197 research outputs found
Probing axion-like particles with the ultraviolet photon polarization from active galactic nuclei in radio galaxies
The mixing of photons with axion-like particles (ALPs) in the large-scale
magnetic field changes the polarization angle of a linearly polarized
photon beam from active galactic nuclei in radio galaxies as it propagates over
cosmological distances. Using available ultraviolet polarization data
concerning these sources we derive a new bound on the product of the photon-ALP
coupling times . We find
GeV nG for ultralight ALPs with eV. We compare
our new bound with the ones present in the literature and we comment about
possible improvements with observations of more sources.Comment: v2: one typo corrected. Added a few comments, matches published
versio
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
Path integral in a magnetic field using the Trotter product formula
The derivation of the Feynman path integral based on the Trotter product
formula is extended to the case where the system is in a magnetic field.Comment: To appear in the American Journal of Physics, 200
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
No axions from the Sun
Preliminary evidence of solar axions in XMM-Newton observations has quite
recently been claimed by Fraser et al. as an interpretation of their detection
of a seasonally-modulated excess of the X-ray background. Within such an
interpretation, these authors also estimate the axion mass to be eV. Since an axion with this mass behaves as a cold dark
matter particle, according to the proposed interpretation the considered
detection directly concerns cold dark matter as well. So, the suggested
interpretation would lead to a revolutionary discovery if confirmed.
Unfortunately, we have identified three distinct problems in this
interpretation of the observed result of Fraser et al. which ultimately imply
that the detected signal - while extremely interesting in itself - cannot have
any relation with hypothetical axions produced by the Sun. Thus, a physically
consistent interpretation of the observed seasonally-modulated X-ray excess
still remains an exciting challenge.Comment: 3 pages, 1 figure, accepted for publication in MNRA
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