Optimising Optimal Image Subtraction

Difference imaging is a technique for obtaining precise relative photometry of variable sources in crowded stellar fields and, as such, constitutes a crucial part of the data reduction pipeline in surveys for microlensing events or transiting extrasolar planets. The Optimal Image Subtraction (OIS) algorithm permits the accurate differencing of images by determining convolution kernels which, when applied to reference images of particularly good quality, provide excellent matches to the point-spread functions (PSF) in other images of the time series to be analysed. The convolution kernels are built as linear combinations of a set of basis functions, conventionally bivariate Gaussians modulated by polynomials. The kernel parameters must be supplied by the user and should ideally be matched to the PSF, pixel-sampling, and S/N of the data to be analysed. We have studied the outcome of the reduction as a function of the kernel parameters using our implementation of OIS within the TRIPP package. From the analysis of noise-free PSF simulations as well as test images from the ISIS OIS package, we derive qualitative and quantitative relations between the kernel parameters and the success of the subtraction as a function of the PSF sizes and sampling in reference and data images and compare the results to those of implementations in the literature. On this basis, we provide recommended parameters for data sets with different S/N and sampling.Comment: 10 pages, 6 figures. Accepted for publication in Astronomische Nachrichten / Astronomical Note

The 400d Galaxy Cluster Survey weak lensing programme: II: Weak lensing study of seven clusters with MMT/Megacam

Evolution in the mass function of galaxy clusters sensitively traces both the expansion history of the Universe and cosmological structure formation. Robust cluster mass determinations are a key ingredient for a reliable measurement of this evolution, especially at high redshift. Weak gravitational lensing is a promising tool for, on average, unbiased mass estimates. This weak lensing project aims at measuring reliable weak lensing masses for a complete X-ray selected sample of 36 high redshift (0.35<z<0.9) clusters. The goal of this paper is to demonstrate the robustness of the methodology against commonly encountered problems, including pure instrumental effects, the presence of bright (8--9 mag) stars close to the cluster centre, ground based measurements of high-z (z~0.8) clusters, and the presence of massive unrelated structures along the line-sight. We select a subsample of seven clusters observed with MMT/Megacam. Instrumental effects are checked in detail by cross-comparison with an archival CFHT/MegaCam observation. We derive mass estimates for seven clusters by modelling the tangential shear with an NFW profile, in two cases with multiple components to account for projected structures in the line-of-sight. We firmly detect lensing signals from all seven clusters at more than $3.5\sigma$ and determine their masses, ranging from $10^{14} M_{\odot}$ to $10^{15} M_{\odot}$, despite the presence of nearby bright stars. We retrieve the lensing signal of more than one cluster in the CL 1701+6414 field, while apparently observing CL 1701+6414 through a massive foreground filament. We also find a multi-peaked shear signal in CL 1641+4001. Shear structures measured in the MMT and CFHT images of CL 1701+6414 are highly correlated.Comment: Accepted for publication in Astronomy & Astrophysics; A&A 546, A7

One electron oxidation of triferrocenylmethanol: Synthesis, metal atom dynamics, electron delocalization, and the crystal structure of [Fc 3COH]+ PF6 -

The title compound 2 was prepared and its crystal structure was determined at 100 K. The neat solid was examined by temperature dependent 57Fe Mössbauer effect (ME) spectroscopy over the interval 92 < T < 318 K, and evidences two diamagnetic Fe(II) sites and one paramagnetic Fe(III) site. The latter shows spin–lattice relaxation, but there is no evidence of electron delocalization among the three iron sites in the above temperature interval. The mean-square-amplitude-of-vibration of the diamagnetic iron site has been determined from the recoil-free fraction ME resonance, and compared to the neutral Fc3COH homologue (1). The ME dynamical data are in good agreement with the Ui,j value at 100 K extracted from the crystallographic results. The ME parameters at 5 K have also been determined with the sample compound embedded in a paraffin wax matrix as well as pelletized with BN

Weak Lensing Mass Determination of Eight X-ray Selected Galaxy Clusters from the <em>400d</em> Survey

Evolution in the mass function of galaxy clusters sensitively traces both the expansion history of the Universe and cosmological structure formation. Hence, measuring the number density of galaxy clusters as a function of redshift provides constraints to cosmological parameters, independent of other methods. Current results from these probes, including clusters of galaxies, are found to agree on a cosmological model dominated by Dark Energy and Cold Dark Matter. Investigating the unknown physical nature of Dark Energy ranks among the foremost questions in current cosmology. In particular, the presence or absence of evolution in Dark Energy density is expressed by the equation-of-state parameter. This thesis presents the first results from the 400d Galaxy Cluster Survey Weak Lensing Programme, in which optical follow-up observations for a sample of relatively distant (0.35 ) X-ray selected galaxy clusters are analysed and presented. Mass determination by weak gravitational lensing uses minute distortions in the images of background galaxies, caused by the relativistic curvature of space-time, to infer the mass of the intervening cluster. The weak lensing follow-up project aims at measuring reliable weak lensing masses for 36 clusters, for which a mass function and resulting cosmological constraints using Chandra X-ray observations have been published. Determining cluster masses by weak lensing makes possible a cross-calibration of the assumptions and systematics related to both the X-ray and weak lensing methods. As the initial phase of the 400d weak lensing programme project, observations of eight clusters were obtained with the Megacam instrument at the 6.5m MMT telescope, which we demonstrated to be well-suited for weak lensing. In this thesis, the successful weak lensing detections of these eight clusters are reported, leading to weak lensing mass estimates which then are compared to X-ray masses. For the pilot object, CL 0030+2618, the data analysis is described in great detail, focussing in particular on the construction of a catalogue of lensed background galaxies by using photometry in three passbands. In a synopsis involving several optical and X-ray methods, the identity of the brightest cluster galaxy is established and found to be consistent with both X-ray and weak lensing cluster centres for CL 0030+2618. Cluster masses are obtained by fitting the tangential weak lensing shear measured as a function of separation from the cluster centre with a profile function derived from the Navarro-Frenk-White Dark Matter density profile. Performing a similar analysis for the seven further clusters and investigating the spatial distribution of the lensing signal, multiple shear peaks and/or clusters are detected in three cases. In a comparison between the weak lensing and hydrostatic X-ray mass estimates for the eight clusters, good agreement and a power-law relation with remarkably small scatter are found. Preliminary scaling relation between the weak lensing masses and published X-ray observables of the eight clusters indicate the potential of the weak lensing survey, once observations are available for the complete 36 cluster sample. The completion of the 400d weak lensing survey is concluded to be feasible and promising

The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62+/-0.48kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma/m=(1.7+/-0.7)x10^{-4}cm^2/g x (t/10^9yrs)^{-2}, where t is the infall duration.Comment: 15 pages, 9 figure

QED effective action at finite temperature

The QED effective Lagrangian in the presence of an arbitrary constant electromagnetic background field at finite temperature is derived in the imaginary-time formalism to one-loop order. The boundary conditions in imaginary time reduce the set of gauge transformations of the background field, which allows for a further gauge invariant and puts restrictions on the choice of gauge. The additional invariant enters the effective action by a topological mechanism and can be identified with a chemical potential; it is furthermore related to Debye screening. In concordance with the real-time formalism, we do not find a thermal correction to Schwinger's pair-production formula. The calculation is performed on a maximally Lorentz covariant and gauge invariant stage.Comment: 9 pages, REVTeX, 1 figure, typos corrected, references added, final version to appear in Phys. Rev.

Chandra follow-up of the SDSS DR8 Redmapper catalog using the MATCha pipeline

For abstract see published article