265,148 research outputs found
Reliable inference of exoplanet light curve parameters using deterministic and stochastic systematics models
Time-series photometry and spectroscopy of transiting exoplanets allow us to
study their atmospheres. Unfortunately, the required precision to extract
atmospheric information surpasses the design specifications of most general
purpose instrumentation, resulting in instrumental systematics in the light
curves that are typically larger than the target precision. Systematics must
therefore be modelled, leaving the inference of light curve parameters
conditioned on the subjective choice of models and model selection criteria.
This paper aims to test the reliability of the most commonly used systematics
models and model selection criteria. As we are primarily interested in
recovering light curve parameters rather than the favoured systematics model,
marginalisation over systematics models is introduced as a more robust
alternative than simple model selection. This can incorporate uncertainties in
the choice of systematics model into the error budget as well as the model
parameters. Its use is demonstrated using a series of simulated transit light
curves. Stochastic models, specifically Gaussian processes, are also discussed
in the context of marginalisation over systematics models, and are found to
reliably recover the transit parameters for a wide range of systematics
functions. None of the tested model selection criteria - including the BIC -
routinely recovered the correct model. This means that commonly used methods
that are based on simple model selection may underestimate the uncertainties
when extracting transmission and eclipse spectra from real data, and
low-significance claims using such techniques should be treated with caution.
In general, no systematics modelling techniques are perfect; however,
marginalisation over many systematics models helps to mitigate poor model
selection, and stochastic processes provide an even more flexible approach to
modelling instrumental systematics.Comment: 15 pages, 2 figures, published in MNRAS, typo in footnote eq
correcte
Exploiting the full potential of photometric quasar surveys: Optimal power spectra through blind mitigation of systematics
We present optimal measurements of the angular power spectrum of the XDQSOz
catalogue of photometric quasars from the Sloan Digital Sky Survey. These
measurements rely on a quadratic maximum likelihood estimator that
simultaneously measures the auto- and cross-power spectra of four redshift
samples, and provides minimum-variance, unbiased estimates even at the largest
angular scales. Since photometric quasars are known to be strongly affected by
systematics such as spatially-varying depth and stellar contamination, we
introduce a new framework of extended mode projection to robustly mitigate the
impact of systematics on the power spectrum measurements. This technique
involves constructing template maps of potential systematics, decorrelating
them on the sky, and projecting out modes which are significantly correlated
with the data. Our method is able to simultaneously process several thousands
of nonlinearly-correlated systematics, and mode projection is performed in a
blind fashion. Using our final power spectrum measurements, we find a good
agreement with theoretical predictions, and no evidence for further
contamination by systematics. Extended mode projection not only obviates the
need for aggressive sky and quality cuts, but also provides control over the
level of systematics in the measurements, enabling the search for small signals
of new physics while avoiding confirmation bias.Comment: 13 pages, 8 figures. v2: version accepted by MNRAS. v3: systematics
templates publicly available on www.earlyuniverse.org/code, no change to
pape
Internal Robustness: systematic search for systematic bias in SN Ia data
A great deal of effort is currently being devoted to understanding,
estimating and removing systematic errors in cosmological data. In the
particular case of type Ia supernovae, systematics are starting to dominate the
error budget. Here we propose a Bayesian tool for carrying out a systematic
search for systematic contamination. This serves as an extension to the
standard goodness-of-fit tests and allows not only to cross-check raw or
processed data for the presence of systematics but also to pin-point the data
that are most likely contaminated. We successfully test our tool with mock
catalogues and conclude that the Union2.1 data do not possess a significant
amount of systematics. Finally, we show that if one includes in Union2.1 the
supernovae that originally failed the quality cuts, our tool signals the
presence of systematics at over 3.8-sigma confidence level.Comment: 14 pages, 15 figures; matches version accepted for publication in
MNRA
Systematics of 2+ states in semi-magic nuclei
We propose a simple systematics of low lying 2+ energy levels and
electromagnetic transitions in semi-magic isotopic chains Z=28,50,82 and
isotonic chains N=28,50,82,126. To this purpose we use a two-level pairing plus
quadrupole Hamiltonian, within the spherical Quasiparticle Random Phase
Approximation (QRPA). We derive a simple relation connecting the 2+ energy with
the pairing gap and quadrupole-quadupole (QQ) interaction strength. It turns
out that the systematics of energy levels and B(E2) values predicted by this
simple model is fulfilled with a reasonable accuracy by all available
experimental data. Both systematics suggest that not only active nucleons but
also those filling closed shells play an important role
Spider Optimization: Probing the Systematics of a Large Scale B-Mode Experiment
Spider is a long-duration, balloon-borne polarimeter designed to measure
large scale Cosmic Microwave Background (CMB) polarization with very high
sensitivity and control of systematics. The instrument will map over half the
sky with degree angular resolution in I, Q and U Stokes parameters, in four
frequency bands from 96 to 275 GHz. Spider's ultimate goal is to detect the
primordial gravity wave signal imprinted on the CMB B-mode polarization. One of
the challenges in achieving this goal is the minimization of the contamination
of B-modes by systematic effects. This paper explores a number of instrument
systematics and observing strategies in order to optimize B-mode sensitivity.
This is done by injecting realistic-amplitude, time-varying systematics in a
set of simulated time-streams. Tests of the impact of detector noise
characteristics, pointing jitter, payload pendulations, polarization angle
offsets, beam systematics and receiver gain drifts are shown. Spider's default
observing strategy is to spin continuously in azimuth, with polarization
modulation achieved by either a rapidly spinning half-wave plate or a rapidly
spinning gondola and a slowly stepped half-wave plate. Although the latter is
more susceptible to systematics, results shown here indicate that either mode
of operation can be used by Spider.Comment: 15 pages, 12 figs, version with full resolution figs available here
http://www.astro.caltech.edu/~lgg/spider_front.ht
Probing the cosmic acceleration from combinations of different data sets
We examine in some detail the influence of the systematics in different data
sets including type Ia supernova sample, baryon acoustic oscillation data and
the cosmic microwave background information on the fitting results of the
Chevallier-Polarski-Linder parametrization. We find that the systematics in the
data sets does influence the fitting results and leads to different evolutional
behavior of dark energy. To check the versatility of Chevallier-Polarski-Linder
parametrization, we also perform the analysis on the Wetterich parametrization
of dark energy. The results show that both the parametrization of dark energy
and the systematics in data sets influence the evolutional behavior of dark
energy.Comment: 15 pages, 5 figures and 1 table, major revision, delete bao a data,
main results unchanged. jcap in press
Characterizing unknown systematics in large scale structure surveys
Photometric large scale structure (LSS) surveys probe the largest volumes in
the Universe, but are inevitably limited by systematic uncertainties. Imperfect
photometric calibration leads to biases in our measurements of the density
fields of LSS tracers such as galaxies and quasars, and as a result in
cosmological parameter estimation. Earlier studies have proposed using
cross-correlations between different redshift slices or cross-correlations
between different surveys to reduce the effects of such systematics. In this
paper we develop a method to characterize unknown systematics. We demonstrate
that while we do not have sufficient information to correct for unknown
systematics in the data, we can obtain an estimate of their magnitude. We
define a parameter to estimate contamination from unknown systematics using
cross-correlations between different redshift slices and propose discarding
bins in the angular power spectrum that lie outside a certain contamination
tolerance level. We show that this method improves estimates of the bias using
simulated data and further apply it to photometric luminous red galaxies in the
Sloan Digital Sky Survey as a case study.Comment: 24 pages, 6 figures; Expanded discussion of results, added figure 2;
Version to be published in JCA
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