7,554 research outputs found
The X-ray light curve of Gamma-ray bursts: clues to the central engine
We present the analysis of a large sample of gamma-ray burst (GRB) X-ray
light curves in the rest frame to characterise their intrinsic properties in
the context of different theoretical scenarios. We determine the morphology,
time scales, and energetics of 64 long GRBs observed by \emph{Swift}/XRT
\emph{without} flaring activity. We furthermore provide a one-to-one comparison
to the properties of GRBs \emph{with} X-ray flares. We find that the steep
decay morphology and its connection with X-ray flares favour a scenario in
which a central engine origin. We show that this scenario can also account for
the shallow decay phase, provided that the GRB progenitor star has a
self-similar structure with a constant envelope-to-core mass ratio . However, difficulties arise for very long duration
( s) shallow phases. Alternatively, a spinning-down magnetar
whose emitted power refreshes the forward shock can quantitatively account for
the shallow decay properties. In particular we demonstrate that this model can
account for the plateau luminosity vs. end time anticorrelation.Comment: 12 pages, 8 figures, accepted for publication in A&
Swift J0525.6+2416 and IGR J04571+4527: two new hard X-ray selected magnetic cataclysmic variables identified with XMM-Newton
IGR J04571+4527 and Swift J0525.6+2416 are two hard X-ray sources detected in
the Swift/BAT and INTEGRAL/IBIS surveys. They were proposed to be magnetic
cataclysmic variables of the Intermediate Polar (IP) type, based on optical
spectroscopy. IGR J04571+4527 also showed a 1218 s optical periodicity,
suggestive of the rotational period of a white dwarf, further pointing towards
an IP classification. We here present detailed X-ray (0.3-10 keV) timing and
spectral analysis performed with XMM-Newton, complemented with hard X-ray
coverage (15-70 keV) from Swift/BAT. These are the first high signal to noise
observations in the soft X-ray domain for both sources, allowing us to identify
the white dwarf X-ray spin period of Swift J0525.6+2416 (226.28 s), and IGR
J04571+4527 (1222.6 s). A model consisting of multi-temperature optically thin
emission with complex absorption adequately fits the broad-band spectrum of
both sources. We estimate a white dwarf mass of about 1.1 and 1.0 solar masses
for IGR J04571+4527 and Swift J0525.6+2416, respectively. The above
characteristics allow us to unambiguously classify both sources as IPs,
confirming the high incidence of this subclass among hard X-ray emitting
Cataclysmic Variables.Comment: 8 pages, 4 figures, 3 tables. Accepted for publication in MNRA
Combining Structural and Non-structural Risk-reduction Measures to Improve Evacuation Safety in Historical Built Environments
Historical city centres are critical-built environments prone to earthquake risk because of the features of the complex network of Architectural Heritage, facing Open Spaces (OSs) and the users hosted by them. Structural measures, such as building retrofitting actions, and non-structural measures, like emergency and evacuation planning, could be jointly set up to improve the safety of the Historical Built Environments since they aim to face the effects of damage across the OSs and so on the users’ movement. This work adopts a simulation-based approach to assess the individual and combined efficiency of these structural and non-structural measures. Risk indices focused on the evacuation process are used to this end, considering the final effects on the users’ movement and safety. Results show how seismic retrofitting strategies could be located in critical “hot-spots” in the urban fabric to additionally support the evacuation plan, thus reducing implementation efforts for the stakeholders
SIMULATING TO EVALUATE, MANAGE AND IMPROVE EARTHQUAKE RESILIENCE IN HISTORICAL CITY CENTERS: APPLICATION TO AN EMERGENCY SIMULATION-BASED METHOD TO THE HISTORIC CENTRE OF COIMBRA
Abstract. Earthquake resilience in historical centres is significantly affected by interactions between the built environment, defined as the network of building heritage and surrounding open spaces, and hosted population. Building vulnerability, earthquake-induced effects and population's exposure mainly influence the first emergency phases. In the immediate post-earthquake evacuation conditions, people should leave their position to gather in assembly points where first responders can rescue them. Thus, joint analyses of building damage and evacuation flows along the evacuation paths become essential to determine the risk levels for the urban scenario and to provide risk-mitigation solutions. This paper tries to reach this goal by adopting a holistic simulation-based approach. A simplified vulnerability assessment method is used to evaluate the seismic performance of masonry façade walls and to estimate debris depth on outdoor spaces. An existing earthquake pedestrians' evacuation simulator is used to evaluate the probable pedestrians' choices in such evacuation post-earthquake damage scenarios. Then, risk indexes, combining damage assessment and evacuation results, are provided to quantify evacuation safety and to outline critical conditions in the urban layout. Finally, the impact resulting from the consideration of a series of resilience-increasing strategies is simulated and discussed from the proposed risk indexes. A part of the historic centre of Coimbra, Portugal, one of the oldest and most relevant Portuguese cities, is used in this work as a pilot case study. Results show how the method could be used by Local Authorities and Civil Protection Bodies to outline, analyse and coordinate resilience-increasing strategies at the urban scale
Limitations on the principle of stationary phase when it is applied to tunneling analysis
Using a recently developed procedure - multiple wave packet decomposition -
here we study the phase time formulation for tunneling/reflecting particles
colliding with a potential barrier. To partially overcome the analytical
difficulties which frequently arise when the stationary phase method is
employed for deriving phase (tunneling) time expressions, we present a
theoretical exercise involving a symmetrical collision between two identical
wave packets and an one-dimensional rectangular potential barrier. Summing the
amplitudes of the reflected and transmitted waves - using a method we call
multiple peak decomposition - is shown to allow reconstruction of the scattered
wave packets in a way which allows the stationary phase principle to be
recovered.Comment: 17 pages, 2 figure
The Amati relation in the "fireshell" model
(Shortened) CONTEXT: [...] AIMS: Motivated by the relation proposed by Amati
and collaborators, we look within the ``fireshell'' model for a relation
between the peak energy E_p of the \nu F_\nu total time-integrated spectrum of
the afterglow and the total energy of the afterglow E_{aft}, which in our model
encompasses and extends the prompt emission. METODS: [...] Within the fireshell
model [...] We can then build two sets of ``gedanken'' GRBs varying the total
energy of the electron-positron plasma E^{e^\pm}_{tot} and keeping the same
baryon loading B of GRB050315. The first set assumes for the effective CBM
density the one obtained in the fit of GRB050315. The second set assumes
instead a constant CBM density equal to the average value of the GRB050315
prompt phase. RESULTS: For the first set of ``gedanken'' GRBs we find a
relation E_p\propto (E_{aft})^a, with a = 0.45 \pm 0.01, whose slope strictly
agrees with the Amati one. Such a relation, in the limit B \to 10^{-2},
coincides with the Amati one. Instead, in the second set of ``gedanken'' GRBs
no correlation is found. CONCLUSIONS: Our analysis excludes the Proper-GRB
(P-GRB) from the prompt emission, extends all the way to the latest afterglow
phases and is independent on the assumed cosmological model, since all
``gedanken'' GRBs are at the same redshift. The Amati relation, on the other
hand, includes also the P-GRB, focuses on the prompt emission only, and is
therefore influenced by the instrumental threshold which fixes the end of the
prompt emission, and depends on the assumed cosmology. This may well explain
the intrinsic scatter observed in the Amati relation.Comment: 4 pages, 5 figures, to appear on A&A Letter
Accurate calculation of polarization-related quantities in semiconductors
We demonstrate that polarization-related quantities in semiconductors can be
predicted accurately from first-principles calculations using the appropriate
approach to the problem, the Berry-phase polarization theory. For III-V
nitrides, our test case, we find polarizations, polarization differences
between nitride pairs, and piezoelectric constants quite close to their
previously established values. Refined data are nevertheless provided for all
the relevant quantities.Comment: RevTeX 4 pages, no figure
First-principles prediction of structure, energetics, formation enthalpy, elastic constants, polarization, and piezoelectric constants of AlN, GaN, and InN: comparison of local and gradient-corrected density-functional theory
A number of diverse bulk properties of the zincblende and wurtzite III-V
nitrides AlN, GaN, and InN, are predicted from first principles within density
functional theory using the plane-wave ultrasoft pseudopotential method, within
both the LDA (local density) and GGA (generalized gradient) approximations to
the exchange-correlation functional. Besides structure and cohesion, we study
formation enthalpies (a key ingredient in predicting defect solubilities and
surface stability), spontaneous polarizations and piezoelectric constants
(central parameters for nanostructure modeling), and elastic constants. Our
study bears out the relative merits of the two density functional approaches in
describing diverse properties of the III-V nitrides (and of the parent species
N, Al, Ga, and In), and leads us to conclude that the GGA approximation,
associated with high-accuracy techniques such as multiprojector ultrasoft
pseudopotentials or modern all-electron methods, is to be preferred in the
study of III-V nitrides.Comment: RevTeX 6 pages, 12 tables, 0 figure
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