11,709 research outputs found
On the nature of the AGILE galactic transient sources
The Italian gamma-ray satellite AGILE has recently reported the detection of
some variable high-energy sources likely of galactic origin. These sources do
not have any obvious counterpart at lower energies. We propose that these
sources are produced in proton-dominated jets of galactic microquasars. We
develop a model for microquasar jets that takes into account both primary
leptons and protons and all relevant radiative processes, including secondary
particle emission and gamma-ray attenuation due to pair creation in the jet. We
obtain spectral energy distributions that correspond to what is observed by
AGILE, with most of the power concentrated between 100 MeV and 10 GeV and
reaching luminosities of erg s. We make detailed spectral
predictions that can be tested by the Fermi gamma-ray telescope in the
immediate future. We conclude that hadronic jets in galactic accreting sources
can be responsible for the variable unidentified gamma-ray sources detected by
AGILE.Comment: 4 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysics (Letters
Ab initio analysis of the x-ray absorption spectrum of the myoglobin-carbon monoxide complex: Structure and vibrations
We present a comparison between Fe K-edge x-ray absorption spectra of
carbonmonoxy-myoglobin and its simulation based on density-functional theory
determination of the structure and vibrations and spectral simulation with
multiple-scattering theory. An excellent comparison is obtained for the main
part of the molecular structure without any structural fitting parameters. The
geometry of the CO ligand is reliably determined using a synergic approach to
data analysis. The methodology underlying this approach is expected to be
especially useful in similar situations in which high-resolution data for
structure and vibrations are available.Comment: 13 pages, 3 figure
Models for gamma-ray production in low-mass microquasars
Unlike high-mass gamma-ray binaries, low-mass microquasars lack external
sources of radiation and matter that could produce high-energy emission through
interactions with relativistic particles. In this work we consider the
synchrotron emission of protons and leptons that populate the jet of a low-mass
microquasar. In our model photohadronic and inverse Compton (IC) interactions
with synchrotron photons produced by both protons and leptons result in a
high-energy tail of the spectrum. We also estimate the contribution from
secondary pairs injected through photopair production. The high-energy emission
is dominated by radiation of hadronic origin, so we can call these objects
proton microquasars.Comment: 4 pages, 2 figures, accepted for publication in the International
Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
September 200
On the nature of the AGILE galactic transient sources
Context. The Italian gamma-ray satellite AGILE has recently reported, the detection of some variable high-energy sources likely of galactic origin. These sources do not have any obvious counterpart at lower energies. Aims. We propose that these sources are produced in proton-dominated jets of galactic microquasars. Methods. We develop a model for microquasar jets that takes into account both primary leptons and protons and all relevant radiative processes, including secondary particle emission and gamma-ray attenuation due to pair creation in the jet. Results. We obtain spectral energy distributions that correspond to what is observed by AGILE, with most of the power concentrated between 100 MeV and 10 GeV and reaching luminosities of 1034-35 erg s-1. We make detailed spectral predictions that can be tested by the Fermi, gamma-ray telescope in the immediate future. Conclusions. We conclude that hadronic jets in galactic accreting sources can be responsible for the variable unidentified gamma-ray sources detected by AGILE.Facultad de Ciencias Astronómicas y Geofísica
Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids
Calculations of the hyperpolarizability are typically much more difficult to
converge with basis set size than the linear polarizability. In order to
understand these convergence issues and hence obtain accurate ab initio values,
we compare calculations of the static hyperpolarizability of the gas-phase
chloroform molecule (CHCl_3) using three different kinds of basis sets:
Gaussian-type orbitals, numerical basis sets, and real-space grids. Although
all of these methods can yield similar results, surprisingly large, diffuse
basis sets are needed to achieve convergence to comparable values. These
results are interpreted in terms of local polarizability and
hyperpolarizability densities. We find that the hyperpolarizability is very
sensitive to the molecular structure, and we also assess the significance of
vibrational contributions and frequency dispersion
Metal Abundances of KISS Galaxies III. Nebular Abundances for Fourteen Galaxies and the Luminosity-Metallicity Relationship for HII Galaxies
We report results from the third in a series of nebular abundance studies of
emission-line galaxies from the KPNO International Spectroscopic Survey (KISS).
Galaxies with coarse metallicity estimates of 12 + log(O/H) less than 8.2 dex
were selected for observation. Spectra of 14 galaxies, which cover the full
optical region from [OII]3727,3729 to beyond [SII]6717,6731, are presented, and
abundance ratios of N, O, Ne, S, and Ar are computed. The auroral [OIII]4363
line is detected in all 14 galaxies. Oxygen abundances determined through the
direct electron temperature T_e method confirm that the sample is metal-poor
with 7.61 <= 12 + log(O/H) <= 8.32. By using these abundances in conjunction
with other T_e-based measurements from the literature, we demonstrate that HII
galaxies and more quiescent dwarf irregular galaxies follow similar
metallicity-luminosity (L-Z) relationships. The primary difference is a
zero-point shift between the correlations such that HII galaxies are brighter
by an average of 0.8 B magnitudes at a given metallicity. This offset can be
used as evidence to argue that low-luminosity HII galaxies typically undergo
factor of two luminosity enhancements, and starbursts that elevate the
luminosities of their host galaxies by 2 to 3 magnitudes are not as common. We
also demonstrate that the inclusion of interacting galaxies can increase the
scatter in the L-Z relation and may force the observed correlation towards
lower metallicities and/or larger luminosities. This must be taken into account
when attempting to infer metal abundance evolution by comparing local L-Z
relations with ones based on higher redshift samples since the fraction of
interacting galaxies should increase with look-back time.Comment: 36 pages, 5 figures. ApJ, in pres
Stochastic estimation of hydraulic transmissivity fields using flow connectivity indicator data
Most methods for hydraulic test interpretation rely on a number of simplified assumptions regarding the homogeneity and isotropy of the underlying porous media. This way, the actual heterogeneity of any natural parameter, such as transmissivity ( math formula), is transferred to the corresponding estimates in a way heavily dependent on the interpretation method used. An example is a long-term pumping test interpreted by means of the Cooper-Jacob method, which implicitly assumes a homogeneous isotropic confined aquifer. The estimates obtained from this method are not local values, but still have a clear physical meaning; the estimated math formula represents a regional-scale effective value, while the log-ratio of the normalized estimated storage coefficient, indicated by math formula, is an indicator of flow connectivity, representative of the scale given by the distance between the pumping and the observation wells. In this work we propose a methodology to use math formula, together with sampled local measurements of transmissivity at selected points, to map the expected value of local math formula values using a technique based on cokriging. Since the interpolation involves two variables measured at different support scales, a critical point is the estimation of the covariance and crosscovariance matrices. The method is applied to a synthetic field displaying statistical anisotropy, showing that the inclusion of connectivity indicators in the estimation method provide maps that effectively display preferential flow pathways, with direct consequences in solute transport
Magnetic patterning of (Ga,Mn)As by hydrogen passivation
We present an original method to magnetically pattern thin layers of
(Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the
hole density, and thereby locally suppress the carrier-mediated ferromagnetic
phase. The sample surface is thus maintained continuous, and the minimal
structure size is of about 200 nm. In micron-sized ferromagnetic dots
fabricated by hydrogen passivation on perpendicularly magnetized layers, the
switching fields can be maintained closer to the continuous film coercivity,
compared to dots made by usual dry etch techniques
First record of Oligochoerus limnophilus (Acoela, Acoelomorpha) from British waters
Primer registro de Oligochoerus limnophilus (Acoela, Acoelomorpha) en aguas británicas
Informamos de la existencia de poblaciones del acelo Oligochoerus limnophilus (Acoelomorpha) en la islas Británicas sobre la base de los especímenes capturados en el río Támesis (también conocido localmente como río Isis) a su paso por Oxford, Inglaterra, ampliando así considerablemente el área de distribución de la especie, restringida hasta ahora al continente europeo. La información gráfica que aportamos, imágenes de especímenes vivos y proyecciones CLSM de estructuras informativas seleccionadas por su valor sistemático, corrobora su estrecha relación con el género Convoluta Ørsted, 1843.We report the occurrence of the acoel Oligochoerus limnophilus (Acoelomorpha) from the British Islands, based on specimens captured in the river Thames (locally known as the river Isis) in Oxford, England, thereby considerably widening the distributional range of the species that had formerly been reported only from continental Europe. We further present live images and CLSM-projections of systematically informative structures that corroborate a close relationship with the genus Convoluta Ørsted, 1843.Primer registre d'Oligochoerus limnophilus (Acoela, Acoelomorpha) en aigües britàniques
Informem de l'existència de poblacions de l'acel Oligochoerus limnophilus (Acoelomorpha) a les illes Britàniques sobre la base dels espècimens capturats al riu Tàmesi (també conegut localment com a riu Isis) al pas per Oxford, Anglaterra, amb la qual cosa ampliem considerablement l'àrea de distribució de l'espècie, restringida fins ara al continent europeu. La informació gràfica que aportem, imatges d'espècimens vius i projeccions CLSM d'estructures informatives seleccionades pel seu valor sistemàtic, corrobora l'estreta relació amb el gènere Convoluta Ørsted, 1843
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