1,210 research outputs found
Theory of photospheric emission from relativistic outflows
In this paper we reexamine the optical depth of ultrarelativistic spherically
symmetric outflows and reevaluate the photospheric radius for each model during
both the acceleration and coasting phases. It is shown that for both the wind
and the shell models there are two asymptotic solutions for the optical depth
during the coasting phase of the outflow. In particular we show that quite
counterintuitively a geometrically thin shell may appear as a thick wind for
photons propagating inside it. For this reason we introduce notions of photon
thick and photon thin outflows, which appear more general and better physically
motivated with respect to winds and shells. Photosphere of relativistic outflow
is a dynamic surface. We study its geometry and find that the photosphere of
photon thin outflow has always a convex shape, while in the photon thick one it
is initially convex (there is always a photon thin layer in any outflow) and
then it becomes concave asymptotically approaching the photosphere of an
infinitely long wind. We find that both instantaneous and time integrated
observed spectra are very close to the thermal one for photon thick outflows,
in line with existing studies. It is our main finding that the photospheric
emission from the photon thin outflow produces non thermal time integrated
spectra, which may be described by the Band function well known in the GRB
literature. We find that energetic GRBs should produce photon thin outflows
with photospheric emission lasting less than one second for the total energy
erg and baryonic loading parameter . It means
that only time integrated spectra may be observed from such GRBs.Comment: Revision of the previous version, new effect is discussed.
Conclusions remain unchange
Maximum Mass-Radius Ratios for Charged Compact General Relativistic Objects
Upper limits for the mass-radius ratio and total charge are derived for
stable charged general relativistic matter distributions. For charged compact
objects the mass-radius ratio exceeds the value 4/9 corresponding to neutral
stars. General restrictions for the redshift and total energy (including the
gravitational contribution) are also obtained.Comment: 6 pages, 2 figures, RevTex. To appear in Europhys. Let
Ionospheric tomography using GNSS reflections
In this paper, we report a preliminary analysis of the impact of Global Navigation Satellite System Reflections (GNSS-R) data on ionospheric monitoring over the oceans. The focus will be on a single polar Low Earth Orbiter (LEO) mission exploiting GNSS-R as well as Navigation (GNSS-N) and Occultation (GNSS-O) total electron content (TEC) measurements. In order to assess impact of the data, we have simulated GNSS-R/O/N TEC data as would be measured from the LEO and from International Geodesic Service (IGS) ground stations, with an electron density (ED) field generated using a climatic ionospheric model. We have also developed a new tomographic approach inspired by the physics of the hydrogen atom and used it to effectively retrieve the ED field from the simulated TEC data near the orbital plane. The tomographic inversion results demonstrate the significant impact of GNSS-R: three-dimensional ionospheric ED fields are retrieved over the oceans quite accurately, even as, in the spirit of this initial study, the simulation and inversion approaches avoided intensive computation and sophisticated algorithmic elements (such as spatio-temporal smoothing). We conclude that GNSS-R data over the oceans can contribute significantly to a Global/GNSS Ionospheric Observation System (GIOS). Index Terms Global Navigation Satellite System (GNSS), Global Navigation Satellite System Reflections (GNSS-R), ionosphere, Low Earth Orbiter (LEO), tomography
Evidence Against Astrophysical Dyadospheres
It is shown how pair production itself would almost certainly prevent the
astrophysical formation of macroscopic dyadospheres, hypothetical regions,
extending many electron Compton wavelengths in all directions, where the
electric field exceeds the critical value for microscopically rapid Schwinger
pair production. Pair production is a self-regulating process that would
discharge a growing electric field, in the example of a hypothetical collapsing
charged stellar core, before it reached 6% of the minimum dyadosphere value,
keeping the pair production rate more than 26 orders of magnitude below the
dyadosphere value, and keeping the efficiency below 0.0002 (M/M_sun)^{1/2}.Comment: 27 pages, LaTeX, shortened version of astro-ph/0605432 accepted 2006
Aug. 17 by The Astrophysical Journal, but also with some significant new
materia
On the structure of the burst and afterglow of Gamma-Ray Bursts I: the radial approximation
We have proposed three paradigms for the theoretical interpretation of
gamma-ray bursts (GRBs). (1) The relative space-time transformation (RSTT)
paradigm emphasizes how the knowledge of the entire world-line of the source
from the moment of gravitational collapse is a necessary condition to interpret
GRB data. (2) The interpretation of the burst structure (IBS) paradigm
differentiates in all GRBs between an injector phase and a beam-target phase.
(3) The GRB-supernova time sequence (GSTS) paradigm introduces the concept of
induced supernova explosion in the supernovae-GRB association. These three
paradigms are illustrated using our theory based on the vacuum polarization
process occurring around an electromagnetic black hole (EMBH theory) and using
GRB 991216 as a prototype. We illustrate the five fundamental eras of the EMBH
theory: the self acceleration of the pair-electromagnetic plasma (PEM
pulse), its interaction with the baryonic remnant of the progenitor star (PEMB
pulse). We then study the approach of the PEMB pulse to transparency, the
emission of the proper GRB (P-GRB) and its relation to the ``short GRBs''.
Finally the three different regimes of the afterglow are described within the
fully radiative and radial approximations. The best fit of the theory leads to
an unequivocal identification of the ``long GRBs'' as extended emission
occurring at the afterglow peak (E-APE). The relative intensities, the time
separation and the hardness ratio of the P-GRB and the E-APE are used as
distinctive observational test of the EMBH theory and the excellent agreement
between our theoretical predictions and the observations are documented. The
afterglow power-law indexes in the EMBH theory are compared and contrasted with
the ones in the literature, and no beaming process is found for GRB 991216.Comment: 96 pages, 40 figures, to appear on Int. Journ. Mod. Phys.
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Connected OFCity Challenge: Addressing the Digital Divide in the Developing World
Over the past 50 years, the development of information and communications technology has provided unprecedented support to the steady economic growth of developed countries. In recent years, some of the largest growth has been reported in emerging economies, which, however, often lack adequate telecommunications infrastructure to further sustain their development. Although a number of service providers and system vendors have started to address the issue, the challenges they encounter are substantially different from those in the developed world, including an unreliable electricity grid, poor fiber infrastructure, low revenue expectations, and often a harsh climate environment. This paper reports use cases and solutions pertinent to the development of the networking infrastructure in emerging economies, provided by organizations directly involved in such activities. After providing some background information on the current state of network infrastructure and the main challenges for Africa and rural China, the paper provides details on two proposed solutions. The first focuses on the provisioning of services and network infrastructure through the development of low-cost data centers, whereas the second proposes cost-effective adaptation of both fiber and hybrid copper-fiber technology to rural areas. The article is concluded with a brief discussion on the complementarity of the two approaches
Thermalization of a nonequilibrium electron-positron-photon plasma
Starting from a nonequilibrium configuration we analyse the essential role of
the direct and the inverse binary and triple interactions in reaching an
asymptotic thermal equilibrium in a homogeneous isotropic
electron-positron-photon plasma. We focus on energies in the range 0.1--10 MeV.
We numerically integrate the integro-partial differential relativistic
Boltzmann equation with the exact QED collisional integrals taking into account
all binary and triple interactions in the plasma. We show that first, when
detailed balance is reached for all binary interactions on a timescale
sec, photons and electron-positron pairs establish
kinetic equilibrium. Successively, when triple interactions fulfill the
detailed balance on a timescale sec, the plasma reaches
thermal equilibrium. It is shown that neglecting the inverse triple
interactions prevents reaching thermal equilibrium. Our results obtained in the
theoretical physics domain also find application in astrophysics and cosmology.Comment: 4 pages, 3 figures, Phys. Rev. Lett., to appea
Emergence of a filamentary structure in the fireball from GRB spectra
It is shown that the concept of a fireball with a definite filamentary
structure naturally emerges from the analysis of the spectra of Gamma-Ray
Bursts (GRBs). These results, made possible by the recently obtained analytic
expressions of the equitemporal surfaces in the GRB afterglow, depend crucially
on the single parameter R describing the effective area of the fireball
emitting the X- and gamma ray radiation. The X- and gamma ray components of the
afterglow radiation are shown to have a thermal spectrum in the co-moving frame
of the fireball and originate from a stable shock front described
self-consistently by the Rankine-Hugoniot equations. Precise predictions are
presented on a correlations between spectral changes and intensity variations
in the prompt radiation verifiable, e.g., by the Swift and future missions. The
highly variable optical and radio emission depends instead on the parameters of
the surrounding medium. The GRB 991216 is used as a prototype for this model.Comment: 9 pages, 3 figures, to appear on International Journal of Modern
Physics
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