683 research outputs found
Observational constraints on annihilation sites in 1E 1740.7-2942 and Nova Muscae
The region of the Galactic center contains several sources which demonstrate
their activity at various wavelengths and particularly above several hundred
keV. Escape of positrons from such a source or several sources into the
interstellar medium, where they slow down and annihilate, can account for the
511 keV narrow line observed from this direction. 1E 1740.7-2942 object has
been proposed as the most likely candidate to be responsible for this variable
source of positrons. Besides, Nova Muscae shows a spectrum which is consistent
with Comptonization by a thermal plasma kT<100 keV in its hard X-ray part,
while a relatively narrow annihilation line observed by SIGMA on January 20-21,
1991 implies that positrons annihilate in a much colder medium.
We estimate the electron number density and the size of the emitting regions
suggesting that annihilation features observed by SIGMA from Nova Muscae and 1E
1740.7-2942 are due to the positron slowing down and annihilation in thermal
plasma. We show that in the case of Nova Muscae the observed radiation is
coming from a pair plasma stream, N(e+)~N(e-), rather than from a gas cloud. We
argue that two models are probably relevant to the 1E source: annihilation in
(hydrogen) plasma N(e+)<~N(e-) at rest, and annihilation in the pair plasma
stream, which involves matter from the source environment.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. of 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia
Diffuse emission measurement with INTEGRAL/SPI as indirect probe of cosmic-ray electrons and positrons
Significant advances have been made in the understanding of the diffuse
Galactic hard X-ray continuum emission using data from the INTEGRAL
observatory. The diffuse hard power-law component seen with the INTEGRAL/SPI
spectrometer has been identified with inverse-Compton emission from
relativistic (GeV) electrons on the cosmic microwave background and Galactic
interstellar radiation field. In the present analysis, SPI data from 2003 to
2009, with a total exposure time of ~ 10^8 s, are used to derive the Galactic
ridge hard X-ray spatial distribution and spectrum between 20 keV and 2.4 MeV.
Both are consistent with predictions from the GALPROP code. The good agreement
between measured and predicted emission from keV to GeV energies suggests that
the correct production mechanisms have been identified. We discuss the
potential of the SPI data to provide an indirect probe of the interstellar
cosmic-ray electron distribution, in particular for energies below a few GeV.Comment: 39 pages, 11 figures. Accepted for publication in The Astrophysical
Journa
Active estimation of a multipath propagation channel with a Bayesian strategy
The active identification of the impulse response of a channel is studied .
First, the connexion between the model used for the channel description
and the optimal identification method is exhihited . In the case of a
multipath channel model, two non linear estimation methods are proposed
: a generalized likelihood maximum method capable ta estimate the
close paths, and a bayesian (a posteriori maximum) method which takes
into account the successive data often available for identification . The performances of' the methods are tested on simulations . Some results
obtained front two underwater acoustic experiments are finally given.On s'intéresse à l'identification active de la réponse impulsionnelle d'un canal. On insiste d'abord sur le lien existant entre le modèle utilisé pour décrire le canal, et la méthode d'estimation optimale correspondante à utiliser. Dans le cas d'un modèle de canal «à trajets multiples», deux méthodes d'identification non linéaires sont proposées: une méthode de maximum de vraisemblance généralisée qui permet d'estimer les retards proches, et une méthode bayésienne (ou encore de maximum a posteriori) (MAP) permettant de tenir compte des données successives acquises pour l'identification: les performances de ces méthodes sont testées sur simulation
Probabilistic representation for solutions of an irregular porous media type equation: the degenerate case
We consider a possibly degenerate porous media type equation over all of
with , with monotone discontinuous coefficients with linear
growth and prove a probabilistic representation of its solution in terms of an
associated microscopic diffusion. This equation is motivated by some singular
behaviour arising in complex self-organized critical systems. The main idea
consists in approximating the equation by equations with monotone
non-degenerate coefficients and deriving some new analytical properties of the
solution
Extreme rainfall variability in Australia: Patterns, drivers and predictability
Leading patterns of observed monthly extreme rainfall variability in Australia are examined using an Empirical Orthogonal Teleconnection (EOT) method. Extreme rainfall variability is more closely related to mean rainfall variability during austral summer than in winter. The leading EOT patterns of extreme rainfall explain less variance in Australia-wide extreme rainfall than is the case for mean rainfall EOTs. We illustrate that, as with mean rainfall, the El Niño-Southern Oscillation (ENSO) has the strongest association with warm-season extreme rainfall variability, while in the cool-season the primary drivers are atmospheric blocking and the subtropical ridge. The Indian Ocean Dipole and Southern Annular Mode also have significant relationships with patterns of variability during austral winter and spring.
Leading patterns of summer extreme rainfall variability have predictability several months ahead from Pacific sea surface temperatures (SSTs) and as much as a year in advance from Indian Ocean SSTs. Predictability from the Pacific is greater for wetter than average summer months than for months that are drier than average, whereas for the Indian Ocean the relationship has greater linearity.
Several cool-season EOTs are associated with mid-latitude synoptic-scale patterns along the south and east coasts. These patterns have common atmospheric signatures denoting moist onshore flow and strong cyclonic anomalies often to the north of a blocking anti-cyclone.
Tropical cyclone activity is observed to have significant relationships with some warm season EOTs.
This analysis shows that extreme rainfall variability in Australia can be related to remote drivers and local synoptic-scale patterns throughout the year
Hard X-ray emission of the microquasar GX 339-4 in the low/hard state
We present the analysis of the high-energy emission of the Galactic black hole binary GX 339-4 in a low/hard state at the beginning of its 2004 outburst. The data from 273 ks of INTEGRAL observations, spread over 4 weeks, are analyzed, along with the existing simultaneous RXTE HEXTE and PCA data. During this period, the flux increases by a factor of ~=3, while the spectral shape is quite unchanged, at least up to 150 keV. The high-energy data allow us to detect the presence of a high-energy cutoff, generally related to thermal mechanisms, and to estimate the plasma parameters in the framework of the Comptonization models. We found an electron temperature of 60-70 keV and an optical depth of around 2.5, with a rather low reflection factor (0.2-0.4). In the last observation, we detected a high-energy excess above 200 keV with respect to thermal Comptonization, while at lower energies the spectrum is practically identical to the previous one taken just 2 days before. This suggests that the low- and high-energy components have a different origin
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