1,084 research outputs found
Polarization of synchrotron emission from relativistic reconfinement shocks
We study the polarization properties of relativistic reconfinement shocks
with chaotic magnetic fields. Using our hydrodynamical model of their
structure, we calculate synthetic polarization maps, longitudinal polarization
profiles and discuss the spatially averaged polarization degree as a function
of jet half-opening angle Theta_j, jet Lorentz factor Gamma_j and observer
inclination angle to the jet axis theta_{obs}. We find, that for theta_{obs} <=
Theta_j the wave electric vectors are parallel in the vicinity of the structure
ends and perpendicular in between, while for theta_{obs} > Theta_j the
polarization can only be perpendicular. The spatially averaged polarization
degree does not exceed 30%. Parallel average polarization, with polarization
degrees lower than 10%, have been found for theta_{obs} < Theta_j under the
condition Gamma_j * Theta_j > 1. As earlier works predicted the parallel
polarization from relativistic conical shocks, we explain our results by
discussing conical shocks with divergent upstream flow.Comment: 8 pages, 10 figures, accepted for publication in MNRA
Microlensing optical depth and event rate in the OGLE-IV Galactic plane fields
Searches for gravitational microlensing events are traditionally concentrated
on the central regions of the Galactic bulge but many microlensing events are
expected to occur in the Galactic plane, far from the Galactic Center. Owing to
the difficulty in conducting high-cadence observations of the Galactic plane
over its vast area, which are necessary for the detection of microlensing
events, their global properties were hitherto unknown. Here, we present results
of the first comprehensive search for microlensing events in the Galactic
plane. We searched an area of almost 3000 square degrees along the Galactic
plane (|b|<7, 0<l<50, 190<l<360 deg) observed by the Optical Gravitational
Lensing Experiment (OGLE) during 2013-2019 and detected 630 events. We
demonstrate that the mean Einstein timescales of Galactic plane microlensing
events are on average three times longer than those of Galactic bulge events,
with little dependence on the Galactic longitude. We also measure the
microlensing optical depth and event rate as a function of Galactic longitude
and demonstrate that they exponentially decrease with the angular distance from
the Galactic Center (with the characteristic angular scale length of 32 deg).
The average optical depth decreases from at l=10 deg to
in the Galactic anticenter. We also find that the optical
depth in the longitude range 240<l<330 deg is asymmetric about the Galactic
equator, which we interpret as a signature of the Galactic warp.Comment: ApJS, in pres
Microlensing optical depth and event rate in the OGLE-IV Galactic plane fields
Searches for gravitational microlensing events are traditionally concentrated on the central regions of the Galactic bulge but many microlensing events are expected to occur in the Galactic plane, far from the Galactic Center. Owing to the difficulty in conducting high-cadence observations of the Galactic plane over its vast area, which are necessary for the detection of microlensing events, their global properties were hitherto unknown. Here, we present results of the first comprehensive search for microlensing events in the Galactic plane. We searched an area of almost 3000 square degrees along the Galactic plane (|b| < 7°, 0° < l < 50°, 190° < l < 360°) observed by the Optical Gravitational Lensing Experiment (OGLE) during 2013–2019 and detected 630 events. We demonstrate that the mean Einstein timescales of Galactic plane microlensing events are on average three times longer than those of Galactic bulge events, with little dependence on the Galactic longitude. We also measure the microlensing optical depth and event rate as a function of Galactic longitude and demonstrate that they exponentially decrease with the angular distance from the Galactic Center (with the characteristic angular scale length of 32°). The average optical depth decreases from 0.5 × 10⁻⁶ at l = 10° to 1.5 × 10⁻⁸ in the Galactic anticenter. We also find that the optical depth in the longitude range 240° < l < 330° is asymmetric about the Galactic equator, which we interpret as a signature of the Galactic warp
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Evaluation of Artery Visualizations for Heart Disease Diagnosis
Heart disease is the number one killer in the United States, and finding indicators of the disease at an early stage is critical for treatment and prevention. In this paper we evaluate visualization techniques that enable the diagnosis of coronary artery disease. A key physical quantity of medical interest is endothelial shear stress (ESS). Low ESS has been associated with sites of lesion formation and rapid progression of disease in the coronary arteries. Having effective visualizations of a patient's ESS data is vital for the quick and thorough non-invasive evaluation by a cardiologist. We present a task taxonomy for hemodynamics based on a formative user study with domain experts. Based on the results of this study we developed HemoVis, an interactive visualization application for heart disease diagnosis that uses a novel 2D tree diagram representation of coronary artery trees. We present the results of a formal quantitative user study with domain experts that evaluates the effect of 2D versus 3D artery representations and of color maps on identifying regions of low ESS. We show statistically significant results demonstrating that our 2D visualizations are more accurate and efficient than 3D representations, and that a perceptually appropriate color map leads to fewer diagnostic mistakes than a rainbow color map.Engineering and Applied Science
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