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Search Results: 1 - 10 of 301851 matches for " J. Becerra-Gonzalez "
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On the origin of the gamma-ray emission from the flaring blazar PKS 1222+216
F. Tavecchio,J. Becerra-Gonzalez,G. Ghisellini,A. Stamerra,G. Bonnoli,L. Foschini,L. Maraschi
Physics , 2011, DOI: 10.1051/0004-6361/201117204
Abstract: The flat spectrum radio quasar PKS 1222+216 was detected in the very high energy gamma-ray band by MAGIC during a highly active gamma-ray phase following an alert by the LAT onboard Fermi. Its relatively hard spectrum without a cut off, together with the observed variability on timescale of ~10 min challenges standard emission models. If the emission originates in a portion of the relativistic jet located inside the BLR, severe absorption of gamma rays above few tens of GeV is expected due to the pair production process. These observations imply the existence of a very compact (R_b ~5 x 10^{14} cm) and very fast blob located far beyond the BLR radius, responsible for the rapidly varying high energy flux. However the long term (days-weeks) coherent evolution of the GeV flux recorded by LAT indicates that there could be also the substantial contribution from another larger emission region. We model the spectral energy distribution of PKS 1222+216 during the epoch of the MAGIC detection assuming three different scenarios,: (1) a one-zone model considering only the emission from a compact blob outside the BLR; (2) a two-zone model considering the compact blob plus an emitting region filling the whole jet cross-section located outside the BLR and (3) a two zone model with the jet emitting region inside the BLR. We find that in all cases the high-energy emission from the compact blob is dominated by the inverse Compton scattering of the IR thermal radiation of the dusty torus. Both regions are matter dominated, with a negligible Poynting flux. These results disfavor models in which the compact blob is the result of reconnection events inside the jet or "needles" of high-energy electrons. Instead, the observational framework and our radiative models could be compatible with scenarios in which the jet is re-collimated and focussed at large distances from the central BH.
MAGIC observation of GRB080430
Stefano Covino,Markus Garczarczyk,Markus Gaug,Angelo Antonelli,Denis Bastieri,Josefa Becerra-Gonzalez,Antonio La Barbera,Alessandro Carosi,Nicola Galante,Francesco Longo,Valeria Scapin,Susanna Spiro,Antonio de Ugarte-Postigo,Alessandra Galli,R. Salvaterra
Physics , 2009,
Abstract: Gamma-ray bursts are cosmological sources emitting radiation from the gamma-rays to the radio band. Substantial observational efforts have been devoted to the study of GRBs during the prompt phase, i.e. the initial burst of high-energy radiation, and during the longer-lasting afterglows. In spite of many successes in interpreting these phenomena there are still several open key questions about the fundamental emission processes, their energetics and the environment. Moreover, independently of their modeling, GRB spectra are remarkably simple, being satisfactorily fitted with power-laws, and therefore offer a very valuable tool to probe the extragalactic background light distribution affecting all high-energy observations of cosmological sources. Observations carried out with Cherenkov telescopes, as MAGIC, can be fundamental for all these scientific topics. GRB080430, being at a rather moderate redshift, z~0.76, and well-studied in the optical, although observed only a few hours after the high-energy event, is a good test case to evaluate the perspective for late-afterglow observations with ground based GeV/TeV observatories.
MAGIC observation of GRB090102
Markus Gaug,Stefano Covino,Markus Garczarczyk,Angelo Antonelli,Denis Bastieri,Antonio La Barbera,Josefa Becerra-Gonzalez,Alessandro Carosi,Nicola Galante,Francesco Longo,Valeria Scapin,Susanna Spiro,for the MAGIC Collaboration
Physics , 2009,
Abstract: On January 2, the MAGIC-I Telescope observed GRB090102 (z=1.55) under particularly good observation conditions. Using the recently upgraded MAGIC-1 sum trigger system, upper limits down to below 50 GeV have been obtained. This is the first time that the new trigger system was fully exploited for a Gamma-ray Burst (GRB) observation and shows the capabilities of the MAGIC observatory for future follow-up observations.
GRB Observations with the MAGIC Telescopes
Markus Garczarczyk,Markus Gaug,Angelo Antonelli,Denis Bastieri,Josefa Becerra-Gonzalez,Stefano Covino,Antonio La Barbera,Alessandro Carosi,Nicola Galante,Francesco Longo,Valeria Scapin,Susanna Spiro,for the MAGIC Collaboration
Physics , 2009,
Abstract: MAGIC is currently the most suitable instrument to perform observations of the prompt an early afterglow emission from Gamma Ray Bursts (GRBs) at energies above 25 GeV. The instrument is designed to have the lowest possible energy threshold and fastest reaction time to GRB Coordinates Network (GCN) alerts. The MAGIC-I telescope started to follow-up GCN alerts in the beginning of 2005. Since then, more than 50 GRB candidates were observed. Just now MAGIC-II, the second telescope has started to operate. Both telescopes together will have a factor of three better sensitivity.
Scientific Prediction in the Beginning of the “Historical Turn”: Stephen Toulmin and Thomas Kuhn  [PDF]
Wenceslao J. Gonzalez
Open Journal of Philosophy (OJPP) , 2013, DOI: 10.4236/ojpp.2013.32053
Abstract:

This paper considers the similarities and differences between Toulmin and Kuhn on the problem of prediction. The context of the analysis is the beginning of the “historical turn” in philosophy of science (i.e., the period before the 1965 international colloquium held at Bedford College). The comparison between these authors takes into account several levels: semantic, logical, epistemological, methodological, ontological, and axiological. The main goal is to analyze whether there are influences of Toulmin in Kuhn regarding scientific prediction or, at least, if the former reached similar positions to the latter on the issue of the role of prediction in science.

QUANTITY AND QUALITY OF b-CAROTENE PRODUCED BY TWO STRAINS OF DUNALIELLA SALINA (TEODORESCO 1905) FROM THE NORTH OF CHILE
GOMEZ,PATRICIA I.; GONZALEZ,MARIELA A.; BECERRA,JOSE;
Boletín de la Sociedad Chilena de Química , 1999, DOI: 10.4067/S0366-16441999000400010
Abstract: the growth and the quantity and quality of carotenes produced by two chilean strains of d. salina (conc-001 and conc-007), grown in two different media (pes and sm) were analyzed. no important differences were found between the two media either in the growth or in the carotenogenesis suggesting that the low-cost media (sm) would be appropriate for large scale culture of d. salina. the strain conc-007 displayed the lower growth rate, but the highest productivity of carotenoid per cell and the highest 9-cis b-carotene content whereas the strain conc-001 exhibited the highest growth rate but a lower accumulation of carotenoid per cell and a low 9-cis b-carotene content. the chilean strains were both more carotenogenic than the reference strain dunaliella bardawil
QUANTITY AND QUALITY OF b-CAROTENE PRODUCED BY TWO STRAINS OF DUNALIELLA SALINA (TEODORESCO 1905) FROM THE NORTH OF CHILE
PATRICIA I. GOMEZ,MARIELA A. GONZALEZ,JOSE BECERRA
Boletín de la Sociedad Chilena de Química , 1999,
Abstract: The growth and the quantity and quality of carotenes produced by two Chilean strains of D. salina (CONC-001 and CONC-007), grown in two different media (PES and SM) were analyzed. No important differences were found between the two media either in the growth or in the carotenogenesis suggesting that the low-cost media (SM) would be appropriate for large scale culture of D. salina. The strain CONC-007 displayed the lower growth rate, but the highest productivity of carotenoid per cell and the highest 9-cis b-carotene content whereas the strain CONC-001 exhibited the highest growth rate but a lower accumulation of carotenoid per cell and a low 9-cis b-carotene content. The Chilean strains were both more carotenogenic than the reference strain Dunaliella bardawil Se analizó el crecimiento así como la cantidad y calidad de los carotenos producidos por dos cepas chilenas de D. salina (CONC-001 y CONC-007) cultivadas en dos medios de distinta formulación (PES y MS). La composición del medio de cultivo no afectó el crecimiento ni la carotenogénesis de las cepas analizadas, sugiriendo que el medio de bajo costo (MS) podría ser apropiado para el cultivo de D. salina a mayor escala. La cepa CONC-007 presentó la menor tasa de crecimiento pero la mayor productividad de carotenoides por célula así como el mayor contenido de 9-cis b-caroteno, el cual corresponde al isómero de mayor interés comercial. Por otro lado, la cepa CONC-001 mostró la mayor tasa de crecimiento pero una menor acumulación de carotenoides por célula y un bajo contenido de 9-cis b-caroteno. Ambas cepas chilenas resultaron ser más carotenogénicas que la cepa de referencia Dunaliella bardawil
Kohn-Luttinger superconductivity in graphene
J. Gonzalez
Physics , 2008, DOI: 10.1103/PhysRevB.78.205431
Abstract: We investigate the development of superconductivity in graphene when the Fermi level becomes close to one of the Van Hove singularities of the electron system. The origin of the pairing instability lies in the strong anisotropy of the e-e scattering at the Van Hove filling, which leads to a channel with attractive coupling when making the projection of the BCS vertex on the symmetry modes with nontrivial angular dependence along the Fermi line. We show that the scale of the superconducting instability may be pushed up to temperatures larger than 10 K, depending on the ability to tune the system to the proximity of the Van Hove singularity.
Higher-order renormalization of graphene many-body theory
J. Gonzalez
Physics , 2012, DOI: 10.1007/JHEP08(2012)027
Abstract: We study the many-body theory of graphene Dirac quasiparticles interacting via the long-range Coulomb potential, taking as a starting point the ladder approximation to different vertex functions. We test in this way the low-energy behavior of the electron system beyond the simple logarithmic dependence of electronic correlators on the high-energy cutoff, which is characteristic of the large-N approximation. We show that the graphene many-body theory is perfectly renormalizable in the ladder approximation, as all higher powers in the cutoff dependence can be absorbed into the redefinition of a finite number of parameters (namely, the Fermi velocity and the weight of the fields) that remain free of infrared divergences even at the charge neutrality point. We illustrate this fact in the case of the vertex for the current density, where a complete cancellation between the cutoff dependences of vertex and electron self-energy corrections becomes crucial for the preservation of the gauge invariance of the theory. The other potentially divergent vertex corresponds to the staggered (sublattice odd) charge density, which is made cutoff independent by a redefinition in the scale of the density operator. This allows to compute a well-defined, scale invariant anomalous dimension to all orders in the ladder series, which becomes singular at a value of the interaction strength marking the onset of chiral symmetry breaking (and gap opening) in the Dirac field theory. The critical coupling we obtain in this way matches with great accuracy the value found with a quite different method, based on the resolution of the gap equation, thus reassuring the predictability of our renormalization approach.
Microscopic model of superconductivity in carbon nanotubes
J. Gonzalez
Physics , 2002, DOI: 10.1103/PhysRevLett.88.076403
Abstract: We propose the model of a manifold of one-dimensional interacting electron systems to account for the superconductivity observed in ropes of nanotubes. We rely on the strong suppression of single-particle hopping between neighboring nanotubes in a disordered rope and conclude that the tunnelling takes place in pairs of electrons, which are formed within each nanotube due to the existence of large superconducting correlations. Our estimate of the transition temperature is consistent with the values that have been measured experimentally in ropes with about 100 metallic nanotubes.
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