2,365 research outputs found
High-energy -ray emission from GRBs
GRBs are nowadays a rather well understood phenomenon in the soft (KeV-MeV)
-ray energy band, while only a few GRBs have been observed at high
photon energies (E_{\gamma} \ut > 1 GeV). It is also widely recognized that
GRBs accelerate protons to relativistic energies and that dense media are often
present nearby the sources. Within this framework we compute in detail the
high-energy -ray flux from the decay of neutral pions produced through
the interaction of accelerate protons with nucleons in the surrounding medium.
We also take into account the local and intergalactic -ray absorption.
The presence of magnetic fields around the GRB sources causes the deflection of
the accelerated protons and so a temporal spread of the produced high-energy
-rays with respect to the signal in the soft -ray band.
Moreover, we analyze the possibility to detect the -ray signal in the
GeV-TeV energy range by the ARGO detector under construction in Tibet.Comment: 9 pages, 7 figures, abstract shortened, to appear in Astronomy and
Astrophysic
Binary brown dwarfs in the galactic halo?
Microlensing events towards the Large Magellanic Cloud entail that a sizable
fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact
Halo Objects), presumably located in the halo of the Galaxy. Within the present
uncertainties, brown dwarfs are a viable candidate for MACHOs. Various reasons
strongly suggest that a large amount of MACHOs should actually consist of
binary brown dwarfs. Yet, this circumstance looks in flat contradiction with
the fact that MACHOs have been detected as unresolved objects so far. We show
that such an apparent paradox does not exist within a model in which MACHOs are
clumped into dark clusters along with cold molecular clouds, since dynamical
friction on these clouds makes binary brown dwarfs very close. Moreover, we
argue that future microlensing experiments with a more accurate photometric
observation can resolve binary brown dwarfs.Comment: Latex file. To appear in Mont. Not. R. Astr. So
Gamma ray emission from a baryonic dark halo
A recent re-analysis of EGRET data by Dixon et al. has led to the discovery
of a statistically significant diffuse -ray emission from the galactic
halo. We show that this emission can naturally be accounted for within a
previously-proposed model for baryonic dark matter, according to which dark
clusters of brown dwarfs and cold self-gravitating clouds populate the
outer galactic halo and can show up in microlensing observations. Basically,
cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark
clusters, giving rise to the observed -ray flux. We derive maps for the
corresponding intensity distribution, which turn out to be in remarkably good
agreement with those obtained by Dixon et al. We also address future prospects
to test our predictions.Comment: 22 pages, 2 figures, slightly shortened version. to appear in New
Journal of Physic
Timing analysis in microlensing
Timing analysis is a powerful tool used to determine periodic features of
physical phenomena. Here we review two applications of timing analysis to
gravitational microlensing events. The first one, in particular cases, allows
the estimation of the orbital period of binary lenses, which in turn enables
the breaking of degeneracies. The second one is a method to measure the
rotation period of the lensed star by observing signatures due to stellar spots
on its surface.Comment: 11 pages, 4 figures. To be published in International Journal of
Modern Physics D (IJMPD
La questione del metodo nella scienza giuridica tributaria italiana e l’origine della nozione di tributo
Cento anni fa in Italia si svolse una disputa tra giuristi ed economisti che riguardò il metodo di studio del fenomeno finanziario e in particolare dei tributi.
La questione del metodo ebbe un incipit didattico. La discussione fu animata dalla circostanza che la legislazione universitaria vigente accomunava in un unico insegnamento la “scienza delle finanze e il diritto finanziario” .
Benvenuto Griziotti ebbe a constatare che la conoscenza dell’attività finanziaria dello Stato era molto progredita per i profili economici e quasi del tutto trascurata per gli aspetti giuridici e asserì che la finanza pubblica dovesse essere studiata unitariamente , comprendendo in una visione integralista sia l’aspetto politico, cheUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
Statistical Mechanics Approach to Inverse Problems on Networks
Statistical Mechanics has gained a central role in modern Inference and Computer Science. Many optimization and inference problems can be cast in a Statistical Mechanics framework, and various concepts and methods developed in this area of Physics can be very helpful not only in the theoretical analysis, but also constitute valuable tools for solving single instance cases of hard inference and computational tasks. In this work, I address various inverse problems on networks, from models of epidemic spreading to learning in neural networks, and apply a variety of methods which have been developed in the context of Disordered Systems, namely Replica and Cavity methods from the theoretical side, and their algorithmic incarnation, Belief Propagation, to solve hard inverse problems which can be formulated in a Bayesian framework
MACHOs as brown dwarfs
Recent observations of microlensing events in the Large Magellanic Cloud
suggest that a sizable fraction of the galactic halo is in the form of Massive
Astrophysical Compact Halo Objects (MACHOs). Although the average MACHO mass is
presently poorly known, the value looks as a realistic
estimate, thereby implying that brown dwarfs are a viable and natural candidate
for MACHOs. We describe a scenario in which dark clusters of MACHOs and cold
molecular clouds (mainly of ) naturally form in the halo at galactocentric
distances larger than 10-20 kpc. Moreover, we discuss various experimental
tests of this picture.Comment: To appear in the proceedings of the workshop DM-ITALIA-9
Observational Features of Black Holes
Recently considered a very attracting possibility to detect retro-MACHOs,
i.e. retro-images of the Sun by a Schwarzschild black hole. In this paper we
discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons
and propose a procedure to measure masses and rotation parameters analyzing
these forms of mirages. In some sense that is a manifestation of gravitational
lens effect in the strong gravitational field near black hole horizon and a
generalization of the retro-gravitational lens phenomenon. We analyze the case
of a Kerr black hole rotating at arbitrary speed for some selected positions of
a distant observer with respect to the equatorial plane of a Kerr black hole.
We discuss glories (mirages) formed near rapidly rotating Kerr black hole
horizons and propose a procedure to measure masses and rotation parameters
analyzing these forms of mirages. Some time ago suggested to search shadows at
the Galactic Center. In this paper we present the boundaries for shadows
calculated numerically. We also propose to use future radio interferometer
RADIOASTRON facilities to measure shapes of mirages (glories) and to evaluate
the black hole spin as a function of the position angle of a distant observer.Comment: Plenary talk presented at Workshop on High Energy Physics&Field
Theory (Protvino, Russia, 2004
- …