958 research outputs found
A GPU-Enabled, High-Resolution Cosmological Microlensing Parameter Survey
In the era of synoptic surveys, the number of known gravitationally lensed
quasars is set to increase by over an order of magnitude. These new discoveries
will enable a move from single-quasar studies to investigations of statistical
samples, presenting new opportunities to test theoretical models for the
structure of quasar accretion discs and broad emission line regions (BELRs). As
one crucial step in preparing for this influx of new lensed systems, a
large-scale exploration of microlensing convergence-shear parameter space is
warranted, requiring the computation of O(10^5) high resolution magnification
maps. Based on properties of known lensed quasars, and expectations from
accretion disc/BELR modelling, we identify regions of convergence-shear
parameter space, map sizes, smooth matter fractions, and pixel resolutions that
should be covered. We describe how the computationally time-consuming task of
producing ~290000 magnification maps with sufficient resolution (10000^2
pixel/map) to probe scales from the inner edge of the accretion disc to the
BELR can be achieved in ~400 days on a 100 teraflop/s high performance
computing facility, where the processing performance is achieved with graphics
processing units. We illustrate a use-case for the parameter survey by
investigating the effects of varying the lens macro-model on accretion disc
constraints in the lensed quasar Q2237+0305. We find that although all
constraints are consistent within their current error bars, models with more
densely packed microlenses tend to predict shallower accretion disc radial
temperature profiles. With a large parameter survey such as the one described
here, such systematics on microlensing measurements could be fully explored.Comment: 30 pages, 3 figures, accepted for publication in Ap
A Robust Determination of the size of quasar accretion disks using gravitational microlensing
Using microlensing measurements from a sample of 27 image-pairs of 19 lensed
quasars we determine a maximum likelihood estimate for the accretion disk size
of an {{\em}average} quasar of light days at rest
frame \AA\ for microlenses with a mean mass of
. This value, in good agreement with previous results from
smaller samples, is roughly a factor of 5 greater than the predictions of the
standard thin disk model. The individual size estimates for the 19 quasars in
our sample are also in excellent agreement with the results of the joint
maximum likelihood analysis.Comment: 6 pages, 3 figures, submitted to Ap
Alternative mechanism for bacteriophage adsorption to the motile bacterium Caulobacter crescentus
2D and 3D cryo-electron microscopy, together with adsorption kinetics assays of ÏCb13 and ÏCbK phage-infected Caulobacter crescentus, provides insight into the mechanisms of infection. ÏCb13 and ÏCbK actively interact with the flagellum and subsequently attach to receptors on the cell pole. We present evidence that the first interaction of the phage with the bacterial flagellum takes place through a filament on the phage head. This contact with the flagellum facilitates concentration of phage particles around the receptor (i.e., the pilus portals) on the bacterial cell surface, thereby increasing the likelihood of infection. Phage head filaments have not been well characterized and their function is described here. Phage head filaments may systematically underlie the initial interactions of phages with their hosts in other systems and possibly represent a widespread mechanism of efficient phage propagation
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