2,080 research outputs found
A fireworks model for Gamma-Ray Bursts
The energetics of the long duration GRB phenomenon is compared with models of
a rotating Black Hole (BH) in a strong magnetic field generated by an accreting
torus. A rough estimate of the energy extracted from a rotating BH with the
Blandford-Znajek mechanism is obtained with a very simple assumption: an
inelastic collision between the rotating BH and the torus. The GRB energy
emission is attributed to an high magnetic field that breaks down the vacuum
around the BH and gives origin to a e+- fireball. Its subsequent evolution is
hypothesized, in analogy with the in-flight decay of an elementary particle, to
evolve in two distinct phases. The first one occurs close to the engine and is
responsible of energizing and collimating the shells. The second one consists
of a radiation dominated expansion, which correspondingly accelerates the
relativistic photon--particle fluid and ends at the transparency time. This
mechanism simply predicts that the observed Lorentz factor is determined by the
product of the Lorentz factor of the shell close to the engine and the Lorentz
factor derived by the expansion. An anisotropy in the fireball propagation is
thus naturally produced, whose degree depends on the bulk Lorentz factor at the
end of the collimation phase.Comment: Accepted for publication in MNRA
How the hydrogen bond in NHF is revealed with Compton scattering
In order to probe electron wave functions involved in the bonding of NHF,
we have performed Compton scattering experiments in an oriented single crystal
and in a powder. Ab initio calculations of the Compton profiles for NHF and
NHCl are used to enlighten the nature of the bonds in the NHF crystal.
As a consequence, we are able to show significant charge transfer in the
ammonium ion which is not observable using other methods. Our study provides a
compelling proof for hydrogen bond formation in NHF.Comment: 4 pages, 5 figures, accepted for publication as a Regular Article in
Physical Review
Recent results in High Energy Gamma-ray Astrophysics
Since June 2008 two gamma-ray satellite experiments, AGILE and Fermi-LAT, have been in contemporaneous operation. Their results allow the study of gamma-ray astrophysics from space to enter in a new era. Starting from the heritage of the EGRET experiment, the science objectives of these two experiments are reviewed. The role of the technological improvement in such achievements is also briefly introduced
Characterization of Thin Film Materials using SCAN meta-GGA, an Accurate Nonempirical Density Functional
We discuss self-consistently obtained ground-state electronic properties of
monolayers of graphene and a number of beyond graphene compounds, including
films of transition-metal dichalcogenides (TMDs), using the recently proposed
strongly constrained and appropriately normed (SCAN) meta-generalized gradient
approximation (meta-GGA) to the density functional theory. The SCAN meta-GGA
results are compared with those based on the local density approximation (LDA)
as well as the generalized gradient approximation (GGA). As expected, the GGA
yields expanded lattices and softened bonds in relation to the LDA, but the
SCAN meta-GGA systematically improves the agreement with experiment. Our study
suggests the efficacy of the SCAN functional for accurate modeling of
electronic structures of layered materials in high-throughput calculations more
generally
Pulsar Bound on the Photon Electric Charge Reexamined
If photons had a small electric charge their path in the galactic
magnetic field would be curved, leading to a time delay between photons of
different frequency from a distant source. Cocconi's previous application of
this argument led to a limit which is too restrictive by a factor of about 200;
the corrected bound is Q_\gamma/e\lapprox10^{-29}.Comment: 3 pages, LaTe
Inelastic X-ray scattering from valence electrons near absorption edges of FeTe and TiSe
We study resonant inelastic x-ray scattering (RIXS) peaks corresponding to
low energy particle-hole excited states of metallic FeTe and semi-metallic
TiSe for photon incident energy tuned near the absorption edge of
Fe and Ti respectively. We show that the cross section amplitudes are well
described within a renormalization group theory where the effect of the core
electrons is captured by effective dielectric functions expressed in terms of
the the atomic scattering parameters of Fe and Ti. This method can be
used to extract the dynamical structure factor from experimental RIXS spectra
in metallic systems.Comment: 6 pages, 4 figure
Anti-ferromagnetism, spin-phonon interaction and the local-density approximation in high-T superconductors
Results from different sets of band calculations for undoped and doped
HgBaCuO show that small changes in localization can lead to very
different ground states.
The normal LDA results are compared with 'modified' LDA results, in which
different linearization energies make the O-p band more localized. The ground
states in the normal calculations are far from the anti-ferromagnetic ones,
while nearly AFM states are found in the modified calculations. The proximity
of an AFM state in the doped system leads to increased , and the
modified band structure has favorable conditions for spin-phonon coupling and
superconductivity mediated by spin fluctuations.Comment: 4 pages, 2 figs., Accepted in J. Physics: Condensed Matter as a
lette
The connection between Gamma-ray bursts and Supernovae Ib/c
It has been established that Gamma-Ray Bursts (GRBs) are connected to Supernovae (SNe) explosions of Type Ib/c. We intend to test whether the hypothesis of Type Ib/c SNe from different massive progenitors can reproduce the local GRB rate as well as the GRB rate as a function of redshift. We aim to predict the GRB rate at very high redshift under different assumptions about galaxy formation and star formation histories in galaxies. We assume different star formation histories in galaxies of different morphological type: ellipticals, spirals and irregulars. We explore different hypotheses concerning the progenitors of Type Ib/c SNe. We find an excellent agreement between the observed GRB local rate and the predicted Type Ib/c SN rate in irregular galaxies, when a range for single Wolf-Rayet stars of 40-100 M_sun is adopted. We also predict the cosmic Type Ib/c SN rate by taking into account all the galaxy types in an unitary volume of the Universe and we compare it with the observed cosmic GRB rate as a function of redshift. By assuming the formation of spheroids at high redshift, we predict a cosmic Type Ib/c SN rate, which is always higher than the GRB rate, suggesting that only a small fraction (0.1-1 %) of Type Ib/c SNe become GRBs. In particular, we find a ratio between the cosmic GRB rate and the cosmic Type Ib/c rate in the range 0.001-0.01, in agreement with previous estimates. Finally, due to the high star formation in spheroids at high redshift, which is our preferred scenario for galaxy formation, we predict more GRBs at high redshift than in the hierarchical scenario for galaxy formation, a prediction which awaits to be proven by future observations
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