2,992 research outputs found
Jets in GRBs: Tests and Predictions for the Structured Jet Model
The two leading interpretations of achromatic breaks that are observed in the
light curves of GRBs afterglow are (i) the manifestation of the edge of a jet,
which has a roughly uniform energy profile and a sharp edge and (ii) a line of
sight effect in jets with a variable energy profile. The first scenario
requires the inner engine to produce a jet with a different opening angle each
explosion, while the latter requires a standard engine. The physical structure
of the jet is a crucial factor in understanding GRB progenitors, and therefore
discriminating the two jet scenarios is particularly relevant. In the
structured jet case, specific predictions can be made for the distribution of
observed break angles , while that distribution is
arbitrary in the first scenario. We derive the theoretical distribution for the
structured jet model. Specifically, we predict the most common angle to be
about 0.12 rad, in rough agreement with the sample. If this agreement would
hold as the sample size increases, it would strengthen the case for the
standard jet hypothesis. We show that a prediction of this model is that the
average viewing angle is an increasing function of the survey sensitivity, and
in particular that a mission like {\em Swift} will find the typical viewing
angle to be about 0.3 rad. The local event rate predicted by this model is
Gpc yr.Comment: 14 pages, 3 figures; accepted to Ap
A Self-Similar Solution for the Propagation of a Relativistic Shock in an Exponential Atmosphere
We derive a fully relativistic, self-similar solution to describe the
propagation of a shock along an exponentially decreasing atmosphere, in the
limit of very large Lorentz factor. We solve the problem in planar symmetry and
compute the acceleration of the shock in terms of the density gradient crossed
during its evolution. We apply our solution to the acceleration of shocks
within the atmosphere of a HyperNova, and show that velocities consistent with
the requirements of GRB models can be achieved with exponential atmospheres
spanning a wide density range.Comment: ApJL in pres
Refereeing the Referees: The Continuing Procedural Muddle in Cases of General Reference Pursuant to Ohio Rule of Civil Procedure 53
The judicial reference device set out in Rule 53 of the Ohio Rules of Civil Procedure was touted in 1974 as a procedure capable of relieving the problem of justice delayed by easing the increasing judicial burden on a besieged judiciary. While judicial reference is certainly no panacea for the problem (if numbers alone are a valid indicator), it is difficult to refute claims that the reference device contributes to a more efficient and speedy administration of justice
Deep VLT infrared observations of X-ray Dim Isolated Neutron Stars
X-ray observations have unveiled the existence of a family of radio-quiet
Isolated Neutron Stars whose X-ray emission is purely thermal, hence dubbed
X-ray Dim Isolated Neutron Stars (XDINSs). While optical observations have
allowed to relate the thermal emission to the neutron star cooling and to build
the neutron star surface thermal map, IR observations are critical to pinpoint
a spectral turnover produced by a so far unseen magnetospheric component, or by
the presence of a fallback disk. The detection of such a turnover can provide
further evidence of a link between this class of isolated neutron stars and the
magnetars, which show a distinctive spectral flattening in the IR.
Here we present the deepest IR observations ever of five XDINSs, which we use
to constrain a spectral turnover in the IR and the presence of a fallback disk.
The data are obtained using the ISAAC instrument at the VLT.
For none of our targets it was possible to identify the IR counterpart down
to limiting magnitudes H = 21.5 - 22.9. Although these limits are the deepest
ever obtained for neutron stars of this class, they are not deep enough to rule
out the existence and the nature of a possible spectral flattening in the IR.
We also derive, by using disk models, the upper limits on the mass inflow rate
in a fallback disk. We find the existence of a putative fallback disk
consistent (although not confirmed) with our observations.Comment: 6 pages, 2 figures, accepted by A&A on 26-06-200
UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers
We study the Gamma Ray Burst (GRB) environment and intervening absorbers by
analyzing the optical absorption features produced by gas surrounding the GRB
or along its line of sight. We analyzed high resolution spectroscopic
observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken
with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum
illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which
has at least four components in the main absorption system. We detect strong
FeII, SiII, and NiII excited absorption lines associated with the bluemost
component only. In addition to the host galaxy, at least two more absorbers
lying along the line of sight to the afterglow have been detected in the
redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost
component in the host galaxy, we derive information about its distance from the
site of the GRB explosion. We do so by assuming that the excited absorption
lines are produced by indirect UV pumping, and compare the data with a time
dependent photo-excitation code. The distance of this component is found to be
280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify
two additional MgII absorbers, one of them with a rest frame equivalent width
larger than 1A. The distance between the GRB and the absorber measured in this
paper confirms that the power of the GRB radiation can influence the conditions
of the interstellar medium up to a distance of at least several hundred pc. For
the intervening absorbers, we confirm the trend that on average one strong
intervening system is found per afterglow, as has been noted in studies
exhibiting an excess of strong MgII absorbers along GRB sightlines compared to
quasars.Comment: 8 Pages, 7 ps figures, A&A in pres
Assessing energy efficiency: econometric evidence and implications for Italian energy policy
Energy efficiency represents one of the key objectives of the Clean Energy Package. In this
context, EU member countries determine the optimal policies for energy efficiency. This papers aims
to develop econometric evidence on country-level energy efficiency performance based on
stochastic frontier analysis, with a specific focus on Italy’s efficiency levels. The analysis is based on
data for a sample of 17 European companies over the period 1996-2016. Overall, Italy’s historical
efficiency and performance ranking is consistently above the sample average and is on an increasing
path since 2012. However, efficiency benchmarking shows that there may be scope for efficiency
savings above and beyond the 2030 targets in the set out in the National Energy Plan. Disaggregate
econometric analysis shows that part of these additional efficiency savings may stem from the
industrial and residential sectors. To the authors’ knowledge, this is the first attempt to directly
compare the efficiency assumptions in a National Action Plan using econometric benchmarking
approaches
performance analysis of integrated systems based on mhd generators
Abstract Magnetohydrodynamic (MHD) power generation is considered an interesting energy conversion system because converts thermal energy into electrical energy without mechanically moving parts. In an MHD generator, a thermal plasma is moving across a magnetic field generating electric power. The heat source required to produce the high-speed gas flow can be supplied by the combustion of a fossil fuel or by using renewable source such as solar energy. The MHD efficiency is usually less than the conventional energy conversion systems (i.e. gas turbine combined cycle, steam power plant) but the availability of thermal power at high temperature can allow plant configurations with high overall efficiency. In this paper two plant configurations based on open-cycle MHD generators fed with coal are presented. The first one is a conventional configuration in which the plasma gas is the products of direct combustion of coal. The second one can be considered an advanced type because the working fluid is the combustion exhausts of syngas generated from coal gasification. In order to evaluate the energy suitability of the proposed systems, a performance analysis has been carried out by means of numerical modeling. Therefore, the operating conditions and the plant configurations for an efficient recovery of the thermal energy available from the MHD exhausts have been defined by a sensitivity analysis carried out varying the preheating temperature of air (or enriched air) sent to the combustion chamber. Results show that high system efficiencies (up to 60%) can be achieved by using the syngas due to a better heat recovery in the high temperature region
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