384 research outputs found
The Submillimeter Array Polarimeter
We describe the Submillimeter Array (SMA) Polarimeter, a polarization
converter and feed multiplexer installed on the SMA. The polarimeter uses
narrow-band quarter-wave plates to generate circular polarization sensitivity
from the linearly-polarized SMA feeds. The wave plates are mounted in rotation
stages under computer control so that the polarization handedness of each
antenna is rapidly selectable. Positioning of the wave plates is found to be
highly repeatable, better than 0.2 degrees. Although only a single polarization
is detected at any time, all four cross correlations of left- and
right-circular polarization are efficiently sampled on each baseline through
coordinated switching of the antenna polarizations in Walsh function patterns.
The initial set of anti-reflection coated quartz and sapphire wave plates
allows polarimetry near 345 GHz; these plates have been have been used in
observations between 325 and 350 GHz. The frequency-dependent
cross-polarization of each antenna, largely due to the variation with frequency
of the retardation phase of the single-element wave plates, can be measured
precisely through observations of bright point sources. Such measurements
indicate that the cross-polarization of each antenna is a few percent or
smaller and stable, consistent with the expected frequency dependence and very
small alignment errors. The polarimeter is now available for general use as a
facility instrument of the SMA.Comment: To appear in Proc. SPIE 7020, 'Millimeter and Submillimeter Detectors
and Instrumentation'. Uses spie.cl
Between history and memory: ambivalent longing in the work of Seth
The work of Canadian cartoonist Seth positions itself between history and
memory, and in doing so gives rise to a range of ambivalent impulses, chief among them
an ambivalent longing for the past. Seth suggests that “the whole process of cartooning
is dealing with memory,” and by consistently drawing attention to the narrative
representation of the past, his comics reveal the extent to which the making of history is
an act of great artifice. In its exploration of what Walter Benjamin calls “the mysterious
work of remembrance—which is really the capacity for endless interpolation into what
has been,” Seth’s work exhibits a complicated nostalgia, well aware of its own
reactionary, restorative and nationalistic inclinations and able to channel them toward
productive ends.
Seth’s ironic, humorous and metafictional approaches to memory, remembrance
and longing for the past reveal that his attitude toward these closely related subjects is
deeply ambivalent. He nimbly mobilises history, (auto)biography, anecdote,
documentary and other parallel modes. This investigation seeks to understand the ways
in which his appropriation of such historicising discourses substantiates the powerful
evocations of longing, loss and memory that characterise his fiction. Memory is here
conceived not just as an invisible, ubiquitous mental phenomenon that reflects our
experience of time and relation to the past, but as a medium, an art – and one which is in
many ways akin to Seth’s mode of expression. The fundamental operation of comics, as
a visual medium, initiates and makes space for narrative interpolations in a way that is
not only comparable to but in a certain sense mimics the historical interpolations of
memory; in both cases, longing is spurred by incompleteness. Seth turns the medium of
memory on itself, using it as an instrument to examine the processes of remembrance
and making history
Variability in GRMHD simulations of Sgr A: Implications for EHT closure phase observations
The observable quantities that carry the most information regarding the
structures of the images of black holes in the interferometric observations
with the Event Horizon Telescope are the closure phases along different
baseline triangles. We use long time span, high cadence, GRMHD+radiative
transfer models of Sgr A to investigate the expected variability of closure
phases in such observations. We find that, in general, closure phases along
small baseline triangles show little variability, except in the cases when one
of the triangle vertices crosses one of a small regions of low visibility
amplitude. The closure phase variability increases with the size of the
baseline triangle, as larger baselines probe the small-scale structures of the
images, which are highly variable. On average, the jet-dominated MAD models
show less closure phase variability than the disk-dominated SANE models, even
in the large baseline triangles, because the images from the latter are more
sensitive to the turbulence in the accretion flow. Our results suggest that
image reconstruction techniques need to explicitly take into account the
closure phase variability, especially if the quality and quantity of data allow
for a detailed characterization of the nature of variability. This also implies
that, if image reconstruction techniques that rely on the assumption of a
static image are utilized, regions of the space that show a high level of
variability will need to be identified and excised.Comment: submitted to apj. 12 pages, 12 figure
Between history and memory: ambivalent longing in the work of Seth
The work of Canadian cartoonist Seth positions itself between history and
memory, and in doing so gives rise to a range of ambivalent impulses, chief among them
an ambivalent longing for the past. Seth suggests that “the whole process of cartooning
is dealing with memory,” and by consistently drawing attention to the narrative
representation of the past, his comics reveal the extent to which the making of history is
an act of great artifice. In its exploration of what Walter Benjamin calls “the mysterious
work of remembrance—which is really the capacity for endless interpolation into what
has been,” Seth’s work exhibits a complicated nostalgia, well aware of its own
reactionary, restorative and nationalistic inclinations and able to channel them toward
productive ends.
Seth’s ironic, humorous and metafictional approaches to memory, remembrance
and longing for the past reveal that his attitude toward these closely related subjects is
deeply ambivalent. He nimbly mobilises history, (auto)biography, anecdote,
documentary and other parallel modes. This investigation seeks to understand the ways
in which his appropriation of such historicising discourses substantiates the powerful
evocations of longing, loss and memory that characterise his fiction. Memory is here
conceived not just as an invisible, ubiquitous mental phenomenon that reflects our
experience of time and relation to the past, but as a medium, an art – and one which is in
many ways akin to Seth’s mode of expression. The fundamental operation of comics, as
a visual medium, initiates and makes space for narrative interpolations in a way that is
not only comparable to but in a certain sense mimics the historical interpolations of
memory; in both cases, longing is spurred by incompleteness. Seth turns the medium of
memory on itself, using it as an instrument to examine the processes of remembrance
and making history
GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*
Synthesis imaging of the black hole in the center of the Milky Way, Sgr A*,
with the Event Horizon Telescope (EHT) rests on the assumption of a stationary
image. We explore the limitations of this assumption using high-cadence GRMHD
simulations of Sgr A*. We employ analytic models that capture the basic
characteristics of the images to understand the origin of the variability in
the simulated visibility amplitudes. We find that, in all simulations, the
visibility amplitudes for baselines oriented perpendicular to the spin axis of
the black hole typically decrease smoothly over baseline lengths that are
comparable to those of the EHT. On the other hand, the visibility amplitudes
for baselines oriented parallel to the spin axis show significant structure
with one or more minima. This suggests that fitting EHT observations with
geometric models will lead to reasonably accurate determination of the
orientation of the black-hole on the plane of the sky. However, in the
disk-dominated models, the locations and depths of the minima in the visibility
amplitudes depend primarily on the width and asymmetry of the crescent-like
images and are highly variable. In the jet-dominated models, the locations of
the minima are determined by the separation of the two image components but
their depths depend primarily on the relative brightness of the two components
and are also variable. This suggests that using time-independent models to
infer additional black-hole parameters, such as the shadow size or the spin
magnitude, will be severely affected by the variability of the accretion flow.Comment: replaced to match published version, new figure added, results
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