28,301 research outputs found
Gamma-Ray Burst Spectral Features: Interpretation as X-ray Emission From A Photoionized Plasma
Numerous reports have been made of features, either in emission or
absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally
interpreted in the context of Galactic neutron star models as cyclotron line
emission and annihilation features, the recent demonstration that
the majority of GRBs lie at cosmological distances make these explanations
unlikely. In this letter, we adopt a relativistic fireball model for
cosmological GRBs in which dense, metal rich blobs or filaments of plasma are
entrained in the relativistic outflow. In the context of this model, we
investigate the conditions under which broadband features, similar to those
detected, can be observed. We find a limited region of parameter space capable
of reproducing the observed GRB spectra. Finally, we discuss possible
constraints further high-energy spectral observations could place on fireball
model parameters.Comment: Accepted for publication in Astrophysical Journal Letters Four pages,
2 figure
Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars
Recent observations by XMM-Newton detected rotational pulsations in the total
brightness and spectrum of several neutron stars. To properly interpret the
data, accurate modeling of neutron star emission is necessary. Detailed
analysis of the shape and strength of the rotational variations allows a
measurement of the surface composition and magnetic field, as well as
constrains the nuclear equation of state. We discuss our models of the spectra
and light curves of two of the most observed neutron stars, RX J1856.5-3754 and
1E 1207.4-5209, and discuss some implications of our results and the direction
of future work.Comment: 5 pages, 6 figures; Proceedings of "40 Years of Pulsars", eds. C.
Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, submitte
Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition
The electronic structure of the molecular compound (TTM-TTP)I_3, which
exhibits a peculiar intra-molecular charge ordering, has been studied using
multi-configuration ab initio calculations. First we derive an effective
Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up
a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine
its full parameters: the transfer integrals, the Coulomb and exchange
interactions. The tight-binding band structure obtained from these transfer
integrals is consistent with the result of the direct band calculation based on
density functional theory. Then, by decomposing the frontier MOs into two
parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be
described by a two-leg ladder model, while the inter-fragment Coulomb energies
are scaled to the inverse of their distances. This result indicates that the
fragment picture that we proposed earlier [M.-L. Bonnet et al.: J. Chem. Phys.
132 (2010) 214705] successfully describes the low-energy properties of this
compound.Comment: 5 pages, 4 figures, published versio
Constrained Allocation Flux Balance Analysis
New experimental results on bacterial growth inspire a novel top-down
approach to study cell metabolism, combining mass balance and proteomic
constraints to extend and complement Flux Balance Analysis. We introduce here
Constrained Allocation Flux Balance Analysis, CAFBA, in which the biosynthetic
costs associated to growth are accounted for in an effective way through a
single additional genome-wide constraint. Its roots lie in the experimentally
observed pattern of proteome allocation for metabolic functions, allowing to
bridge regulation and metabolism in a transparent way under the principle of
growth-rate maximization. We provide a simple method to solve CAFBA efficiently
and propose an "ensemble averaging" procedure to account for unknown protein
costs. Applying this approach to modeling E. coli metabolism, we find that, as
the growth rate increases, CAFBA solutions cross over from respiratory,
growth-yield maximizing states (preferred at slow growth) to fermentative
states with carbon overflow (preferred at fast growth). In addition, CAFBA
allows for quantitatively accurate predictions on the rate of acetate excretion
and growth yield based on only 3 parameters determined by empirical growth
laws.Comment: 21 pages, 6 figures (main) + 33 pages, various figures and tables
(supporting); for the supplementary MatLab code, see
http://tinyurl.com/h763es
Custo de produção de trigo e de aveia: estimativa para a safra 2004.
bitstream/CNPT-2010/40482/1/p-co117.pd
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