764 research outputs found
Gamma ray astronomy and baryonic dark matter
Recently, Dixon et al. have re-analyzed the EGRET data, finding a
statistically significant diffuse -ray emission from the galactic halo.
We show that this emission can naturally be explained within a
previously-proposed model for baryonic dark matter, in which -rays are
produced through the interaction of high-energy cosmic-ray protons with cold
clouds clumped into dark clusters - these dark clusters supposedly
populate the outer galactic halo and can show up in microlensing observations.
Our estimate for the halo -ray flux turns out to be in remarkably good
agreement with the discovery by Dixon et al. We also address future prospects
to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999
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
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
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
Constraints on Cold H_2 Clouds from Gravitational Microlensing Searches
It has been proposed that the Galaxy might contain a population of cold
clouds in numbers sufficient to account for a substantial fraction of the total
mass of the Galaxy. These clouds would have masses of the order of 10^{-3}
Solar mass and sizes of the order of 10 AU. We consider here the lensing
effects of such clouds on the light from background stars. A semianalytical
formalism for calculation of the magnification event rate produced by such
gaseous lensing is developed, taking into account the spatial distribution of
the dark matter in the Galaxy, the velocity distribution of the lensing clouds
and source stars, and motion of the observer. Event rates are calculated for
the case of gaseous lensing of stars in the Large Magellanic Cloud and results
are directly compared with the results of the search for gravitational
microlensing events undertaken by the MACHO collaboration. The MACHO experiment
strongly constrains the properties of the proposed molecular clouds, but does
not completely rule them out. Future monitoring programs will either detect or
more strongly constrain this proposed population.Comment: 36 pages, 9 figures, 1 table, typos corrected, minor change
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
A catalogue sample of low mass galaxies observed in X-rays with central candidate black holes
We present a sample of -ray selected candidate black holes in 51 low mass
galaxies with {and mass up to M} obtained by
cross-correlating the NASA-SLOAN Atlas with the 3XMM catalogue. {We have also
searched in the available catalogues for radio counterparts of the black hole
candidates and find that 19 of the previously selected sources have also a
radio counterpart.} Our results show that about of the galaxies of our
sample host { an -ray source} (associated to a radio counterpart) spatially
coincident with the galaxy center, in agreement with { other recent works}. For
these {\it nuclear} sources, the -ray/radio fundamental plane relation
allows one to estimate the mass of the (central) candidate black holes which
results to be in the range M (with median
value of M and eight candidates having mass
below M). This result, while suggesting that -ray
emitting black holes in low-mass galaxies may have had a key role in the
evolution of such systems, makes even more urgent to explain how such massive
objects formed in galaxies. {Of course, dedicated follow-up observations both
in the -ray and radio bands, as well as in the optical, are necessary in
order to confirm our resultsComment: 15 Pages, 2 Figures, 3 Table
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