131 research outputs found
Re-born fireballs in Gamma-Ray Bursts
No full textWe consider the interaction between a relativistic fireball and material assumed to be still located just outside the progenitor star. Only a small fraction of the expected mass is sufficient to decelerate the fireball efficiently, leading to dissipation of most of its kinetic energy. Since the scattering optical depths are still large at distances comparable to the progenitor radius, the dissipated energy is trapped in the system, accelerating it to relativistic velocities. The process resembles the birth of another fireball at radii R similar to 10(11) cm, not far from the transparency radius, and with starting bulk Lorentz factors Gamma(c) similar to 10. As seen in the observer frame, this `re- generated' fireball appears collimated within an angle theta = 1/Gamma(c). If the central engine works intermittently, the funnel can, at least partially, refill and the process can repeat itself. We discuss how this idea can help to solve some open issues of the more conventional internal shock scenario for interpreting gamma-ray burst propertie
La qualità dell’osso trabecolare valutata con il trabecular bone score (tbs) è fortemente alterata nelle osteoporosi secondarie.
No full textThe superficial vein-only DIEP flap
No full textWe read with interest this retrospective investigation by Nigro et al.,1 although we find that the conclusion is insufficiently supported. While discussing the results, the authors mention that “these values are not significantly different”; they refer to a statistical analysis by means of the t test, but do not present any P values
Formation And Evolution Of Disk Galaxies
No full textdel the formation of disks in centrifugal equilibrium within growing CDM halos (Avila-Reese et al. 1998; Firmani & Avila-Reese 2000). Thus, the SF history of the galaxy models is driven by the gas infall rate (cosmological accretion) and the disk gas surface density determined mainly by the spin parameter of the halo. Bulge forms from secular evolution of the stellar disk. From our models, we conclude that the main properties of disk galaxies (exponential surface Instituto de Astronom'ia, UNAM, A.P. 70-268, 04510 M'exico, D.F., M'exico Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy brightness (SB) profiles, nearly flat rotation curves, local and global color indexes, gas fractions f g , etc.) as well as their correlations (Tully-Fisher (TF) and magnitude-radius relations, the HS, etc.) are the result of the combination of 3 cosmological factors and their dispersions: mass, mass aggregation history (MAH) and . The last 2 determine the intensive prop
Sworn testimony of the model evidence: Gaussian Mixture Importance (GAME) sampling
No full textWhat is the “best” model? The answer to this question lies in part in the eyes of the beholder, nevertheless a good model must blend rigorous theory with redeeming qualities such as parsimony and quality of fit. Model selection is used to make inferences, via weighted averaging, from a set of K candidate models, (Formula presented.), and help identify which model is most supported by the observed data, (Formula presented.). Here, we introduce a new and robust estimator of the model evidence, (Formula presented.), which acts as normalizing constant in the denominator of Bayes’ theorem and provides a single quantitative measure of relative support for each hypothesis that integrates model accuracy, uncertainty, and complexity. However, (Formula presented.) is analytically intractable for most practical modeling problems. Our method, coined GAussian Mixture importancE (GAME) sampling, uses bridge sampling of a mixture distribution fitted to samples of the posterior model parameter distribution derived from MCMC simulation. We benchmark the accuracy and reliability of GAME sampling by application to a diverse set of multivariate target distributions (up to 100 dimensions) with known values of (Formula presented.) and to hypothesis testing using numerical modeling of the rainfall-runoff transformation of the Leaf River watershed in Mississippi, USA. These case studies demonstrate that GAME sampling provides robust and unbiased estimates of the evidence at a relatively small computational cost outperforming commonly used estimators. The GAME sampler is implemented in the MATLAB package of DREAM and simplifies considerably scientific inquiry through hypothesis testing and model selection.Water Resource
The peak luminosity - peak energy correlation in GRBs
No full textWe derive the peak luminosity - peak energy (L_iso - E_peak) correlation using 22 long Gamma-Ray Bursts (GRBs) with firm redshift measurements. We find that its slope is similar to the correlation between the time integrated isotropic emitted energy E_iso and E_peak (Amati et al. 2002). For the 15 GRBs in our sample with estimated jet opening angle we compute the collimation corrected peak luminosity L_gamma, and find that it correlates with E_peak. This has, however, a scatter larger than the correlation between E_peak and E_gamma (the time integrated emitted energy, corrected for collimation; Ghirlanda et al. 2004), which we ascribe to the fact that the opening angle is estimated through the global energetics. We have then selected a large sample of 442 GRBs with pseudo--redshifts, derived through the lag-luminosity relation, to test the existence of the L_iso-E_peak correlation. With this sample we also explore the possibility of a correlation between time resolved quantities, namely L_iso,p and the peak energy at the peak of emission E_peak,p
On the Meaningfulness of “Big Data Quality” (Invited Paper)
No full textIn this paper, we discuss the application of concept of data quality to big data by highlighting how much complex is to define it in a general way. Already data quality is a multidimensional concept, difficult to characterize in precise definitions even in the case of well-structured data. Big data add two further dimensions of complexity: (i) being “very” source specific, and for this we adopt the interesting UNECE classification, and (ii) being highly unstructured and schema-less, often without golden standards to refer to or very difficult to access. After providing a tutorial on data quality in traditional contexts, we analyze big data by providing insights into the UNECE classification, and then, for each type of data source, we choose a specific instance of such a type (notably deep Web data, sensor-generated data, and Twitters/short texts) and discuss how quality dimensions can be defined in these cases. The overall aim of the paper is therefore to identify further research directions in the area of big data quality, by providing at the same time an up-to-date state of the art on data quality. © 2015, The Author(s)
Cliques are Too Strict for Representing Communities: Finding Large k-plexes in Real Networks
No full textk-plexes are a formal yet flexible way of defining communities in networks. They generalize the notion of cliques and are more appropriate in most real cases: while a node of a clique C is connected to all other nodes of C, a node of a k-plex may miss up to k connections. Unfortunately, computing all maximal k-plexes is a gruesome task and state-of-the-art algorithms can only process small-size networks. In this paper we propose a new approach for enumerating large k-plexes in networks that speeds up the search by several orders of magnitude, leveraging on efficient techniques for the computation of maximal cliques
Robust entity resolution using random graphs
No full textEntity resolution (ER) seeks to identify which records in a data set refer to the same real-world entity. Given the diversity of ways in which entities can be represented, matched and distinguished, ER is known to be a challenging task for automated strategies, but relatively easier for expert humans. In our work, we abstract the knowledge of experts with the notion of a binary oracle. Our oracle can answer questions of the form "do records u and v refer to the same entity?" under a? exible error model, allowing for some questions to be more di?cult to answer correctly than others. Our contribution is a general error correction tool that can be leveraged by a variety of hybrid-human machine ER algorithms, based on a formal way for selecting indirect "control queries". In our experiments we demonstrate that correction-less ER algorithms equipped with our tool can perform even better than recent ER algorithms speci?cally designed for correcting errors. Our control queries are selected among those that provide strongest connectivity between records of each cluster, based on the concept of graph expanders (which are sparse graphs with formal connectivity properties). We give formal performance guarantees for our toolkit and provide experiments on real and synthetic data
Short versus long gamma-ray bursts: spectra, energetics, and luminosities
No full textWe compare the spectral properties of 79 short and 79 long Gamma-Ray Bursts (GRBs) detected by BATSE and selected with the same limiting peak flux. Short GRBs have a low-energy spectral component harder and a peak energy slightly higher than long GRBs, but no difference is found when comparing short GRB spectra with those of the first 1-2 sec emission of long GRBs. These results confirm earlier findings for brighter GRBs. The bolometric peak flux of short GRBs correlates with their peak energy in a similar way to long bursts. Short and long GRBs populate different regions of the bolometric fluence-peak energy plane, short bursts being less energetic by a factor similar to the ratio of their durations. If short and long GRBs had similar redshift distributions, they would have similar luminosities yet different energies, which correlate with the peak energy E_peak for the population of long GRBs. We also test whether short GRBs are consistent with the E_peak-E_iso and E_peak-L_iso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E_peak,obs: while short GRBs are inconsistent with the E_peak-E_iso correlation of long GRBs, they could follow the E_peak-L_iso correlation of long bursts. All the above indications point to short GRBs being similar to the first phases of long bursts. This suggests that a similar central engine (except for its duration) operates in GRBs of different durations
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