169,829 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
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
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
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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
Fast enumeration of large k-Plexes
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 (i) methods for strongly reducing the search space and (ii) efficient techniques for the computation of maximal cliques. Several experiments show that our strategy is effective and is able to increase the size of the networks for which the computation of large k-plexes is feasible from a few hundred to several hundred thousand nodes
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