Searcharxiv⌕ Search
Home/Archive/arXiv

arXiv · 0802.1214

Handbook for the GREAT08 Challenge: An image analysis competition for cosmological lensing

Sarah Bridle·John Shawe-Taylor·Adam Amara·Douglas Applegate·Sreekumar T. Balan·Joel Berge·Gary Bernstein·Hakon Dahle·Thomas Erben·Mandeep Gill·Alan Heavens·Catherine Heymans·F. William High·Henk Hoekstra·Mike Jarvis·Donnacha Kirk·Thomas Kitching·Jean-Paul Kneib·Konrad Kuijken·David Lagatutta·Rachel Mandelbaum·Richard Massey·Yannick Mellier·Baback Moghaddam·Yassir Moudden·Reiko Nakajima·Stephane Paulin-Henriksson·Sandrine Pires·Anais Rassat·Alexandre Refregier·Jason Rhodes·Tim Schrabback·Elisabetta Semboloni·Marina Shmakova·Ludovic van Waerbeke·Dugan Witherick·Lisa Voigt·David Wittman
View original source ↗Read source document ↗Export metadata ↓

Abstract

The GRavitational lEnsing Accuracy Testing 2008 (GREAT08) Challenge focuses on a problem that is of crucial importance for future observations in cosmology. The shapes of distant galaxies can be used to determine the properties of dark energy and the nature of gravity, because light from those galaxies is bent by gravity from the intervening dark matter. The observed galaxy images appear distorted, although only slightly, and their shapes must be precisely disentangled from the effects of pixelisation, convolution and noise. The worldwide gravitational lensing community has made significant progress in techniques to measure these distortions via the Shear TEsting Program (STEP). Via STEP, we have run challenges within our own community, and come to recognise that this particular image analysis problem is ideally matched to experts in statistical inference, inverse problems and computational learning. Thus, in order to continue the progress seen in recent years, we are seeking an infusion of new ideas from these communities. This document details the GREAT08 Challenge for potential participants. Please visit http://www.great08challenge.info for the latest information.

Explore related subjects

astro-phstat.AP

Keep this discovery

BibTeXRIS

Sarah Bridle, John Shawe-Taylor, Adam Amara, Douglas Applegate, Sreekumar T. Balan, Joel Berge, Gary Bernstein, Hakon Dahle, Thomas Erben, Mandeep Gill, Alan Heavens, Catherine Heymans, F. William High, Henk Hoekstra, Mike Jarvis, Donnacha Kirk, Thomas Kitching, Jean-Paul Kneib, Konrad Kuijken, David Lagatutta, Rachel Mandelbaum, Richard Massey, Yannick Mellier, Baback Moghaddam, Yassir Moudden, Reiko Nakajima, Stephane Paulin-Henriksson, Sandrine Pires, Anais Rassat, Alexandre Refregier, Jason Rhodes, Tim Schrabback, Elisabetta Semboloni, Marina Shmakova, Ludovic van Waerbeke, Dugan Witherick, Lisa Voigt, David Wittman. 2009-06-15. Handbook for the GREAT08 Challenge: An image analysis competition for cosmological lensing. https://doi.org/10.1214/08-aoas222

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2009

Memory effects in turbulent transport

In the mean-field theory of magnetic fields, turbulent transport, i.e. the turbulent electromotive force, is described by a combination of the alpha effect and turbulent magnetic diffusion, which are usually assumed to be proportional respectively to the mean field and its spatial derivatives. For a passive scalar there is just turbulent diffusion, where the mean flux of concentration depends on the gradient of the mean concentration. However, these proportionalities are approximations that are valid only if the mean field or the mean concentration vary slowly in time. Examples are presented where turbulent transport possesses memory, i.e. where it depends crucially on the past history of the mean field. Such effects are captured by replacing turbulent transport coefficients with time integral kernels, resulting in transport coefficients that depend effectively on the frequency or the growth rate of the mean field itself. In this paper we perform numerical experiments to find the characteristic timescale (or memory length) of this effect as well as simple analytical models of the integral kernels in the case of passive scalar concentrations and kinematic dynamos. The integral kernels can then be used to find self-consistent growth or decay rates of the mean fields. In mean-field dynamos the growth rates and cycle periods based on steady state values of alpha effect and turbulent diffusivity can be quite different from the actual values.

astro-ph↗
arXiv2009

Magnetic Reconnection During the Two-Phase Evolution of a Solar Eruptive Flare

We present a detailed multi-wavelength analysis and interpretation of the evolution of an M7.6 flare on October 24, 2003. The X-ray observations of the flare taken from the RHESSI spacecraft reveal two phases of the flare evolution. The first phase is characterized by the altitude decrease of the X-ray looptop (LT) source for $\sim$11 minutes. Such a long duration of the descending LT source motion is reported for the first time. The EUV loops, located below the X-ray LT source, also undergo contraction with similar speed ($\sim$15 km s$^{-1}$) in this interval. During the second phase the two distinct hard X-ray footpoints (FP) sources are observed which correlate well with UV and H$α$ flare ribbons. The X-ray LT source now exhibits upward motion. The RHESSI spectra during the first phase are soft and indicative of hot thermal emission from flaring loops with temperatures $T>25$ MK at the early stage. On the other hand, the spectra at high energies ($\varepsilon \gtrsim$25 keV) follow hard power laws during the second phase ($γ= 2.6-2.8$). We show that the observed motion of the LT and FP sources can be understood as a consequence of three-dimensional magnetic reconnection at a separator in the corona. During the first phase of the flare, the reconnection releases an excess of magnetic energy related to the magnetic tensions generated before a flare by the shear flows in the photosphere. The relaxation of the associated magnetic shear in the corona by the reconnection process explains the descending motion of the LT source. During the second phase, the ordinary reconnection process dominates describing the energy release in terms of the standard model of large eruptive flares.

astro-ph↗
arXiv2009

Proper motion L and T dwarf candidate members of the Pleiades

We present the results of a deep optical-near-infrared multi-epoch survey covering 2.5 square degrees of the Pleiades open star cluster to search for new very-low-mass brown dwarf members. A significant (~ 5 year) epoch difference exists between the optical (CFH12k I-, Z-band) and near infrared (UKIRT WFCAM J-band) observations. We construct I,I-Z and Z,Z-J colour magnitude diagrams to select candidate cluster members. Proper motions are computed for all candidate members and compared to the background field objects to further refine the sample. We recover all known cluster members within the area of our survey. In addition, we have discovered 9 new candidate brown dwarf cluster members. The 7 faintest candidates have red Z-J colours and show blue near-infrared colours. These are consistent with being L and T-type Pleiads. Theoretical models predict their masses to be around 11 Jupiter masses. There is 1 errata for this paper

astro-ph↗