3,619 research outputs found
Slow and fast components in the X-ray light curves of Gamma-Ray Bursts
Gamma-ray burst light curves show quite different patterns: from very simple
to extremely complex. We present a temporal and spectral study of the light
curves in three energy bands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the
Wide Field Cameras on board BeppoSAX. For some events the time profiles are
characterized by peaks superposed on a slowly evolving pedestal, which in some
cases becomes less apparent at higher energies. We describe this behaviour with
the presence of two components (slow and fast) having different variability
time scales. We modelled the time evolution of slow components by means of an
analytical function able to describe asymmetric rising and decaying profiles.
The residual light curves, after the subtraction of the slow components,
generally show structures more similar to the original curves in the highest
energy band. Spectral study of these two components was performed evaluating
their hardness ratios, used also to derive photon indices. Slow components are
found generally softer than the fast ones suggesting that their origin is
likely different. Being typical photon indices lower than those of the
afterglows there is no evidence that the emission processes are similar.
Another interesting possibility is that slow components can be related to the
presence of a hot photosphere having a thermal spectrum with kT around a few
keV superposed to a rapid variable non-thermal emission of the fast component.Comment: 16 pages, 20 figures (18 color, 2 B&W), accepted for publication in
Astronomy and Astrophysic
Full-Configuration-Interaction Study of the Metal-Insulator Transition in Model Systems: Li<sub>N</sub> Linear Chains (N=2,4,6,8)
International audienceThe precursor of the metal-insulator transition is studied at ab initio level in linear chains of equally spaced lithium atoms. In particular, full configuration interaction calculations (up to 1×109 determinants) are performed, in order to take into account the different nature of the wave function at different internuclear distances. Several indicators of the Metal-Insulator transition (minimum of the energy gap, maximum of the localization tensor or of the polarizability) are considered and discussed. It is shown that the different indicators give concordant results, showing a rapid change in the nature of the wave function at an internuclear distance of about 7bohrs
The viscosity of shoshonitic melts (Vulcanello Peninsula, Aeolian Islands, Italy): insight on the magma ascent in dikes
The viscosity of shoshonitic melts from Vulcanello Peninsula (Vulcano Island, Italy) is
experimentally determined at temperatures between 733 K and 1673 K. The water content of the
melts varies from 0.03 to 4.75 wt% H2O. The micropenetration technique is employed at ambient
pressure in the high viscosity range (109-1012 Pa·s). Falling sphere(s) experiments are performed
at 500 and 2000 MPa in the low viscosity range (100.5-103 Pa·s). Results show a decrease of about
2 orders of magnitude in viscosity if ~ 3 wt% of water is added to the dry melt at 1300 K. At high
temperature the viscosity of Vulcanello melts is intermediate between that of andesitic and
basaltic melts. In contrast, at low temperatures (≤1050 K), the shoshonitic melt is characterized
by a lower viscosity with respect to the two previous melts. Based on our new data set, a
calculation model is proposed to predict the viscosity of the shoshonitic melts as a function of
temperature and water content. The viscosity data are used to constrain the ascent velocity of
shoshonitic magmas from Vulcanello within dikes. Using petrological data (temperature and
crystal content of the magma) and volcanological information (geometrical parameters of the
eruptive fissure and depth of magma storage), we estimate the time scale for the ascent of magma
from the main reservoir to the surface. Results show time scales in the order of hours to few days.
We conclude that the rapid ascent of poorly evolved melts from Moho depths should be taken
into account for the hazard assessment of Vulcano Island
The onset and solidification path of a basaltic melt by in situ differential scanning calorimetry (DSC) and ex situ investigations
The in situ differential scanning calorimetry (DSC) technique has been applied to investigate the solidification paths of a basaltic liquid. The starting glass was heated up to 1300◦C, kept at this superliquidus temperature for 2 h and cooled at rates (∆T/∆t) of 7, 60, 180, 1000, and 1800◦C/h, down to 800 and 600◦C. Glass transition temperature (Tg), crystallization temperature (Tx_HR) and melting temperature (Tm) were measured by in situ DSC spectra on heating. Tx measured along the cooling paths (Tx_CR) shows exothermic peaks that change from a single symmetric shape (7 and 60◦C/h) to multi-component patterns (180, 1000, and 1800◦C/h). The recovered products characterized by field emission gun source of the scanning electron microscopy and electron probe micro-analyzer-wavelength dispersive spectrometers show a phase assemblage of spinel (sp), clinopyroxene (cpx), melilite (mel), plagioclase (plg), and glass. Moreover, crystal size distributions (CSDs) and growth rates (Gmax and GCSD) were also determined. The crystal content slightly increases from 7 to 1800◦C/h. Faceted sp are present in all the run products with an amount always <2 area%. Cpx increases from 7 to 1800◦C/h, changing its texture from almost faceted to dendritic between 60 and 180◦C/h. The area% of mel follows an asymmetric Gaussian trend, while plg nucleates only at 7◦C/h with a content <2 area%. The coupling of DSC and SEM outcomes indicate that sp nucleate first, followed by cpx and mel (and/or plg). The increment of ∆T/∆t causes an increase of the CSD slope (m) and crystal population density per size (n0 ), as well as a decrease of the crystal size, for both cpx and sp. The log-linear CSD segments with different slopes at 7 and 60◦C/h suggest multiple nucleation events and crystal growth by coarsening. Gmax and GCSD for cpx and sp directly measured on the actual crystallization time by DSC spectra, both increase with the increasing of ∆T/∆t. The onset temperature of crystallization (Txi ) decreases as ∆T/∆t increases, following an exponential trend that defines the uppermost portion of a time-transformation-temperature-like curve. This analytical model allows us to quantitatively model the kinetic crystallization paths of dry basalts
Intrinsic solidification behaviour of basaltic to rhyolitic melts: a cooling rate experimental study
Dynamic cooling-induced solidification experiments were run using six silicate glasses
along the basalt - rhyolite join (B100= 100 wt % of basalt, R100= 100 wt % of rhyolite), i.e.
B100, B80R20, B60R40, B40R60, B20R80 and R100; the glasses directly quenched from 1300 °C
after a dwell of 120 minutes (experiment E0) contain 50-400 ppm H2O, << 1 area% μm-sized
bubble, and Fe2+/Fetot between 0.34 and 0.46. Experiments were performed in Pt capsules at
room pressure and fO2 of air, between 1300 and 800 °C using three different cooling rates of
0.0167, 3 and 30 °C/min; these cooling rates were run two times: E1-E2 experiments at
0.0167°C/min, S1-E3 at 3 °C/min, and E4-E5 at 30 °C/min. In experiments E1 to E5, samples
were annealed for 120 minutes at 1300 °C, whereas in the experiment S1 the samples were
firstly heated for 30 minutes at 1400 °C followed by a dwell time of 2400 minutes at 1300°C
before cooling. In the experiments a preferential crystallization was not observed at the
melt/gas interface. B100, B80R20 and B60R40 run-products have a low tendency to preferentially
crystallize on Pt walls, while B40R60, B20R80 and R100 are not affected by the presence of Pt
substrata. All run-products show very homogeneous textures, except for B60R40 and B40R60 at
0.0167°C/min in the E1 experiment. The duplicates of B40R60 and B60R40 at 0.0167°C/min
and B100 at 30 °C/min show relatively large differences in crystal content (> 4 and < 14
area%). B40R60 and B60R40 duplicated run-products have the same amount of earlycrystallized
clinopyroxene and spinel, but different contents in lately-formed plagioclase. The
run-products with the same starting composition from E3-S1 (3 °C/min) show a high
reproducibility in terms of crystal shape, size, and amount (< 4 area%). This demonstrates
that the crystallization path is not affected by the different heat treatment above the liquidus
temperature, i.e. the time scale of structural re-equilibration (relaxation) and chemical rehomogenization
are shorter than our experimental time scale. Possible chemicalheterogeneities on a length scale of several micrometers for R100 and several hundreds of
micrometers for B100 can be removed at 1300 °C within 120 minutes. A heat treatment at
1300 °C for 120 minutes significantly reduces the amount of μm-sized bubbles, potentially
responsible for the onset of nucleation and unreveals the intrinsic solidification of silicate
melts. The experimental reproducibility is low when the cooling path intersects the tip of the
time-temperature-transformation (TTT) curves, i.e. when the nucleation rate is near its
maximum (Imax). In that case, even small thermal variations in cooling rate and local
composition can have large effects on phase abundance and crystal size. Dynamic
crystallization experiments can be properly interpreted and compared only if they are
texturally homogeneous and the physico-chemical state of the superheated silicate liquid is
known. The solidification conditions used in this study mirror those of aphyric lavas and
dikes emplaced at shallower crustal levels
Retrieving magma composition from TIR spectra: implications for terrestrial planets investigations
Emissivity and reflectance spectra have been investigated on two series of silicate glasses, having compositions belonging to alkaline and subalkaline series, covering the most common terrestrial igneous rocks. Glasses were synthesized starting from natural end-members outcropping at Vulcano Island (Aeolian Islands, Italy) and on Snake River Plain (USA). Results show that the shift of the spectra, by taking Christiansen feature (CF) as a reference point, is correlated with SiO2 content, the SCFM factor and/or the degree of polymerization state via the NBO/T and temperature. The more evolved is the composition, the more polymerized the structure, the shorter the wavelength at which CF is observable. CF shift is also dependent on temperature. The shape of the spectra discriminates alkaline character, and it is related to the evolution of Qn structural units. Vulcano alkaline series show larger amount of Q4 and Q3 species even for mafic samples compared to the subalkaline Snake River Plain series. Our results provide new and robust insights for the geochemical characterization of volcanic rocks by remote sensing, with the outlook to infer origin of magmas both on Earth as well as on terrestrial planets or rocky bodies, from emissivity and reflectance spectra
E835 at FNAL: Charmonium Spectroscopy in Annihilations
I present preliminary results on the search for in its
and decay modes. We observe an excess of \eta_c\gamma{\cal P} \sim 0.001M=3525.8 \pm 0.2 \pm 0.2
\Gamma\leq10.6\pm 3.7\pm3.4(br) <
\Gamma_{\bar{p}p}B_{\eta_c\gamma} < 12.8\pm 4.8\pm4.5(br) J/\psi\pi^0$ mode.Comment: Presented at the 6th International Conference on Hyperons, Charm and
Beauty Hadrons (BEACH 2004), Chicago(Il), June 27-July 3,200
SÃntesis de nanoparticulas de Ni2P : Su aplicación como fase activa en reacciones de hidrogenación
En los últimos años se han comenzado a estudiar las propiedades catalÃticas de los fosfuros de metales de transición, principalmente en reacciones de hidrodesulfurización e hidrodeoxigenación. Sin embargo, los estudios de reacciones de hidrogenación selectiva empleando estas fases son prácticamente inexistentes. El objetivo del presente trabajo es investigar las propiedades del fosfuro de nÃquel (Ni2P) como fase activa para la hidrogenación quimioselectiva de acetofenona. Debido a que muchos estudios han demostrado que la morfologÃa de las partÃculas de Ni2P afecta la performance catalÃtica, nos hemos propuesto utilizar nanopartÃculas (NPs) de Ni2P presintetizadas con una estrecha distribución de tamaño de partÃcula.Facultad de Ciencias Exacta
SÃntesis de nanoparticulas de Ni2P : Su aplicación como fase activa en reacciones de hidrogenación
En los últimos años se han comenzado a estudiar las propiedades catalÃticas de los fosfuros de metales de transición, principalmente en reacciones de hidrodesulfurización e hidrodeoxigenación. Sin embargo, los estudios de reacciones de hidrogenación selectiva empleando estas fases son prácticamente inexistentes. El objetivo del presente trabajo es investigar las propiedades del fosfuro de nÃquel (Ni2P) como fase activa para la hidrogenación quimioselectiva de acetofenona. Debido a que muchos estudios han demostrado que la morfologÃa de las partÃculas de Ni2P afecta la performance catalÃtica, nos hemos propuesto utilizar nanopartÃculas (NPs) de Ni2P presintetizadas con una estrecha distribución de tamaño de partÃcula.Facultad de Ciencias Exacta
SÃntesis de nanoparticulas de Ni2P : Su aplicación como fase activa en reacciones de hidrogenación
En los últimos años se han comenzado a estudiar las propiedades catalÃticas de los fosfuros de metales de transición, principalmente en reacciones de hidrodesulfurización e hidrodeoxigenación. Sin embargo, los estudios de reacciones de hidrogenación selectiva empleando estas fases son prácticamente inexistentes. El objetivo del presente trabajo es investigar las propiedades del fosfuro de nÃquel (Ni2P) como fase activa para la hidrogenación quimioselectiva de acetofenona. Debido a que muchos estudios han demostrado que la morfologÃa de las partÃculas de Ni2P afecta la performance catalÃtica, nos hemos propuesto utilizar nanopartÃculas (NPs) de Ni2P presintetizadas con una estrecha distribución de tamaño de partÃcula.Facultad de Ciencias Exacta
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