2,921 research outputs found
Effect of spatial resolution on simulated rainfall over western Philippines
This study evaluated the reproducibility of simulated rainfall over western Philippines at different resolutions using a regional climate model, Advanced Research Weather Research and Forecasting Model (WRF-ARW). Four sets of experiments with distinct horizontal mesh sizes of 25, 12.5, 8 and 5 km were done for the summer monsoon period of June to August over 31 years from 1982 to 2012. The spatial distribution of monthly rainfall amount improved for higher resolution simulations. The experiment run at 12.5 km was able to reproduce monthly rainfall distribution closest to observed rainfall. The domains of 8-km and 5-km resolutions, which were further downscaled from a coarser initial domain, performed better in capturing the monthly rainfall distribution than the 25 km-single domain setup. In addition, high rainfall amount was simulated over an offshore area apart from the coastline in the windward direction of the Asian summer monsoon westerlies. This offshore rainfall was also observed in other regions of the monsoon Asia
Challenging GRB models through the broadband dataset of GRB060908
Context: Multiwavelength observations of gamma-ray burst prompt and afterglow
emission are a key tool to disentangle the various possible emission processes
and scenarios proposed to interpret the complex gamma-ray burst phenomenology.
Aims: We collected a large dataset on GRB060908 in order to carry out a
comprehensive analysis of the prompt emission as well as the early and late
afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from
a number of different ground-based optical/NIR and millimeter telescopes
allowed us to follow the afterglow evolution from about a minute from the
high-energy event down to the host galaxy limit. We discuss the physical
parameters required to model these emissions. Results: The prompt emission of
GRB060908 was characterized by two main periods of activity, spaced by a few
seconds of low intensity, with a tight correlation between activity and
spectral hardness. Observations of the afterglow began less than one minute
after the high-energy event, when it was already in a decaying phase, and it
was characterized by a rather flat optical/NIR spectrum which can be
interpreted as due to a hard energy-distribution of the emitting electrons. On
the other hand, the X-ray spectrum of the afterglow could be fit by a rather
soft electron distribution. Conclusions: GRB060908 is a good example of a
gamma-ray burst with a rich multi-wavelength set of observations. The
availability of this dataset, built thanks to the joint efforts of many
different teams, allowed us to carry out stringent tests for various
interpretative scenarios showing that a satisfactorily modeling of this event
is challenging. In the future, similar efforts will enable us to obtain
optical/NIR coverage comparable in quality and quantity to the X-ray data for
more events, therefore opening new avenues to progress gamma-ray burst
research.Comment: A&A, in press. 11 pages, 5 figure
Late evolution of the X-ray afterglow of GRB 030329
The X-ray afterglow of the Gamma-Ray Burst GRB 030329, associated to SN2003dh
at z=0.1685, has been observed with XMM-Newton 258 days after the burst
explosion. A source with flux of (6.2 +/- 2.3) 10^{-16} erg cm^{-2} s^{-1}
(0.5-2 keV) has been detected at the GRB position. This measurement, together
with a re-analysis of the previous X-ray observations, indicates a flattening
of the X-ray light curve ~40 days after the burst. This is in remarkable
agreement with the scenario invoking the presence of two jets with different
opening angles. The wider jet should be responsible for the observed flattening
due to its transition into the non-relativistic Sedov-Taylor phase.Comment: 5 pages, 3 figures, accepted for publication in A&A, revised
discussion and one table adde
Long gamma-ray bursts without visible supernovae: a case study of redshift estimators and alleged novel objects
It has been argued that the observational limits on a supernova (SN)
associated with GRB060614 convincingly exclude a SN akin to SN1998bw as its
originator, and provide evidence for a new class of long-duration GRBs. We
discuss this issue in the contexts of indirect 'redshift estimators' and of the
fireball and cannonball models of GRBs. The latter explains the unusual
properties of GRB060614: at its debated but favoured low redshift (0.125) they
are predicted, as opposed to exceptional, if the associated core-collapse SN is
of a recently discovered, very faint type. We take the occasion to discuss the
'association' between GRBs and SNe.Comment: 19 pages, 2 figures. To be published in Ap
The short GRB070707 afterglow and its very faint host galaxy
We present the results from an ESO/VLT campaign aimed at studying the
afterglow properties of the short/hard gamma ray burst GRB 070707. Observations
were carried out at ten different epochs from ~0.5 to ~80 days after the event.
The optical flux decayed steeply with a power-law decay index greater than 3,
later levelling off at R~27.3 mag; this is likely the emission level of the
host galaxy, the faintest yet detected for a short GRB. Spectroscopic
observations did not reveal any line features/edges that could unambiguously
pinpoint the GRB redshift, but set a limit z < 3.6. In the range of allowed
redshifts, the host has a low luminosity, comparable to that of long-duration
GRBs. The existence of such faint host galaxies suggests caution when
associating short GRBs with bright, offset galaxies, where the true host might
just be too dim for detection. The steepness of the decay of the optical
afterglow of GRB 070707 challenges external shock models for the optical
afterglow of short/hard GRBs. We argue that this behaviour might results from
prolonged activity of the central engine or require alternative scenarios.Comment: 6 pages, 5 figures, accepted by A&
Measurement of the hadronic photon structure function F_{2}^{γ} at LEP2
The hadronic structure function of the photon F_{2}^{γ} (x, Q²) is measured as a function of Bjorken x and of the photon virtuality Q² using deep-inelastic scattering data taken by the OPAL detector at LEP at e⁺e⁻ centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F_{2}^{γ} are extended to an average Q² of 〈Q²〉=780 GeV² using data in the kinematic range 0.15<x<0.98. The Q² evolution of F_{2}^{γ} is studied for 12.1<〈Q²〉<780 GeV² using three ranges of x. As predicted by QCD, the data show positive scaling violations in F_{2}^{γ} with F_{2}^{γ} (Q²)/α = (0.08±0.02⁺⁰·⁰⁵_₀.₀₃) + (0.13±0.01⁺⁰·⁰¹_₀.₀₁) lnQ², where Q² is in GeV², for the central x region 0.10–0.60. Several parameterisations of F_{2}^{γ} are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data
Measurement of the charm structure function F_{2,c)^{γ} of the photon at LEP
The production of charm quarks is studied in deep-inelastic electron–photon scattering using data recorded by the OPAL detector at LEP at nominal e⁺e⁻ centre-of-mass energies from 183 to 209 GeV. The charm quarks have been identified by full reconstruction of charged D* mesons using their decays into D⁰π with the D⁰ observed in two decay modes with charged particle final states, Kπ and Kπππ. The cross-section σ^{D*} for production of charged D* in the reaction e⁺e⁻→e⁺e⁻D*Χ is measured in a restricted kinematical region using two bins in Bjorken x, 0.00140.1 the perturbative QCD calculation at next-to-leading order agrees perfectly with the measured cross-section. For x<0.1 the measured cross-section is 43.8±14.3±6.3±2.8 pb with a next-to-leading order prediction of 17.0⁺²·⁹_₂.₃ pb
The electromagnetic model of Gamma Ray Bursts
I describe electromagnetic model of gamma ray bursts and contrast its main
properties and predictions with hydrodynamic fireball model and its
magnetohydrodynamical extension. The electromagnetic model assumes that
rotational energy of a relativistic, stellar-mass central source
(black-hole--accretion disk system or fast rotating neutron star) is converted
into magnetic energy through unipolar dynamo mechanism, propagated to large
distances in a form of relativistic, subsonic, Poynting flux-dominated wind and
is dissipated directly into emitting particles through current-driven
instabilities. Thus, there is no conversion back and forth between internal and
bulk energies as in the case of fireball model. Collimating effects of magnetic
hoop stresses lead to strongly non-spherical expansion and formation of jets.
Long and short GRBs may develop in a qualitatively similar way, except that in
case of long bursts ejecta expansion has a relatively short, non-relativistic,
strongly dissipative stage inside the star. Electromagnetic and fireball models
(as well as strongly and weakly magnetized fireballs) lead to different early
afterglow dynamics, before deceleration time. Finally, I discuss the models in
view of latest observational data in the Swift era.Comment: solicited contribution to Focus Issue of New Journal of Physics, 27
pages, 4 figure
Procedural feature generation for volumetric terrains using voxel grammars
© 2018 Terrain generation is a fundamental requirement of many computer graphics simulations, including computer games, flight simulators and environments in feature films. There has been a considerable amount of research in this domain, which ranges between fully automated and semi-automated methods. Voxel representations of 3D terrains can create rich features that are not found in other forms of terrain generation techniques, such as caves and overhangs. In this article, we introduce a semi-automated method of generating features for volumetric terrains using a rule-based procedural generation system. Features are generated by selecting subsets of a voxel grid as input symbols to a grammar, composed of user-created operators. This results in overhangs and caves generated from a set of simple rules. The feature generation runs on the CPU and the GPU is utilised to extract a robust mesh from the volumetric dataset
Measurement of triple gauge boson couplings from W⁺W⁻ production at LEP energies up to 189 GeV
A measurement of triple gauge boson couplings is presented, based on W-pair data recorded by the OPAL detector at LEP during 1998 at a centre-of-mass energy of 189 GeV with an integrated luminosity of 183 pb⁻¹. After combining with our previous measurements at centre-of-mass energies of 161–183 GeV we obtain κ = 0.97_{-0.16}^{+0.20}, g_{1}^{z} = 0.991_{-0.057}^{+0.060} and λ = -0.110_{-0.055}^{+0.058}, where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to their Standard Model values. These results are consistent with the Standard Model expectations
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