223 research outputs found
Are there MACHOs in the Milky Way halo?
Microlensing searches aim to detect compact halo dark matter via its
gravitational lensing effect on stars within the Large Magellanic Cloud. The
most recent results have led to the claim that roughly one fifth of the
galactic halo dark matter may be in the form of compact, solar-mass objects. We
analyze this hypothesis by considering the goodness-of-fit of the best-fit halo
dark matter solutions to the observational data. We show that the distribution
of the durations of the observed microlensing events is narrower than that
expected to result from a standard halo lens population at 90 to 95%
confidence. Only when the fraction of expected background (non-halo) events is
significantly increased does the discrepancy between the observed and
theoretical event duration distributions disappear. This indicates that it is
possible that most of the lenses responsible for the observed microlensing
events are not located within the Milky Way halo.Comment: 5 pages, 2 figures, minor changes to discussion and additional
references, version to appear in Astronomy and Astrophysic
Not enough stellar mass Machos in the Galactic halo
We present an update of results from the search for microlensing towards the
Large Magellanic Cloud (LMC) by EROS (Experience de Recherche d'Objets
Sombres). We have now monitored 25 million stars over three years. Because of
the small number of observed microlensing candidates (four), our results are
best presented as upper limits on the amount of dark compact objects in the
halo of our Galaxy. We discuss critically the candidates and the possible
location of the lenses, halo or LMC . We compare our results to those of the
MACHO group. Finally, we combine these new results with those from our search
towards the Small Magellanic Cloud as well as earlier ones from the EROS1 phase
of our survey. The combined data is sensitive to compact objects in the broad
mass range solar masses. The derived upper limit on the
abundance of stellar mass MACHOs rules out such objects as the dominant
component of the Galactic halo if their mass is smaller than 2 solar masses.Comment: 7 pages, 4 figures, presented at the XIX International Conference on
Neutrino Physics and Astrophysics, Sudbury, Canada, June 200
Global Analysis of Data on the Proton Structure Function g1 and Extraction of its Moments
Inspired by recent measurements with the CLAS detector at Jefferson Lab, we
perform a self-consistent analysis of world data on the proton structure
function g1 in the range 0.17 < Q2 < 30 (GeV/c)**2. We compute for the first
time low-order moments of g1 and study their evolution from small to large
values of Q2. The analysis includes the latest data on both the unpolarized
inclusive cross sections and the ratio R = sigmaL / sigmaT from Jefferson Lab,
as well as a new model for the transverse asymmetry A2 in the resonance region.
The contributions of both leading and higher twists are extracted, taking into
account effects from radiative corrections beyond the next-to-leading order by
means of soft-gluon resummation techniques. The leading twist is determined
with remarkably good accuracy and is compared with the predictions obtained
using various polarized parton distribution sets available in the literature.
The contribution of higher twists to the g1 moments is found to be
significantly larger than in the case of the unpolarized structure function F2.Comment: 18 pages, 13 figures, to appear in Phys. Rev.
Impact of Higher Order and Soft Gluon Corrections on the Extraction of Higher Twist Effects in DIS
The impact of recently calculated next-to-next-to-leading order QCD
corrections and soft gluon resummations on the extraction of higher twist
contributions to the deep-inelastic structure function F_2 is studied using the
BCDMS and SLAC data. It is demonstrated to which extent the need for higher
twist terms is diminishing due to these higher order effects in the kinematical
region, 0.35 \le x \le 0.85 and Q^2>1.2 GeV^2, investigated. In addition,
theoretical uncertainties in the extraction of higher twist contributions are
discussed, and comparisons to results obtained previously are made.Comment: 16 pages, 3 figure
Neural Network Parametrization of Deep-Inelastic Structure Functions
We construct a parametrization of deep-inelastic structure functions which
retains information on experimental errors and correlations, and which does not
introduce any theoretical bias while interpolating between existing data
points. We generate a Monte Carlo sample of pseudo-data configurations and we
train an ensemble of neural networks on them. This effectively provides us with
a probability measure in the space of structure functions, within the whole
kinematic region where data are available. This measure can then be used to
determine the value of the structure function, its error, point-to-point
correlations and generally the value and uncertainty of any function of the
structure function itself. We apply this technique to the determination of the
structure function F_2 of the proton and deuteron, and a precision
determination of the isotriplet combination F_2[p-d]. We discuss in detail
these results, check their stability and accuracy, and make them available in
various formats for applications.Comment: Latex, 43 pages, 22 figures. (v2) Final version, published in JHEP;
Sect.5.2 and Fig.9 improved, a few typos corrected and other minor
improvements. (v3) Some inconsequential typos in Tab.1 and Tab 5 corrected.
Neural parametrization available at http://sophia.ecm.ub.es/f2neura
Compact object detection in self-lensing binary systems with a main-sequence star
Detecting compact objects by means of their gravitational lensing effect on
an observed companion in a binary system has already been suggested almost four
decades ago. However, these predictions were made even before the first
observations of gravitational lensing, whereas nowadays gravitational
microlensing surveys towards the Galactic bulge yield almost 1000 events per
year where one star magnifies the light of a more distant one. With a specific
view on those experiments, we therefore carry out simulations to assess the
prospects for detection of the transient periodic magnification of the
companion star, which lasts typically only a few hours binaries involving a
main-sequence star. We find that detectability is given by the achievability of
dense monitoring with the required photometric accuracy. In sharp contrast to
earlier expectations by other authors, we find that main-sequence stars are not
substantially less favourable targets to observe this effect than white dwarfs.
The requirement of an almost edge-on orbit leads to a probability of the order
of for spotting the signature of an existing compact object
in a binary system with this technique. Assuming an abundance of such systems
about 0.4 per cent, a high-cadence monitoring every 15~min with 5 per cent
photometric accuracy would deliver a signal rate per target star of \gamma
\sim 4 \times 10^{-7}~\mbox{yr}^{-1} at a recurrence period of about 6 months.
With microlensing surveys having demonstrated the capability to monitor about
stars, one is therefore provided with the chance to detect
roughly semi-annually recurring self-lensing signals from several compact
compacts in a binary system. If the photometric accuracy was pushed down to 0.3
per cent, 10 times as many signals would become detectable.Comment: 7 pages, 5 figures, accepted in MNRA
EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation
We report the discovery of eleven beat Cepheids in the Small Magellanic
Cloud, using data obtained by the EROS microlensing survey. Four stars are
beating in the fundamental and first overtone mode (F/1OT), seven are beating
in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is
systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the
SMC Cepheids is the same as the LMC one.Comment: 4 pages, Latex file with 4 .ps figures. accepted for publication in A
A Letter
Galactic Bulge Microlensing Optical Depth from EROS-2
We present a new EROS-2 measurement of the microlensing optical depth toward
the Galactic Bulge. Light curves of clump-giant stars
distributed over of the Bulge were monitored during seven Bulge
seasons. 120 events were found with apparent amplifications greater than 1.6
and Einstein radius crossing times in the range 5 {\rm d}.
This is the largest existing sample of clump-giant events and the first to
include northern Galactic fields. In the Galactic latitude range
1.4\degr<|b|<7.0\degr, we find with . These results are in good
agreement with our previous measurement, with recent measurements of the MACHO
and OGLE-II groups, and with predictions of Bulge models.Comment: accepted A&A, minor revision
The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant () determination
Recent HST determinations of the expansion's rate of the Universe (the Hubble
constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I
are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996,
Tanvir et al., 1995). The three groups obtain different vales for H_0. We note
that most of this discrepancy stems from the asumption (by both groups) that
the Period-Luminosity relation is independent of metallicity. We come to this
conclusion as a result of our study of the Period-Luminosity relation of 481
Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud
and the Small Magellanic Cloud obtained as a by-product of the EROS
microlensing survey. We find that the derived interstellar absorption
corrections are particularly sensitive to the metallicity and when our result
is applied to recent estimates based on HST Cepheids observations it makes the
low-H_0 values higher and the high-H_0 value lower, bringing those discrepant
estimates into agrement around .Comment: 4 pages, Latex, with 2 .ps accepted for publication astronomy and
astrophysics Letter
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