78,777 research outputs found
Photo-z optimization for measurements of the BAO radial direction
Baryon Acoustic Oscillations (BAO) in the radial direction offer a method to
directly measure the Universe expansion history, and to set limits to space
curvature when combined to the angular BAO signal. In addition to spectroscopic
surveys, radial BAO might be measured from accurate enough photometric
redshifts obtained with narrow-band filters. We explore the requirements for a
photometric survey using Luminous Red Galaxies (LRG) to competitively measure
the radial BAO signal and discuss the possible systematic errors of this
approach. If LRG were a highly homogeneous population, we show that the photo-z
accuracy would not substantially improve by increasing the number of filters
beyond , except for a small fraction of the sources detected at high
signal-to-noise, and broad-band filters would suffice to achieve the target
for measuring radial BAO. Using the LRG spectra
obtained from SDSS, we find that the spectral variability of LRG substantially
worsens the achievable photometric redshift errors, and that the optimal system
consists of 30 filters of width . A
is generally necessary at the filters on the red side of the
break to reach the target photometric accuracy. We estimate that a
5-year survey in a dedicated telescope with etendue in excess of 60 would be necessary to obtain a high enough density of galaxies to
measure radial BAO with sufficiently low shot noise up to . We
conclude that spectroscopic surveys have a superior performance than
photometric ones for measuring BAO in the radial direction.Comment: Replaced with minor editorial comments and one extra figure. Results
unchange
Supernova Constraints and Systematic Uncertainties from the First Three Years of the Supernova Legacy Survey
We combine high-redshift Type Ia supernovae from the first three years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-z, 93 SDSS, 242 SNLS, and 14 Hubble Space Telescope). SN data alone require cosmic acceleration at >99.999% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least z = 1.4) in a flat universe, we find w = –0.91^(+0.16)_(–0.20)(stat)^(+0.07)_(–0.14)(sys) from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between host-galaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematic covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems
Chaotic dynamics and the role of covariance inflation for reduced rank Kalman filters with model error
The ensemble Kalman filter and its variants have shown to be robust for data assimilation in high dimensional geophysical models, with localization, using ensembles of extremely small size relative to the model dimension. However, a reduced rank representation of the estimated covariance leaves a large dimensional complementary subspace unfiltered. Utilizing the dynamical properties of the filtration for the backward Lyapunov vectors, this paper explores a previously unexplained mechanism, providing a novel theoretical interpretation for the role of covariance inflation in ensemble-based Kalman filters. Our derivation of the forecast error evolution describes the dynamic upwelling of the unfiltered error from outside of the span of the anomalies into the filtered subspace. Analytical results for linear systems explicitly describe the mechanism for the upwelling, and the associated recursive Riccati equation for the forecast error, while nonlinear approximations are explored numerically
Optoelectronic Reservoir Computing
Reservoir computing is a recently introduced, highly efficient bio-inspired
approach for processing time dependent data. The basic scheme of reservoir
computing consists of a non linear recurrent dynamical system coupled to a
single input layer and a single output layer. Within these constraints many
implementations are possible. Here we report an opto-electronic implementation
of reservoir computing based on a recently proposed architecture consisting of
a single non linear node and a delay line. Our implementation is sufficiently
fast for real time information processing. We illustrate its performance on
tasks of practical importance such as nonlinear channel equalization and speech
recognition, and obtain results comparable to state of the art digital
implementations.Comment: Contains main paper and two Supplementary Material
A New Method to Calibrate the Magnitudes of Type Ia Supernovae at Maximum Light
We present a new empirical method for fitting multicolor light curves of Type
Ia supernovae. Our method combines elements from two widely used techniques in
the literature: the delta_m15 template fitting method and the Multicolor
Light-Curve Shape method. An advantage of our technique is the ease of adding
new colors, templates, or parameters to the fitting procedure. We use a large
sample of published light curves to calibrate the relations between the
absolute magnitudes at maximum and delta_m15 in BVRI filters. We find that
individual subsamples from a given survey or publication have significantly
tighter relationships between light curve shape and luminosity than the
relationship derived from the sum of all the samples, pointing to uncorrected
systematic errors in the photometry, mainly in BV filters. Using our method, we
calculate luminosity distances and host galaxy reddening to 89 SNe in the
Hubble flow and construct a low-z Hubble diagram. The dispersion of the SNe in
the Hubble diagram is 0.20 mag, or an error of ~9% in distance to a single SN.
Our technique produces similar or smaller dispersion in the low-z Hubble
diagram than other techniques in the literature.Comment: 43 pages, 16 figures, 6 tables, accepted by ApJ. For additional
material go to
http://www.astronomy.ohio-state.edu/~prieto/paper_dm15/dm15.htm
Estimations of non-linearities in structural vibrations of string musical instruments
Under the excitation of strings, the wooden structure of string instruments
is generally assumed to undergo linear vibrations. As an alternative to the
direct measurement of the distortion rate at several vibration levels and
frequencies, we characterise weak non-linearities by a signal-model approach
based on cascade of Hammerstein models. In this approach, in a chain of two
non-linear systems, two measurements are sufficient to estimate the non-linear
contribution of the second (sub-)system which cannot be directly linearly
driven, as a function of the exciting frequency. The experiment consists in
exciting the instrument acoustically. The linear and non-linear contributions
to the response of (a) the loudspeaker coupled to the room, (b) the instrument
can be separated. Some methodological issues will be discussed. Findings
pertaining to several instruments - one piano, two guitars, one violin - will
be presented.Comment: 11th Congr\`es Fran\c{c}ais d'Acoustique, Nantes : France (2012
A Search for Planets Transiting the M Dwarf Debris Disk Host, AU Microscopii
We present high cadence, high precision multi-band photometry of the young,
M1Ve, debris disk star, AU Microscopii. The data were obtained in three
continuum filters spanning a wavelength range from 4500\AA to 6600\AA, plus
H, over 28 nights in 2005. The lightcurves show intrinsic stellar
variability due to starspots with an amplitude in the blue band of 0.051
magnitudes and a period of 4.847 days. In addition, three large flares were
detected in the data which all occur near the minimum brightness of the star.
We remove the intrinsic stellar variability and combine the lightcurves of all
the filters in order to search for transits by possible planetary companions
orbiting in the plane of the nearly edge-on debris disk. The combined final
lightcurve has a sampling of 0.35 minutes and a standard deviation of 6.8
millimags (mmag). We performed Monte Carlo simulations by adding fake transits
to the observed lightcurve and find with 95% significance that there are no
Jupiter mass planets orbiting in the plane of the debris disk on circular
orbits with periods, P days. In addition, there are no young
Neptune-like planets (with radii 2.5 smaller than the young Jupiter) on
circular orbits with periods, P days.Comment: accepted to MNRA
Transmission enhancement through square coaxial apertures arrays in metallic film: when leaky modes filter infrared light
We consider arrays of square coaxial apertures in a gold layer and study
their diffractive behavior in the far infrared region. These structures exhibit
a resonant transmission enhancement that is used to design tunable bandpass
filters. We provide a study of their spectral features and show by a modal
analysis that the resonance peak is due to the excitation of leaky modes of the
open photonic structure. Fourier transform infrared (FTIR) spectrophotometry
transmission measurements of samples deposited on Si substrate show good
agreement with numerical results and demonstrate angular tolerance up to 30
degrees of the fabricated filters.Comment: 4 pages, 3 figure
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