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Your search keyword '"Fraser, George W."' showing total 53 results
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53 results on '"Fraser, George W."'

1. The Lunar X-ray Observatory (LXO).

Author

Porter, F. Scott, Abbey, Tony F., Bannister, Nigel P., Carter, Jenny A., Collier, Michael, Cravens, Tom, Evans, Mike, Fraser, George W., Galeazzi, Massimiliano, Hills, Kent, Kuntz, Kip, Read, Andrew, Robertson, Ina P., Sembay, Steve, Sibeck, David G., Snowden, Steve, Stubbs, Timothy, and Travnicek, Pavel

Published
2008
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3. Proposed mission concept for the Astrophysical Plasmadynamic Explorer (APEX): an EUV high-resolution spectroscopic SMEX.

Author

Kowalski, Michael P., Cruddace, Raymond G., Wood, Kent S., Yentis, Daryl J., Wolff, Michael T., Laming, J. M., Gursky, Herbert, Carruthers, George R., Barbee Jr., Troy W., Kordas, Joseph F., Mauche, Chris W., Fritz, Gilbert G., Varlese, Steve J., Barstow, Martin A., Fraser, George W., Siegmund, Oswald H. W., Welsh, Barry Y., Brickhouse, Nancy S., Dupree, Andrea K., and Brown, Alex

Published
2003
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4. LOBSTER-ISS: an imaging x-ray all-sky monitor for the International Space Station.

Author

Fraser, George W., Brunton, Adam N., Bannister, Nigel P., Pearson, James F., Ward, Martin, Stevenson, Tim J., Watson, D. J., Warwick, Bob, Whitehead, S., O'Brian, Paul, White, Nicholas, Jahoda, Keith, Black, Kevin, Hunter, Stanley D., Deines-Jones, Phil, Priedhorsky, William C., Brumby, Steven P., Borozdin, Konstantin N., Vestrand, Thomas, and Fabian, A. C.

Published
2002
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6. Efficiency calibration of the four multilayer-coated holographic ion-etched flight gratings for a sounding rocket high-resolution spectrometer.

Author

Kowalski, Michael P., Gursky, Herbert, Rife, Jack C., Yentis, Daryl J., Cruddace, Raymond G., Barbee Jr., Troy W., Goldstein, William H., Kordas, Joseph F., Heidemann, Klaus F., Fritz, Gilbert G., Hunter, William R., Barstow, Martin A., Fraser, George W., Bannister, Nigel P., Lapington, Jonathan S., Tandy, J. A., and Sanderson, B. S.

Published
2001
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10. In-flight performance and calibration of the Chandra high-resolution camera spectroscopic readout (HRC-S).

Author

Kraft, Ralph P., Chappell, John H., Kenter, Almus T., Meehan, Gary R., Murray, Stephen S., Zombeck, Martin V., Donnelly, R. H., Drake, Jeremy J., Johnson, C. Olivia, Juda, Michael, Patnaude, Daniel, Pease, Deron O., Ratzlaff, Peter W., Wargelin, Bradford J., Zhao, Ping, Austin, Gerald K., Fraser, George W., Pearson, James F., Lees, John E., and Brunton, Adam N.

Published
2000
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15. XIPE: the X-ray imaging polarimetry explorer.

Author

Soffitta, Paolo, Barcons, Xavier, Bellazzini, Ronaldo, Braga, João, Costa, Enrico, Fraser, George W., Gburek, Szymon, Huovelin, Juhani, Matt, Giorgio, Pearce, Mark, Poutanen, Juri, Reglero, Victor, Santangelo, Andrea, Sunyaev, Rashid A., Tagliaferri, Gianpiero, Weisskopf, Martin, Aloisio, Roberto, Amato, Elena, Attiná, Primo, and Axelsson, Magnus

Subjects
ASTRONOMICAL polarimetry, X-ray imaging, SOLAR flares, ACCRETION (Astrophysics), MAGNETIC reconnection, NEUTRON stars
Abstract

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 10 5 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time. [ABSTRACT FROM AUTHOR]

Published
2013
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16. Background simulations for the Large Area Detector onboard LOFT.

Author

Campana, Riccardo, Feroci, Marco, Del Monte, Ettore, Mineo, Teresa, Lund, Niels, and Fraser, George W.

Subjects
COSMIC rays, GRAVITATIONAL fields, MONTE Carlo method, ENERGY bands, ASTRONOMICAL observations, NUCLEAR density, NUCLEAR counters
Abstract

The Large Observatory For X-ray Timing (LOFT), currently in an assessment phase in the framework the ESA M3 Cosmic Vision programme, is an innovative medium-class mission specifically designed to answer fundamental questions about the behaviour of matter, in the very strong gravitational and magnetic fields around compact objects and in supranuclear density conditions. Having an effective area of ∼10 m 2 at 8 keV, LOFT will be able to measure with high sensitivity very fast variability in the X-ray fluxes and spectra. A good knowledge of the in-orbit background environment is essential to assess the scientific performance of the mission and optimize the design of its main instrument, the Large Area Detector (LAD). In this paper the results of an extensive Geant-4 simulation of the instrumentwillbe discussed, showing the main contributions to the background and the design solutions for its reduction and control. Our results show that the current LOFT/LAD design is expected to meet its scientific requirement of a background rate equivalent to 10 mCrab in 2‒30 keV, achieving about 5 mCrab in the most important 2–10 keV energy band. Moreover, simulations show an anticipated modulation of the background rate as small as 10 % over the orbital timescale. The intrinsic photonic origin of the largest background component also allows for an efficient modelling, supported by an in-flight active monitoring, allowing to predict systematic residuals significantly better than the requirement of 1 %, and actually meeting the 0.25 % science goal. [ABSTRACT FROM AUTHOR]

Published
2013
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18. Recording from the same neurons chronically in motor cortex.

Author

Fraser, George W. and Schwartz, Andrew B.

Abstract

Two rhesus monkeys were implanted with silicon arrays of 96 microelectrodes. Neural activity was recorded periodically over a period of weeks to months. We have developed a method to determine whether single units in two separate recording sessions represent the same neuron. Pairwise cross-correlograms, the autocorrelogram, waveform shape, and mean firing rate were used together as identifying features of a neuron. When two units recorded on separate days were compared using these features, their similarity scores tended to be either high, indicating two recordings from the same neuron, or low, indicating different neurons. Although these metrics are individually weak, together they produce a strong classifier. Some neurons were recorded for 100 days. These monkeys performed a center-out reaching task, and we found that the firing properties of chronically recorded neurons were stable over time. [ABSTRACT FROM AUTHOR]

Published
2012
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42. Functional network reorganization during learning in a brain-computer interface paradigm.

Author

Jarosiewicz, Beata, Chase, Steven M., Fraser, George W., Velliste, Meel, Kass, Robert E., and Schwartz, Andrew B.

Subjects
NEURAL transmission, LEARNING, NEUROPHYSIOLOGY, BRAIN-computer interfaces, NEURONS, MOTOR cortex
Abstract

Efforts to study the neural correlates of learning are hampered by the size of the network in which learning occurs. To understand the importance of learning-related changes in a network of neurons, it is necessary to understand how the network acts as a whole to generate behavior. Here we introduce a paradigm in which the output of a cortical network can be perturbed directly and the neural basis of the compensatory changes studied in detail. Using a brain-computer interface, dozens of simultaneously recorded neurons in the motor cortex of awake, behaving monkeys are used to control the movement of a cursor in a three-dimensional virtual-reality environment. This device creates a precise, well-defined mapping between the firing of the recorded neurons and an expressed behavior (cursor movement). In a series of experiments, we force the animal to relearn the association between neural firing and cursor movement in a subset of neurons and assess how the network changes to compensate. We find that changes in neural activity reflect not only an alteration of behavioral strategy but also the relative contributions of individual neurons to the population error signal. [ABSTRACT FROM AUTHOR]

Published
2008
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