829 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
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
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
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
Signatures of rotating binaries in micro-lensing experiments
Gravitational microlensing offers a powerful method with which to probe a
variety of binary-lens systems, as the binarity of the lens introduces
deviations from the typical (single-lens) Paczy\'nski behaviour in the event
light curves. Generally, a static binary lens is considered to fit the observed
light curve and, when the orbital motion is taken into account, an
oversimplified model is usually employed. In this paper, we treat the
binary-lens motion in a realistic way and focus on simulated events that are
fitted well by a Paczy\'nski curve. We show that an accurate timing analysis of
the residuals (calculated with respect to the best-fitting Paczy\'nski model)
is usually sufficient to infer the orbital period of the binary lens. It goes
without saying that the independently estimated period may be used to further
constrain the orbital parameters obtained by the best-fitting procedure, which
often gives degenerate solutions. We also present a preliminary analysis of the
event OGLE-2011-BLG-1127 / MOA-2011-BLG-322, which has been recognized to be
the result of a binary lens. The period analysis results in a periodicity of
\simeq 12 days, which confirms the oscillation of the observed data around the
best-fitting model. The estimated periodicity is probably associated with an
intrinsic variability of the source star, and therefore there is an opportunity
to use this technique to investigate either the intrinsic variability of the
source or the effects induced by the binary-lens orbital motion.Comment: In press on MNRAS, 2014. 8 pages, 4 figures. On-line material
available on the Journal web-pag
MHOs and molecular clouds in dark galactic halos
We outline a scenario in which dark clusters of Massive Halo Objects (MHOs) and molecular clouds form in the halo at galactocentric distances larger than 10–20 kpc, provided baryons are a major constituent of the halo. Possible signatures of the presence of molecular clouds in our galaxy are discussed. We also discuss how molecular clouds as well as MHOs can be observed directly in the nearby M31 galaxy
Observing molecular hydrogen clouds and dark massive objects in galactic halos
Molecular hydrogen clouds can contribute substantially to the galactic halo<
dark matter and may lead to the birth of massive halo objects (MHOs) observed
indirectly by microlensing. We present a method to detect these molecular
clouds in the halo of M31 using the Doppler shift effect. We also consider the
possibility to directly observe MHOs in the halo of M31 via their infrared
emission.Comment: 7 pages, postscript file, to appear in Astron. & Astrophy
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