5,565 research outputs found
Subsampling needlet coefficients on the sphere
In a recent paper, we analyzed the properties of a new kind of spherical
wavelets (called needlets) for statistical inference procedures on spherical
random fields; the investigation was mainly motivated by applications to
cosmological data. In the present work, we exploit the asymptotic uncorrelation
of random needlet coefficients at fixed angular distances to construct
subsampling statistics evaluated on Voronoi cells on the sphere. We illustrate
how such statistics can be used for isotropy tests and for bootstrap estimation
of nuisance parameters, even when a single realization of the spherical random
field is observed. The asymptotic theory is developed in detail in the high
resolution sense.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ164 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Asymptotics for spherical needlets
We investigate invariant random fields on the sphere using a new type of
spherical wavelets, called needlets. These are compactly supported in frequency
and enjoy excellent localization properties in real space, with
quasi-exponentially decaying tails. We show that, for random fields on the
sphere, the needlet coefficients are asymptotically uncorrelated for any fixed
angular distance. This property is used to derive CLT and functional CLT
convergence results for polynomial functionals of the needlet coefficients:
here the asymptotic theory is considered in the high-frequency sense. Our
proposals emerge from strong empirical motivations, especially in connection
with the analysis of cosmological data sets.Comment: Published in at http://dx.doi.org/10.1214/08-AOS601 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
How alternative food networks work in a metropolitan area? An analysis of Solidarity Purchase Groups in Northern Italy
Our paper focuses on Solidarity Purchase Group (SPG) participants located in a highly urbanized area, with the aim to investigate the main motivations underlining their participation in a SPG and provide a characterization of them. To this end, we carried out a survey of 795 participants involved in 125 SPGs in the metropolitan area of Milan (Italy). Taking advantage of a questionnaire with 39 questions, we run a factor analysis and a two-step cluster analysis to identify different profiles of SPG participants. Our results show that the system of values animating metropolitan SPG practitioners does not fully conform to that traditionally attributed to an alternative food network (AFN). In fact, considerations linked to food safety and healthiness prevail on altruistic motives such as environmental sustainability and solidarity toward small producers. Furthermore, metropolitan SPGs do not consider particularly desirable periurban and local food products. Observing the SPGs from this perspective, it emerges as such initiatives can flourish also in those places where the lack of connection with the surrounding territory is counterbalanced by the high motivation to buy products from trusted suppliers who are able to guarantee genuine and safe products, not necessarily located nearby
Compressed correlation functions and fast aging dynamics in metallic glasses
We present x-ray photon correlation spectroscopy measurements of the atomic
dynamics in a Zr67Ni33 metallic glass, well below its glass transition
temperature. We find that the decay of the density fluctuations can be well
described by compressed, thus faster than exponential, correlation functions
which can be modeled by the well-known Kohlrausch-Williams-Watts function with
a shape exponent {\beta} larger than one. This parameter is furthermore found
to be independent of both waiting time and wave-vector, leading to the
possibility to rescale all the correlation functions to a single master curve.
The dynamics in the glassy state is additionally characterized by different
aging regimes which persist in the deep glassy state. These features seem to be
universal in metallic glasses and suggest a non diffusive nature of the
dynamics. This universality is supported by the possibility of describing the
fast increase of the structural relaxation time with waiting time using a
unique model function, independently of the microscopic details of the system.Comment: 7 pages, 4 figures. To be published in J. Chem. Phy
New flaring of an ultraluminous X-ray source in NGC 1365
We have studied a highly variable ultraluminous X-ray source (ULX) in the
Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations
between 2002 and 2006. In 2006 April, the source peaked at a luminosity ~ 3 x
10^{40} erg/s in the 0.3-10 keV band (similar to the maximum luminosity found
by ASCA in 1995), and declined on an e-folding timescale ~ 3 days. The X-ray
spectrum is always dominated by a broad power-law-like component. When the
source is seen at X-ray luminosities ~ 10^{40} erg/s, an additional soft
thermal component (which we interpret as emission from the accretion disk)
contributes ~ 1/4 of the X-ray flux; when the luminosity is higher, ~ 3 x
10^{40} erg/s, the thermal component is not detected and must contribute < 10%
of the flux. At the beginning of the decline, ionized absorption is detected
around 0.5-2 keV; it is a possible signature of a massive outflow. The
power-law is always hard, with a photon index Gamma ~ 1.7 (and even flatter at
times), as is generally the case with bright ULXs. We speculate that this
source and perhaps most other bright ULXs are in a high/hard state: as the
accretion rate increases well above the Eddington limit, more and more power is
extracted from the inner region of the inflow through non-radiative channels,
and is used to power a Comptonizing corona, jet or wind. The observed thermal
component comes from the standard outer disk; the transition radius between
outer standard disk and Comptonizing inner region moves further out and to
lower disk temperatures as the accretion rate increases. This produces the
observed appearance of a large, cool disk. Based on X-ray luminosity and
spectral arguments, we suggest that this accreting black hole has a likely mass
~ 50-150 Msun (even without accounting for possible beaming).Comment: 14 pages, to appear in MNRA
Rotation in galaxy clusters from MUSIC simulations with the kinetic Sunyaev-Zel'dovich effect
We propose in this work its application for the detection of possible coherent rotational motions in the hot intra-cluster medium. We select a sample of massive, relaxed and rotating galaxy clusters from Marenostrum-mUltidark SImulations of galaxy Clusters (MUSIC), and we produce mock maps of the temperature distortion produced by the kinetic Sunyaev-Zel'dovich effect by exploring six different lines of sight, in the best observational condition. These maps are compared with the expected signal computed from a suitable theoretical model in two cases: (i) focusing only on the contribution from the rotation, and (ii) accounting also for the cluster bulk motion. We find that the parameters of the model assumed for the radial profile of the rotational velocity, averaged over the considered lines of sight, are in agreement within two standard deviations at most with independent estimates from the simulation data, without being significantly affected by the presence of the cluster bulk term. The amplitude of the rotational signal is, on average, of the order of 23 per cent of the total signal accounting also for the cluster bulk motion, and its values are consistent with the literature. The projected bulk velocity of the cluster is also recovered at the different lines of sight, with values in agreement with the simulation dataASB acknowledges funding from Sapienza UniversitĂ di Roma - Progetti per Avvio alla Ricerca Anno 2017, prot. AR11715C82402BC
Comparative Evaluation of Packet Classification Algorithms for Implementation on Resource Constrained Systems
This paper provides a comparative evaluation of a number of known classification algorithms that have been considered for both software and hardware implementation. Differently from other sources, the comparison has been carried out on implementations based on the same principles and design choices. Performance measurements are obtained by feeding the implemented classifiers with various traffic traces in the same test scenario. The comparison also takes into account implementation feasibility of the considered algorithms in resource constrained systems (e.g. embedded processors on special purpose network platforms). In particular, the comparison focuses on achieving a good compromise between performance, memory usage, flexibility and code portability to different target platforms
Network Virtual Machine (NetVM): A New Architecture for Efficient and Portable Packet Processing Applications
A challenge facing network device designers, besides increasing the speed of network gear, is improving its programmability in order to simplify the implementation of new applications (see for example, active networks, content networking, etc). This paper presents our work on designing and implementing a virtual network processor, called NetVM, which has an instruction set optimized for packet processing applications, i.e., for handling network traffic. Similarly to a Java Virtual Machine that virtualizes a CPU, a NetVM virtualizes a network processor. The NetVM is expected to provide a compatibility layer for networking tasks (e.g., packet filtering, packet counting, string matching) performed by various packet processing applications (firewalls, network monitors, intrusion detectors) so that they can be executed on any network device, ranging from expensive routers to small appliances (e.g. smart phones). Moreover, the NetVM will provide efficient mapping of the elementary functionalities used to realize the above mentioned networking tasks upon specific hardware functional units (e.g., ASICs, FPGAs, and network processing elements) included in special purpose hardware systems possibly deployed to implement network devices
Time Driven Priority Router Implementation and First Experiments
This paper reports on the implementation of Time-Driven Priority (TDP) scheduling on a FreeBSD platform. This work is part of a TDP prototyping and demonstration project aimed at showing the implications of TDP deployment in packet-switched networks, especially benefits for real-time applications. This paper focuses on practical aspects related to the implementation of the technology on a Personal Computer (PC)-based router and presents the experimental results obtained on a testbed network. The basic building blocks of a TDP router are described and implementation choices are discussed. The relevant results achieved and here presented can be categorized into two types: qualitative results, including the successful integration of all needed blocks and the insight obtained on the complexity related to the implementation of a TDP router, and quantitative ones, including measures of achievable network utilization and of jitter experienced on a fully-loaded TDP network. The outcome demonstrates the effectiveness of the presented implementation while confirming TDP points of strengt
Spherical Needlets for CMB Data Analysis
We discuss Spherical Needlets and their properties. Needlets are a form of
spherical wavelets which do not rely on any kind of tangent plane approximation
and enjoy good localization properties in both pixel and harmonic space;
moreover needlets coefficients are asymptotically uncorrelated at any fixed
angular distance, which makes their use in statistical procedures very
promising. In view of these properties, we believe needlets may turn out to be
especially useful in the analysis of Cosmic Microwave Background (CMB) data on
the incomplete sky, as well as of other cosmological observations. As a final
advantage, we stress that the implementation of needlets is computationally
very convenient and may rely completely on standard data analysis packages such
as HEALPix.Comment: 7 pages, 7 figure
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