We apply the method to real data, and we find an excellent agreement with the
results obtained with simulated data. We find that the gamma-ray DBSCAN can be
successfully used in the detection of clusters in gamma-ray data. The
significance returned by our algorithm is strongly correlated with that
provided by the Maximum Likelihood analysis with standard Fermi-LAT software,
and can be used to safely remove spurious clusters. The positional accuracy of
the reconstructed cluster centroid compares to that returned by standard
Maximum Likelihood analysis, allowing to look for astrophysical counterparts in
narrow regions, minimizing the chance probability in the counterpart
association. We find that gamma-ray DBSCAN is a powerful tool in the detection
of clusters in gamma-ray data, this method can be used both to look for
point-like sources, and extended sources, and can be potentially applied to any
astrophysical field related with detection of clusters in data.Comment: Accepted for publication in A&
European and United States farmersâ markets: similarities, differences and potential developments
Farmersâ markets have always been the usual way of buying and selling rural products in the Western world. With the advent of supermarkets, farmersâ markets rapidly disappeared in many nations. However, in countries such as France and Italy, which place a high priority on food origin and regional specialisation, some farmersâ markets continued to exist, partly due to their mechanisms to identify and promote locally grown foods. The consumerâs desire to re-establish a bond with local food products, local growers and producers, together with the growing concern for food freshness and healthiness have been key drivers for the renaissance of farmersâ markets occurred in the latest years in many European countries and in the United States. Several studies, conducted across Europe and in the U.S., have shown rising consumersâ consideration of farmersâ markets as important sources of household food shopping and increasing attention of farmers to this sale channel, in a period in which their share of the âfood dollarâ is continuing to decrease. The objectives of this paper are to contribute to extend understanding of the main features of the European and United States farmersâ markets and raise critical questions about their potential development in the modern food system.Farmersâ markets, Local foods, Europe, U.S.A., Agricultural and Food Policy,
Private Transport Access Among Older People: Identifying The Disadvantaged
Private transport is important in enabling older people living in the community to maintain their independence and social networks. Access to this resource remains a major concern for older people. This study examines the demographic risk factors that restrict older people's access to private transport. The findings lead to policy recommendations directed towards self-reliance. Analysis, based on the study's household survey consisting of a sample of noninstitutionalised older Gold Coast people (N=401), reveals that there is a sizable group (29%) who do not drive. Of single older women, 21% report that the inability to drive causes significant hardship. Being female, aged over 80 years, receiving a full government pension and possessing a disability are significant factors to the inability to drive. Within coupled households the preference for male drivers may lead to the depreciation of women's driving skills. Since these women are likely to become widowed, they eventually lose their primary source of transport. Programmes delaying the surrender of licences - such as campaigns encouraging married older women not to surrender their driver licences prematurely - will alleviate the pressure of the growing demand for government subsidised transport services and promote greater independence among the older people.
Gravitational waves and pulsar timing: stochastic background, individual sources and parameter estimation
Massive black holes are key ingredients of the assembly and evolution of
cosmic structures. Pulsar Timing Arrays (PTAs) currently provide the only means
to observe gravitational radiation from massive black hole binary systems with
masses >10^7 solar masses. The whole cosmic population produces a signal
consisting of two components: (i) a stochastic background resulting from the
incoherent superposition of radiation from the all the sources, and (ii) a
handful of individually resolvable signals that raise above the background
level and are produced by sources sufficiently close and/or massive.
Considering a wide range of massive black hole binary assembly scenarios, we
investigate both the level and shape of the background and the statistics of
resolvable sources. We predict a characteristic background amplitude in the
interval h_c(f = 10^-8 Hz)~5*10^-16 - 5*10^-15, within the detection range of
the complete Parkes PTA. We also quantify the capability of PTAs of measuring
the parameters of individual sources, focusing on monochromatic signals
produced by binaries in circular orbit. We investigate how the results depend
on the number and distribution of pulsars in the array, by computing the
variance-covariance matrix of the parameter measurements. For plausible Square
Kilometre Array (SKA) observations (100 pulsars uniformly distributed in the
sky), and assuming a coherent signal-to-noise ratio of 10, the sky position of
massive black hole binaries can be located within a ~40deg^2 error box, opening
promising prospects for detecting a putative electromagnetic counterpart to the
gravitational wave emission. The planned SKA, can plausibly observe these
unique systems, although the number of detections is likely to be small.
(Abridged)Comment: 11 pages, 4 figures. Submitted to CQGra. Proceedings to the AMALDI8
conferenc
Measuring the parameters of massive black hole binary systems with Pulsar Timing Array observations of gravitational waves
The observation of massive black hole binaries (MBHBs) with Pulsar Timing
Arrays (PTAs) is one of the goals of gravitational wave astronomy in the coming
years. Massive (>10^8 solar masses) and low-redshift (< 1.5) sources are
expected to be individually resolved by up-coming PTAs, and our ability to use
them as astrophysical probes will depend on the accuracy with which their
parameters can be measured. In this paper we estimate the precision of such
measurements using the Fisher-information-matrix formalism. We restrict to
"monochromatic" sources. In this approximation, the system is described by
seven parameters and we determine their expected statistical errors as a
function of the number of pulsars in the array, the array sky coverage, and the
signal-to-noise ratio (SNR) of the signal. At fixed SNR, the gravitational wave
astronomy capability of a PTA is achieved with ~20 pulsars; adding more pulsars
(up to 1000) to the array reduces the source error-box in the sky \Delta\Omega
by a factor ~5 and has negligible consequences on the statistical errors on the
other parameters. \Delta\Omega improves as 1/SNR^2 and the other parameters as
1/SNR. For a fiducial PTA of 100 pulsars uniformly distributed in the sky and a
coherent SNR = 10, we find \Delta\Omega~40 deg^2, a fractional error on the
signal amplitude of ~30% (which constraints only very poorly the chirp mass -
luminosity distance combination M_c^{5/3}/D_L), and the source inclination and
polarization angles are recovered at the ~0.3 rad level. The ongoing Parkes PTA
is particularly sensitive to systems located in the southern hemisphere, where
at SNR = 10 the source position can be determined with \Delta\Omega ~10 deg^2,
but has poorer performance for sources in the northern hemisphere. (Abridged)Comment: 20 pages, 12 figures, 2 color figures, submitted to Phys. Rev.
Searching for continuous gravitational wave sources in binary systems
We consider the problem of searching for continuous gravitational wave
sources orbiting a companion object. This issue is of particular interest
because the LMXB's, and among them Sco X-1, might be marginally detectable with
2 years coherent observation time by the Earth-based laser interferometers
expected to come on line by 2002, and clearly observable by the second
generation of detectors. Moreover, several radio pulsars, which could be deemed
to be CW sources, are found to orbit a companion star or planet, and the
LIGO/VIRGO/GEO network plans to continuously monitor such systems. We estimate
the computational costs for a search launched over the additional five
parameters describing generic elliptical orbits using match filtering
techniques. These techniques provide the optimal signal-to-noise ratio and also
a very clear and transparent theoretical framework. We provide ready-to-use
analytical expressions for the number of templates required to carry out the
searches in the astrophysically relevant regions of the parameter space, and
how the computational cost scales with the ranges of the parameters. We also
determine the critical accuracy to which a particular parameter must be known,
so that no search is needed for it. In order to disentangle the computational
burden involved in the orbital motion of the CW source, from the other source
parameters (position in the sky and spin-down), and reduce the complexity of
the analysis, we assume that the source is monochromatic and its location in
the sky is exactly known. The orbital elements, on the other hand, are either
assumed to be completely unknown or only partly known. We apply our theoretical
analysis to Sco X-1 and the neutron stars with binary companions which are
listed in the radio pulsar catalogue.Comment: 31 pages, LaTeX, 6 eps figures, submitted to PR
Robust Moment Closure Method for the Chemical Master Equation
The Chemical Master Equation (CME) is used to stochastically model
biochemical reaction networks, under the Markovian assumption. The low-order
statistical moments induced by the CME are often the key quantities that one is
interested in. However, in most cases, the moments equation is not closed; in
the sense that the first moments depend on the higher order moments, for
any positive integer . In this paper, we develop a moment closure technique
in which the higher order moments are approximated by an affine function of the
lower order moments. We refer to such functions as the affine Moment Closure
Functions (MCF) and prove that they are optimal in the worst-case context, in
which no a priori information on the probability distribution is available.
Furthermore, we cast the problem of finding the optimal affine MCF as a linear
program, which is tractable. We utilize the affine MCFs to derive a finite
dimensional linear system that approximates the low-order moments. We quantify
the approximation error in terms of the induced norm of some
linear system. Our results can be effectively used to approximate the low-order
moments and characterize the noise properties of the biochemical network under
study
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