1,738 research outputs found
Updating the Chandra HETGS Efficiencies using In-Orbit Observations
The efficiencies of the gratings in the High Energy Transmission Grating
Spectrometer (HETGS) were updated using in-flight observations of bright
continuum sources. The procedure first involved verifying that fluxes obtained
from the +1 and -1 orders match, which checks that the contaminant model and
the CCD quantum efficiencies agree. Then the fluxes derived using the high
energy gratings (HEGs) were compared to those derived from the medium energy
gratings (MEGs). The flux ratio was fit to a low order polynomial, which was
allocated to the MEGs above 1 keV or the HEGs below 1 keV. The resultant
efficiencies were tested by examining fits to blazar spectra.Comment: 10 pages, 7 figures; to appear in the SPIE proceedings, volume 844
Polarimetry with a soft x-ray spectrometer
An approach for measuring linear X-ray polarization over a broad-band using
conventional spectroscopic optics is described. A set of multilayer-coated
flats reflect the dispersed X-rays to the instrument detectors. The intensity
variation as a function of energy and position angle is measured to determine
three Stokes parameters: I, Q, and U. By laterally grading the multilayer
optics and matching the dispersion of the gratings, one may take advantage of
high multilayer reflectivities and achieve modulation factors over 80% over the
entire 0.2 to 0.8 keV band. A sample design is shown that could be used with a
small orbiting mission.Comment: Accepted for publication in the proceedings of the SPIE, volume 701
A Soft X-ray Polarimeter Designed for Broad-band X-ray Telescopes
A novel approach for measuring linear X-ray polarization over a broad-band
using conventional imaging optics and cameras is described. A new type of high
efficiency grating, called the critical angle transmission grating is used to
disperse soft X-rays radially from the telescope axis. A set of
multilayer-coated paraboloids re-image the dispersed X-rays to rings in the
focal plane. The intensity variation around these rings is measured to
determine three Stokes parameters: I, Q, and U. By laterally grading the
multilayer optics and matching the dispersion of the gratings, one may take
advantage of high multilayer reflectivities and achieve modulation factors over
50% over the entire 0.2 to 0.8 keV band. A sample design is shown that could be
used with the Constellation-X optics.Comment: 11 pages, 7 figures, presented at conference 6688 of the SPI
Broad-Band Soft X-ray Polarimetry
We developed an instrument design capable of measuring linear X-ray
polarization over a broad-band using conventional spectroscopic optics. A set
of multilayer-coated flats reflects the dispersed X-rays to the instrument
detectors. The intensity variation with position angle is measured to determine
three Stokes parameters: I, Q, and U -- all as a function of energy. By
laterally grading the multilayer optics and matching the dispersion of the
gratings, one may take advantage of high multilayer reflectivities and achieve
modulation factors > 50% over the entire 0.2 to 0.8 keV band. This instrument
could be used in a small orbiting mission or scaled up for the International
X-ray Observatory. Laboratory work has begun that would demonstrate the
capabilities of key components.Comment: 6 pages, 3 figures (2 color); to appear in proceedings of "The Coming
of Age of X-ray Polarimetry
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The Marine Corps subculture
This thesis analyzes the subculture within the United States Marine Corps. It attempts to bring the traditional literature of criminal subculture and the subculture of violence together with literature of occupational subculture to explain many of the behaviors exhibited by Marines
Further Development of Event-Based Analysis of X-ray Polarization Data
An event-based maximum likelihood method for handling X-ray polarimetry data
is extended to include the effects of background and nonuniform sampling of the
possible position angle space. While nonuniform sampling in position angle
space generally introduces cross terms in the uncertainties of polarization
parameters that could create degeneracies, there are interesting cases that
engender no bias or parameter covariance. When including background in
Poisson-based likelihood formulation, the formula for the minimum detectable
polarization (MDP) has nearly the same form as for the case of Gaussian
statistics derived by Elsner et al. (2012) in the limiting case of an
unpolarized signal. A polarized background is also considered, which
demonstrably increases uncertainties in source polarization measurements. In
addition, a Kolmogorov-style test of the event position angle distribution is
proposed that can provide an unbinned test of models where the polarization
angle in Stokes space depends on event characteristics such as time or energy.Comment: 8 pages, accepted for publication in the Astrophysical Journa
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Lexical organization in deaf children who use British Sign Language: Evidence from a semantic fluency task
We adapted the semantic fluency task into British Sign Language (BSL). In Study 1, we present data from twenty-two deaf signers aged four to fifteen. We show that the same ‘cognitive signatures’ that characterize this task in spoken languages are also present in deaf children, for example, the semantic clustering of responses. In Study 2, we present data from thirteen deaf children with Specific Language Impairment (SLI) in BSL, in comparison to a subset of children from Study 1 matched for age and BSL exposure. The two groups' results were comparable in most respects. However, the group with SLI made occasional word-finding errors and gave fewer responses in the first 15 seconds. We conclude that deaf children with SLI do not differ from their controls in terms of the semantic organization of the BSL lexicon, but that they access signs less efficiently
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