217 research outputs found
Baryonic acoustic oscillations simulations for the Large Synoptic Survey Telescope (LSST)
The baryonic acoustic oscillations are features in the spatial distribution
of the galaxies which, if observed at different epochs, probe the nature of the
dark energy. In order to be able to measure the parameters of the dark energy
equation of state to high precision, a huge sample of galaxies has to be used.
The Large Synoptic Survey Telescope will survey the optical sky with 6 filters
from 300nm and 1100nm, such that a catalog of galaxies with photometric
redshifts will be available for dark energy studies. In this article, we will
give a rough estimate of the impact of the photometric redshift uncertainties
on the computation of the dark energy parameter through the reconstruction of
the BAO scale from a simulated photometric catalog.Comment: 4 pages, 2 figures, 10th Rencontres de Blois proceedin
Onto what planes should Coulomb stress perturbations be resolved?
[1] Coulomb stress maps are produced by computing the tensorial stress perturbation due to an earthquake rupture and resolving this tensor onto planes of a particular orientation. It is often assumed that aftershock fault planes are ‘‘optimally oriented’’; in other words, the regional stress and coseismic stress change are used to compute the orientation of planes most likely to fail and the coseismic stress is resolved onto these orientations. This practice assumes that faults capable of sustaining aftershocks exist at all orientations, an assumption contradicted by the observation that aftershock focal mechanisms have strong preferred orientations consistent with mapped structural trends. Here we systematically investigate the best planes onto which stress should be resolved for the Landers, Hector Mine, Loma Prieta, and Northridge earthquakes by quantitatively comparing observed aftershock distributions with stress maps based on optimally oriented planes (two- and three-dimensional), main shock orientation, and regional structural trend. We find that the best model differs between different tectonic regions but that in all cases, models that incorporate the regional stress field tend to produce stress maps that best fit the observed aftershock distributions, although not all such models do so equally well. Our results suggest that when the regional stress field is poorly defined, or in structurally complex areas, the best model may be to fix the strike of the planes upon which the stress is to be resolved to that of the main shock but allow the dip and rake to vary
Study of tropospheric correction for intercontinental GPS common-view time transfer
Current practice is to incorporate general empirical models of the troposphere, which depend only on the station height and the elevation of the satellite, in GPS time receivers used for common-view time transfer. Comparisons of these models with a semi-empirical model based on weather measurements show differences of several nanoseconds. This paper reports on a study of tropospheric correction during GPS common-view time transfer over a short baseline of about 700 km, and three long baselines of 6400 km, 9000 km and 9600 km. It is shown that the use of a general empirical model of the troposphere within a region where the climate is similar does not affect time transfer by more than a few hundreds of picoseconds. For the long distance links, differences between the use of a general empirical model and the use of a semi-empirical model reach several nanoseconds
Photometric selection of high-redshift type Ia supernovae
We present a method for selecting high-redshift type Ia supernovae (SNe Ia)
located via rolling SN searches. The technique, using both color and magnitude
information of events from only 2-3 epochs of multi-band real-time photometry,
is able to discriminate between SNe Ia and core collapse SNe. Furthermore, for
the SNe Ia, the method accurately predicts the redshift, phase and light-curve
parameterization of these events based only on pre-maximum-light data. We
demonstrate the effectiveness of the technique on a simulated survey of SNe Ia
and core-collapse SNe, where the selection method effectively rejects most
core-collapse SNe while retaining SNe Ia. We also apply the selection code to
real-time data acquired as part of the Canada-France-Hawaii Telescope Supernova
Legacy Survey (SNLS). During the period May 2004 to January 2005 in the SNLS,
440 SN candidates were discovered of which 70 were confirmed spectroscopically
as SNe Ia and 15 as core-collapse events. For this test dataset, the selection
technique correctly identifies 100% of the identified SNe II as non-SNe Ia with
only a 1-2% false rejection rate. The predicted parameterization of the SNe Ia
has a precision of |delta_z|/(1+z_spec)<0.09 in redshift, and +/- 2-3
rest-frame days in phase, providing invaluable information for planning
spectroscopic follow-up observations. We also investigate any bias introduced
by this selection method on the ability of surveys such as SNLS to measure
cosmological parameters (e.g., w and omega matter), and find any effect to be
negligible.Comment: Accepted for publication in A
The Supernova Legacy Survey 3-year sample: Type Ia Supernovae photometric distances and cosmological constraints
We present photometric properties and distance measurements of 252 high
redshift Type Ia supernovae (0.15 < z < 1.1) discovered during the first three
years of the Supernova Legacy Survey (SNLS). These events were detected and
their multi-colour light curves measured using the MegaPrime/MegaCam instrument
at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four
one-square degree fields in four bands. Follow-up spectroscopy was performed at
the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and
to measure their redshifts. Systematic uncertainties arising from light curve
modeling are studied, making use of two techniques to derive the peak
magnitude, shape and colour of the supernovae, and taking advantage of a
precise calibration of the SNLS fields. A flat LambdaCDM cosmological fit to
231 SNLS high redshift Type Ia supernovae alone gives Omega_M = 0.211 +/-
0.034(stat) +/- 0.069(sys). The dominant systematic uncertainty comes from
uncertainties in the photometric calibration. Systematic uncertainties from
light curve fitters come next with a total contribution of +/- 0.026 on
Omega_M. No clear evidence is found for a possible evolution of the slope
(beta) of the colour-luminosity relation with redshift.Comment: (The SNLS Collaboration) 40 pages, 32 figures, Accepted in A&
The effect of progenitor age and metallicity on luminosity and 56Ni yield in Type Ia supernovae
Timmes, Brown & Truran found that metallicity variations could theoretically
account for a 25% variation in the mass of 56Ni synthesized in Type Ia
supernovae (SNe Ia), and thus account for a large fraction of the scatter in
observed SN Ia luminosities. Higher-metallicity progenitors are more
neutron-rich, producing more stable burning products relative to radioactive
56Ni. We develop a new method for estimating bolometric luminosity and 56Ni
yield in SNe Ia and use it to test the theory with data from the Supernova
Legacy Survey. We find that the average 56Ni yield does drop in SNe Ia from
high metallicity environments, but the theory can only account for 7%--10% of
the dispersion in SN Ia 56Ni mass, and thus luminosity. This is because the
effect is dominant at metallicities significantly above solar, whereas we find
that SN hosts have predominantly subsolar or only moderately above-solar
metallicities. We also show that allowing for changes in O/Fe with the
metallicity [Fe/H] does not have a major effect on the theoretical prediction
of Timmes, Brown & Truran, so long as one is using the O/H as the independent
variable. Age may have a greater effect than metallicity -- we find that the
luminosity weighted age of the host galaxy is correlated with 56Ni yield, and
thus more massive progenitors give rise to more luminous explosions. This is
hard to understand if most SNe Ia explode when the primaries reach the
Chandrasekhar mass. Finally, we test the findings of Gallagher et al., that the
residuals of SNe Ia from the Hubble diagram are correlated with host galaxy
metallicity, and we find no such correlation.Comment: ApJ, accepted, 34 pages, 11 figures, apologies for one column format
-- necessary for the equation
Type Ia Supernovae Rates and Galaxy Clustering from the CFHT Supernova Legacy Survey
The Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS) has created
a large homogeneous database of intermediate redshift (0.2 < z < 1.0) type Ia
supernovae (SNe Ia). The SNLS team has shown that correlations exist between SN
Ia rates, properties, and host galaxy star formation rates. The SNLS SN Ia
database has now been combined with a photometric redshift galaxy catalog and
an optical galaxy cluster catalog to investigate the possible influence of
galaxy clustering on the SN Ia rate, over and above the expected effect due to
the dependence of SFR on clustering through the morphology-density relation. We
identify three cluster SNe Ia, plus three additional possible cluster SNe Ia,
and find the SN Ia rate per unit mass in clusters at intermediate redshifts is
consistent with the rate per unit mass in field early-type galaxies and the SN
Ia cluster rate from low redshift cluster targeted surveys. We also find the
number of SNe Ia in cluster environments to be within a factor of two of
expectations from the two component SNIa rate model.Comment: 21 pages, 2 figures, 6 tables, accepted for publication in A
- …