8,622 research outputs found
An AdS/QCD model from Sen's tachyon action
We construct a new, simple phenomenological model along the lines of AdS/QCD.
The essential new ingredient is the brane-antibrane effective action including
the open string tachyon proposed by Sen. Chiral symmetry breaking happens
because of tachyon dynamics. We fit a large number of low-spin meson masses at
the 10%-15% level. The only free parameters involved in the fits correspond to
the overall QCD-scale and the quark masses. Several aspects of previous models
are qualitatively improved.Comment: 9 pages, references added, discussions improve
Optical spectroscopy of microquasar candidates at low galactic latitudes
We report optical spectroscopic observations of a sample of 6 low-galactic
latitude microquasar candidates selected by cross-identification of X-ray and
radio point source catalogs for |b|<5 degrees. Two objects resulted to be of
clear extragalactic origin, as an obvious cosmologic redshift has been measured
from their emission lines. For the rest, none exhibits a clear stellar-like
spectrum as would be expected for genuine Galactic microquasars. Their
featureless spectra are consistent with being extragalactic in origin although
two of them could be also highly reddened stars. The apparent non-confirmation
of our candidates suggests that the population of persistent microquasar
systems in the Galaxy is more rare than previously believed. If none of them is
galactic, the upper limit to the space density of new Cygnus X-3-like
microquasars within 15 kpc would be 1.1\times10^{-12} per cubic pc. A similar
upper limit for new LS 5039-like systems within 4 kpc is estimated to be
5.6\times10^{-11} per cubic pc.Comment: 7 pages, 7 figures. Published in A&A, see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...413..309
Leptonic emission from microquasar jets: from radio to very high-energy gamma-rays
Microquasars are sources of very high-energy gamma-rays and, very probably,
high-energy gamma-ray emitters. We propose a model for a jet that can allow to
give accurate observational predictions for jet emission at different energies
and provide with physical information of the object using multiwavelength data.Comment: 2 pages, 1 figure. Proceedings of the conference: "International
Astronomical Union Symposium No. 230: Populations of High Energy Sources in
Galaxies". Edited by Evert J.A. Meurs & Giuseppina Fabbian
Three-dimensional Ising model confined in low-porosity aerogels: a Monte Carlo study
The influence of correlated impurities on the critical behaviour of the 3D
Ising model is studied using Monte Carlo simulations. Spins are confined into
the pores of simulated aerogels (diffusion limited cluster-cluster aggregation)
in order to study the effect of quenched disorder on the critical behaviour of
this magnetic system. Finite size scaling is used to estimate critical
couplings and exponents. Long-range correlated disorder does not affect
critical behavior. Asymptotic exponents differ from those of the pure 3D Ising
model (3DIS), but it is impossible, with our precision, to distinguish them
from the randomly diluted Ising model (RDIS).Comment: 10 pages, 10 figures. Submitted to Physical Review
EVN and MERLIN observations of microquasar candidates at low galactic latitudes
In an attempt to increase the number of known microquasars, Paredes et al.
(2002) have presented a long-term project focused on the search for new objects
of this type. They performed a cross-identification between X-ray and radio
catalogs under very restrictive selection criteria for sources with |b|<5
degrees, and obtained a sample of 13 radio-emitting X-ray sources. Follow-up
observations of 6 of these sources with the VLA provided accurate coordinates,
which were used to discover optical counterparts for all of them. We have
observed these six sources with the EVN and MERLIN at 5 GHz. Five of the six
objects have been detected and imaged, presenting different morphologies: one
source has a two-sided jet, three sources have one-sided jets, and one source
is compact. With all the presently available information, we conclude that two
of the sources are promising microquasar candidates in our Galaxy.Comment: 9 pages, 5 figures. Published in A&A, see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...394..983
Gravitational microlensing of gamma-ray blazars
We present a detailed study of the effects of gravitational microlensing on
compact and distant -ray blazars. These objects have -ray
emitting regions which are small enough as to be affected by microlensing
effects produced by stars lying in intermediate galaxies. We analyze the
temporal evolution of the gamma-ray magnification for sources moving in a
caustic pattern field, where the combined effects of thousands of stars are
taken into account using a numerical technique. We propose that some of the
unidentified -ray sources (particularly some of those lying at high
galactic latitude whose gamma-ray statistical properties are very similar to
detected -ray blazars) are indeed the result of gravitational lensing
magnification of background undetected Active Galactic Nuclei (AGNs).Comment: 30 pages, 27 figures. Four figures are being submitted only as .gif
files, and should be printed separately. The abstract below has been
shortened from the actual version appearing in the pape
Hispanics: Does Our Language (Spanish) Define Who We Are?
Use of the pan-ethnic denomination Hispanic to refer to Spanishspeakers assumes a homogeneous group. Hispanics literally constitute 20 national origins with different characteristics and idiosyncrasies. Because individuals’ national origin may influence their traditions, customs, values, and beliefs, differences in nationality should be considered in designing research
Nonthermal processes and neutrino emission from the black hole GRO J0422+32 in a bursting state
GRO J0422+32 is a member of the class of low-mass X-ray binaries (LMXBs). It
was discovered during an outburst in 1992. During the entire episode a
persistent power-law spectral component extending up to MeV was
observed, which suggests that nonthermal processes should have occurred in the
system. We study relativistic particle interactions and the neutrino production
in the corona of GRO J0422+32, and explain the behavior of GRO J0422+32 during
its recorded flaring phase. We have developed a magnetized corona model to fit
the spectrum of GRO J0422+32 during the low-hard state. We also estimate
neutrino emission and study the detectability of neutrinos with 1 km
detectors, such as IceCube. The short duration of the flares ( hours) and
an energy cutoff around a few TeV in the neutrino spectrum make neutrino
detection difficult. There are, however, many factors that can enhance neutrino
emission. The northern-sky coverage and full duty cycle of IceCube make it
possible to detect neutrino bursts from objects of this kind through
time-dependent analysis.Comment: 12 pages, 11 figures, accepted for publication in A&
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