4,112 research outputs found
Comparing the performance of stellar variability filters for the detection of planetary transits
We have developed a new method to improve the transit detection of
Earth-sized planets in front of solar-like stars by fitting stellar
microvariability by means of a spot model. A large Monte Carlo numerical
experiment has been designed to test the performance of our approach in
comparison with other variability filters and fitting techniques for stars of
different magnitudes and planets of different radius and orbital period, as
observed by the space missions CoRoT and Kepler. Here we report on the results
of this experiment.Comment: 4 pages, 3 postscript figures, Transiting Planets Proceeding IAU
Symposium No.253, 200
The Clustering Characteristics of HI-Selected Galaxies from the 40% ALFALFA Survey
The 40% Arecibo Legacy Fast ALFA (ALFALFA) survey catalog (\alpha.40) of
approximately 10,150 HI-selected galaxies is used to analyze the clustering
properties of gas-rich galaxies. By employing the Landy-Szalay estimator and a
full covariance analysis for the two-point galaxy-galaxy correlation function,
we obtain the real-space correlation function and model it as a power law,
\xi(r) = (r/r_0)^(-\gamma), on scales less than 10 h^{-1} Mpc. As the largest
sample of blindly HI-selected galaxies to date, \alpha.40 provides detailed
understanding of the clustering of this population. We find \gamma = 1.51 +/-
0.09 and r_0 = 3.3 +0.3, -0.2 h^{-1} Mpc, reinforcing the understanding that
gas-rich galaxies represent the most weakly clustered galaxy population known;
we also observe a departure from a pure power law shape at intermediate scales,
as predicted in \Lambda CDM halo occupation distribution models. Furthermore,
we measure the bias parameter for the \alpha.40 galaxy sample and find that HI
galaxies are severely antibiased on small scales, but only weakly antibiased on
large scales. The robust measurement of the correlation function for gas-rich
galaxies obtained via the \alpha.40 sample constrains models of the
distribution of HI in simulated galaxies, and will be employed to better
understand the role of gas in environmentally-dependent galaxy evolution.Comment: 30 pages, 10 figures, accepted by Ap
Kepler Presearch Data Conditioning I - Architecture and Algorithms for Error Correction in Kepler Light Curves
Kepler provides light curves of 156,000 stars with unprecedented precision.
However, the raw data as they come from the spacecraft contain significant
systematic and stochastic errors. These errors, which include discontinuities,
systematic trends, and outliers, obscure the astrophysical signals in the light
curves. To correct these errors is the task of the Presearch Data Conditioning
(PDC) module of the Kepler data analysis pipeline. The original version of PDC
in Kepler did not meet the extremely high performance requirements for the
detection of miniscule planet transits or highly accurate analysis of stellar
activity and rotation. One particular deficiency was that astrophysical
features were often removed as a side-effect to removal of errors. In this
paper we introduce the completely new and significantly improved version of PDC
which was implemented in Kepler SOC 8.0. This new PDC version, which utilizes a
Bayesian approach for removal of systematics, reliably corrects errors in the
light curves while at the same time preserving planet transits and other
astrophysically interesting signals. We describe the architecture and the
algorithms of this new PDC module, show typical errors encountered in Kepler
data, and illustrate the corrections using real light curve examples.Comment: Submitted to PASP. Also see companion paper "Kepler Presearch Data
Conditioning II - A Bayesian Approach to Systematic Error Correction" by Jeff
C. Smith et a
On Active Galactic Nuclei as Sources of Ultra-High Energy Cosmic Rays
We measure the correlation between sky coordinates of the Swift BAT catalogue
of active galactic nuclei with the arrival directions of the highest energy
cosmic rays detected by the Auger Observatory. The statistically complete, hard
X-ray catalogue helps to distinguish between AGN and other source candidates
that follow the distribution of local large-scale structure. The positions of
the full catalogue are marginally uncorrelated with the cosmic ray arrival
directions, but when weighted by their hard X-ray flux, AGN within 100 Mpc are
correlated at a significance level of 98 per cent. This correlation sharply
decreases for sources beyond ~100 Mpc, suggestive of a GZK suppression. We
discuss the implications for determining the mechanism that accelerates
particles to these extreme energies in excess of 10^19 eV.Comment: 5 pages, 2 figures. Accepted for publication in MNRA
GPI PSF subtraction with TLOCI: the next evolution in exoplanet/disk high-contrast imaging
To directly image exoplanets and faint circumstellar disks, the noisy stellar
halo must be suppressed to a high level. To achieve this feat, the angular
differential imaging observing technique and the least-squares Locally
Optimized Combination of Images (LOCI) algorithm have now become the standard
in single band direct imaging observations and data reduction. With the
development and commissioning of new high-order high-contrast adaptive optics
equipped with integral field units, the image subtraction algorithm needs to be
modified to allow the optimal use of polychromatic images, field-rotated images
and archival data. A new algorithm, TLOCI (for Template LOCI), is designed to
achieve this task by maximizing a companion signal-to-noise ratio instead of
simply minimizing the noise as in the original LOCI algorithm. The TLOCI
technique uses an input spectrum and template Point Spread Functions (PSFs,
generated from unocculted and unsaturated stellar images) to optimize the
reference image least-squares coefficients to minimize the planet
self-subtraction, thus maximizing its throughput per wavelength, while
simultaneously providing a maximum suppression of the speckle noise. The new
algorithm has been developed using on-sky GPI data and has achieved impressive
contrast. This paper presents the TLOCI algorithm, on-sky performance, and will
discuss the challenges in recovering the planet spectrum with high fidelity.Comment: 13 pages, 8 figures, to appear in Proceedings of SPIE 914
A closer look at the X-ray transient XTE J1908+094: identification of two new near-infrared candidate counterparts
We had reported in Chaty, Mignani, Israel (2002) on the near-infrared (NIR)
identification of a possible counterpart to the black hole candidate XTE
J1908+094 obtained with the ESO/NTT. Here, we present new, follow-up, CFHT
adaptive optics observations of the XTE J1908+094 field, which resolved the
previously proposed counterpart in two objects separated by about 0.8".
Assuming that both objects are potential candidate counterparts, we derive that
the binary system is a low-mass system with a companion star which could be
either an intermediate/late type (A-K) main sequence star at a distance of 3-10
kpc, or a late-type (K) main sequence star at a distance of 1-3 kpc.
However, we show that the brighter of the two objects (J ~ 20.1, H ~ 18.7, K' ~
17.8) is more likely to be the real counterpart of the X-ray source. Its
position is more compatible with our astrometric solution, and colours and
magnitudes of the other object are not consistent with the lower limit of 3 kpc
derived independently from the peak bolometric flux of XTE J1908+094. Further
multi-wavelength observations of both candidate counterparts are crucial in
order to solve the pending identification.Comment: accepted for publication in MNRAS, 5 pages, 3 figure
The extended ROSAT-ESO Flux Limited X-ray Galaxy Cluster Survey (REFLEX II)\\ II. Construction and Properties of the Survey
Galaxy clusters provide unique laboratories to study astrophysical processes
on large scales and are important probes for cosmology. X-ray observations are
currently the best means of detecting and characterizing galaxy clusters. In
this paper we describe the construction of the REFLEX II galaxy cluster survey
based on the southern part of the ROSAT All-Sky Survey. REFLEX II extends the
REFLEX I survey by a factor of about two down to a flux limit of erg s cm (0.1 - 2.4 keV). We describe the determination
of the X-ray parameters, the process of X-ray source identification, and the
construction of the survey selection function. The REFLEX II cluster sample
comprises currently 915 objects. A standard selection function is derived for a
lower source count limit of 20 photons in addition to the flux limit. The
median redshift of the sample is . Internal consistency checks and
the comparison to several other galaxy cluster surveys imply that REFLEX II is
better than 90\% complete with a contamination less than 10\%.Comment: Astronomy and Astrophysics Vol. 555, A30 - 15 pages, 20 figure
A model for cost efficient Workforce Organizational Dynamics and its optimization
This paper presents a workforce planning model scalable to an entire
hierarchical organization. Its main objective is to design a cost optimal
target which leverages flexible workforce solutions while ensuring an efficient
promotional flux. The value of this paper lies in its proposal of an adequate
flexibility rate using various solution types and in its discussion about
external hiring ratios. The mathematical structures of the models are analyzed
and numerical simulations illustrate the theoretical background
Bringing global gyrokinetic turbulence simulations to the transport timescale using a multiscale approach
The vast separation dividing the characteristic times of energy confinement
and turbulence in the core of toroidal plasmas makes first-principles
prediction on long timescales extremely challenging. Here we report the
demonstration of a multiple-timescale method that enables coupling global
gyrokinetic simulations with a transport solver to calculate the evolution of
the self-consistent temperature profile. This method, which exhibits resiliency
to the intrinsic fluctuations arising in turbulence simulations, holds
potential for integrating nonlocal gyrokinetic turbulence simulations into
predictive, whole-device models.Comment: 7 pages, 3 figure
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