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 $u-v$ 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 unchange