85 research outputs found
A Proposal to Search for Transparent Hidden Matter Using Optical Scintillation
It is proposed to search for scintillation of extragalactic sources through
the last unknown baryonic structures. Appropriate observation of the
scintillation process described here should allow one to detect column density
stochastic variations in cool Galactic molecular clouds of order of -- that is
-- per transverse distance
Positioning and orienting a static cylindrical radio-reflector for wide field surveys
Several projects in radioastronomy plan to use large static cylindrical
reflectors with an extended lobe sampling a sector of the rotating sky. This
study provides the exact mathematical expression of the transit time of a
celestial object within the acceptance lobe of such a cylindrical device. The
mathematical approach, based on the stereographic projection, allows one to
study the optimisation of the position and orientation of the radio-reflector,
and should provide exact coefficients for the spatial Fourier Transform of the
radio signal along the cylinder axis.Comment: 15 pages, 13 figure
The OSER project
The OSER project (Optical Scintillation by Extraterrestrial Refractors) is
proposed to search for scintillation of extragalactic sources through the
galactic -- disk or halo -- transparent clouds, the last
unknown baryonic structures. This project should allow one to detect column
density stochastic variations in cool Galactic molecular clouds of order of
per transverse
distance.Comment: 6 pages, 9 figures, 1 tabl
Simulation of optical interstellar scintillation
Stars twinkle because their light propagates through the atmosphere. The same
phenomenon is expected on a longer time scale when the light of remote stars
crosses an interstellar turbulent molecular cloud, but it has never been
observed at optical wavelengths. The aim of the study described in this paper
is to fully simulate the scintillation process, starting from the molecular
cloud description as a fractal object, ending with the simulations of
fluctuating stellar light curves. Fast Fourier transforms are first used to
simulate fractal clouds. Then, the illumination pattern resulting from the
crossing of background star light through these refractive clouds is calculated
from a Fresnel integral that also uses fast Fourier transform techniques.
Regularisation procedure and computing limitations are discussed, along with
the effect of spatial and temporal coherency (source size and wavelength
passband). We quantify the expected modulation index of stellar light curves as
a function of the turbulence strength --characterised by the diffraction radius
-- and the projected source size, introduce the timing aspects, and
establish connections between the light curve observables and the refractive
cloud. We extend our discussion to clouds with different structure functions
from Kolmogorov-type turbulence. Our study confirms that current telescopes of
~4m with fast-readout, wide-field detectors have the capability of discovering
the first interstellar optical scintillation effects. We also show that this
effect should be unambiguously distinguished from any other type of variability
through the observation of desynchronised light curves, simultaneously measured
by two distant telescopes.Comment: 11 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Blending from binarity in microlensing searches toward the Large Magellanic Cloud
Studies of gravitational microlensing effects require the estimation of their
detection efficiency as soon as one wants to quantify the massive compact
objects along the line of sight of source targets. This is particularly
important for setting limits on the contribution of massive compact objects to
the Galactic halo. These estimates of detection efficiency must not only
account for the blending effects of accidentally superimposed sources in
crowded fields, but also for possible mixing of light from stars belonging to
multiple gravitationally bound stellar systems.
Until now, only blending due to accidental alignment of stars had been
studied, in particular as a result of high-resolution space images. In this
paper, we address the impact of unresolved binary sources that are physically
gravitationally bound and not accidentally aligned, in the case of microlensing
detection efficiencies toward the Large Magellanic Cloud (LMC).
We used the Gaia catalog of nearby stars to constrain the local binarity
rate, which we extrapolated to the distance of the LMC. Then we estimated an
upper limit to the impact of this binarity on the detection efficiency of
microlensing effects, as a function of lens mass.
We find that a maximum of 6.2\% of microlensing events on LMC sources due to
halo lenses heavier than could be affected as a result of the
sources belonging to unresolved binary systems. This number is the maximum
fraction of events for which the source is a binary system separated by about
one angular Einstein radius or more in a configuration where light-curve
distortion could affect the efficiency of some detection algorithms. For events
caused by lighter lenses on LMC sources, our study shows that the chances of
blending effects by binary systems is likely to be higher and should be studied
in more detail to improve the accuracy of efficiency calculations.Comment: 11 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
A new method to improve photometric redshift reconstruction. Applications to the Large Synoptic Survey Telescope
In the next decade, the LSST will become a major facility for the
astronomical community. However accurately determining the redshifts of the
observed galaxies without using spectroscopy is a major challenge.
Reconstruction of the redshifts with high resolution and well-understood
uncertainties is mandatory for many science goals, including the study of
baryonic acoustic oscillations. We investigate different approaches to
establish the accuracy that can be reached by the LSST six-band photometry. We
construct a realistic mock galaxy catalog, based on the GOODS survey luminosity
function, by simulating the expected apparent magnitude distribution for the
LSST. To reconstruct the photometric redshifts (photo-z's), we consider a
template-fitting method and a neural network method. The photo-z reconstruction
from both of these techniques is tested on real CFHTLS data and also on
simulated catalogs. We describe a new method to improve photo-z reconstruction
that efficiently removes catastrophic outliers via a likelihood ratio
statistical test. This test uses the posterior probability functions of the fit
parameters and the colors. We show that the photometric redshift accuracy will
meet the stringent LSST requirements up to redshift after a selection
that is based on the likelihood ratio test or on the apparent magnitude for
galaxies with in at least 5 bands. The former selection has the
advantage of retaining roughly 35% more galaxies for a similar photo-z
performance compared to the latter. Photo-z reconstruction using a neural
network algorithm is also described. In addition, we utilize the CFHTLS
spectro-photometric catalog to outline the possibility of combining the neural
network and template-fitting methods. We conclude that the photo-z's will be
accurately estimated with the LSST if a Bayesian prior probability and a
calibration sample are used.Comment: 19 pages, 25 figures, accepted for publication in Astronomy and
Astrophysics Astronomy and Astrophysics, 201
Slitless spectrophotometry with forward modelling: principles and application to atmospheric transmission measurement
In the next decade, many optical surveys will aim to tackle the question of
dark energy nature, measuring its equation of state parameter at the permil
level. This requires trusting the photometric calibration of the survey with a
precision never reached so far, controlling many sources of systematic
uncertainties. The measurement of the on-site atmospheric transmission for each
exposure, or on average for each season or for the full survey, can help reach
the permil precision for magnitudes. This work aims at proving the ability to
use slitless spectroscopy for standard star spectrophotometry and its use to
monitor on-site atmospheric transmission as needed, for example, by the Vera C.
Rubin Observatory Legacy Survey of Space and Time supernova cosmology program.
We fully deal with the case of a disperser in the filter wheel, which is the
configuration chosen in the Rubin Auxiliary Telescope. The theoretical basis of
slitless spectrophotometry is at the heart of our forward model approach to
extract spectroscopic information from slitless data. We developed a publicly
available software called Spectractor (https://github.com/LSSTDESC/Spectractor)
that implements each ingredient of the model and finally performs a fit of a
spectrogram model directly on image data to get the spectrum. We show on
simulations that our model allows us to understand the structure of
spectrophotometric exposures. We also demonstrate its use on real data, solving
specific issues and illustrating how our procedure allows the improvement of
the model describing the data. Finally, we discuss how this approach can be
used to directly extract atmospheric transmission parameters from data and thus
provide the base for on-site atmosphere monitoring. We show the efficiency of
the procedure on simulations and test it on the limited data set available.Comment: 30 pages, 36 figures, submitted to Astronomy and Astrophysic
Microlensing as a probe of the Galactic structure; 20 years of microlensing optical depth studies
Microlensing is now a very popular observational astronomical technique. The
investigations accessible through this effect range from the dark matter
problem to the search for extra-solar planets. In this review, the techniques
to search for microlensing effects and to determine optical depths through the
monitoring of large samples of stars will be described. The consequences of the
published results on the knowledge of the Milky-Way structure and its dark
matter component will be discussed. The difficulties and limitations of the
ongoing programs and the perspectives of the microlensing optical depth
technique as a probe of the Galaxy structure will also be detailed.Comment: Accepted for publication in General Relativity and Gravitation.
General Relativity and Gravitation in press (2010) 0
Supersymmetric Dark Matter
There is almost universal agreement among astronomers that most of the mass
in the Universe and most of the mass in the Galactic halo is dark. Many lines
of reasoning suggest that the dark matter consists of some new, as yet
undiscovered, weakly-interacting massive particle (WIMP). There is now a vast
experimental effort being surmounted to detect WIMPS in the halo. The most
promising techniques involve direct detection in low-background laboratory
detectors and indirect detection through observation of energetic neutrinos
from annihilation of WIMPs that have accumulated in the Sun and/or the Earth.
Of the many WIMP candidates, perhaps the best motivated and certainly the most
theoretically developed is the neutralino, the lightest superpartner in many
supersymmetric theories. We review the minimal supersymmetric extension of the
Standard Model and discuss prospects for detection of neutralino dark matter.
We review in detail how to calculate the cosmological abundance of the
neutralino and the event rates for both direct- and indirect-detection schemes,
and we discuss astrophysical and laboratory constraints on supersymmetric
models. We isolate and clarify the uncertainties from particle physics, nuclear
physics, and astrophysics that enter at each step in the calculation. We
briefly review other related dark-matter candidates and detection techniques.Comment: The complete postscript file is available at
ftp://ftp.npac.syr.edu/pub/users/jungman/susyreview/susyreview.ps.Z The TeX
source and figures (plain TeX; macros included) are at
ftp://ftp.npac.syr.edu/pub/users/jungman/susyreview/susyreview.tar.Z Full
paper NOT submitted to lanl archive: table of contents only. To appear in
Physics Report
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