958 research outputs found
Supernova Remnants and Plerions in the Compton Gamma-Ray Observatory Era
Due to observations made by the Compton Gamma-Ray Observatory over the last
six years, it appears that a number of galactic supernova remnants may be
candidates for sources of cosmic gamma-rays. These include shell-type remnants
such as IC443 and Cygni, which have no known parent pulsars, but have
significant associations with unidentified EGRET sources, and others that
appear to be composite, where a pulsar is embedded in a shell (e.g. W44 and
Vela), or are purely pulsar-driven, such as the Crab Nebula. This review
discusses our present understanding of gamma-ray production in plerionic and
non-plerionic supernova remnants, and explores the relationship between such
emission and that in other wavebands. Focuses include models of the Crab and
Vela nebulae, the composite nature of W44, the relationship of shell-type
remnants to cosmic ray production, the relative importance of shock-accelerated
protons and electrons, constraints on models placed by TeV, X-ray and radio
observations, and the role of electrons injected directly into the remnants by
parent pulsars.Comment: 21 pages, including 4 eps figures, invited review, to appear in Proc.
4th Compton Symposium, (1997) ed. Dermer, C. D. & Kurfess, J. D. (AIP, New
York
Advanced Conducting Project
Shenandoah by Frank Ticheli -- Angel Band: III. Finale by Walter S. Hartley -- Petite Symphonie: No. 2 Andante Cantabile by Charles Gounod -- Amazing Grace by Frank Ticheli -- Fire Dance by David Shaffer -- Royal Crown March by Bruce Pearson and Barrie Gott -- Prophesy by Sean O\u27Loughlin -- Music from The Hunger Games by Michael Brown -- The Magic of Harry Potter by Michael Story -- On a Hymnsong of Philip Bliss by David Holsinger
A magnetometer based payload for a PTOL UAV with application in geophysical surveys
Includes bibliographical references.Applying the principles of physics to studying the Earth has given rise to the field of geophysics, which has been recognised as a separate discipline since the 19th century. The practical implementation of this field has led to a separate branch, aptly named exploration geophysics. Exploration geophysics aims to measure various naturally occurring phenomena associated with the Earth in order to make predictions about what might lie beneath the Earth’s surface. One of the fundamental phenomena associated with the Earth is the magnetic field or geomagnetic field. By localising magnetic anomalies within the geomagnetic field one can make predictions or inferences about the localised geophysical makeup and potential ore bodies, hydrocarbon deposits or archaeological artefacts that might exist below the surface. The fundamental sensor used to perform these surveys is the magnetometer. The concept of an unmanned aerial vehicle (UAV) has been around since 1915, with the first manufactured UAV appearing in 1916. Subsequent to the realisation of the UAV in the 1950s by Ryan Aeronautical for military reconnaissance, the idea of using UAV platforms to perform dull, dirty and dangerous functions has become common-place in the military environment. The first practical use of a UAV came in the 1991 Gulf War. The subsequent appearance of UAVs in the civilian realm can largely be attributed to the advent of low cost, high power density, lithium based batteries in the 1990s and the growth of the radio controlled (RC) hobbyist market
Testing the Production of Scintillation Arcs with the Pulsar B1133+16
Pulsars are extremely dense, highly magnetized stars that emit pulses of radio emission every millisecond or so. The arrival times of their radio signals at Earth observatories can be used as a clock precise enough to detect gravitational waves. Performing such a detection requires the mitigation of interference effects from the interstellar medium: the slightly ionized, mostly hydrogen gas that the radio waves traverse as they travel from the pulsar to Earth. We investigate radio wave delays using a powerful tool: scintillation arcs, fluctuations in frequency and time of the pulsar signal intensity that are manifested as parabolic arcs in the pulsar’s secondary spectrum. While scintillation arcs were first observed by Oberlin students almost two decades ago, the structures that cause them are still unknown. We explore the unique capabilities of the pulsar B1133+16 for testing models of scintillation arcs. Using measurements of scintillation arc curvatures over 96 days, we test a model for the annual modulation of arc curvature due to Earth’s orbit. The formalism for determining the width of a pulsar image from a scintillation arc width is used to design an empirical test of scintillation arc models based on the frequency dependence of arc widths. This formalism is then explored in the framework of a simple, one-dimensional model for the production of scintillation arcs
Balancing Power Auctions - Theoretical and Empirical Analyses
The European-wide goal of increasing the share of final electricity consumption from renewable sources to 27% by 2030 increases the supply volatility of the power system considerably. To manage this volatility, ancillary services for the power system become increasingly important. The most important short-term ancillary service is balancing power: it balances the electricity demand and supply instantaneously and, thus, ensures a constant frequency over time in alternating current grids. In most liberalized electricity markets worldwide, the procurement of balancing power is organized with the help of auctions. This thesis analyzes balancing power auctions theoretically and empirically and focuses on the current Austrian-German and the future European-wide Secondary Balancing Power (aFRR) auction design
Dynamics of stochastic integrate-and-fire networks
The neural dynamics generating sensory, motor, and cognitive functions are
commonly understood through field theories for neural population activity.
Classic neural field theories are derived from highly simplified models of
individual neurons, while biological neurons are highly complex cells.
Integrate-and-fire neuron models balance biophysical detail and analytical
tractability. Here, we develop a statistical field theory for networks of
integrate-and-fire neurons with stochastic spike emission. This reveals an
exact mapping to a self-consistent renewal process and a new mean field theory
for the activity in these networks. The mean field theory has a rate-dependent
leak, approximating the spike-driven resets of the membrane voltage. This gives
rise to bistability between quiescent and active states in homogenous and
excitatory-inhibitory pulse-coupled networks. The field-theoretic framework
also exposes fluctuation corrections to the mean field theory. We find that due
to the spike reset, fluctuations suppress activity. We then examine the roles
of spike resets and recurrent inhibition in stabilizing network activity. We
calculate the phase diagram for inhibitory stabilization and find that an
inhibition-stabilized regime occurs in wide regions of parameter space,
consistent with experimental reports of inhibitory stabilization in diverse
brain regions. Fluctuations narrow the region of inhibitory stabilization,
consistent with their role in suppressing activity through spike resets.Comment: A previous version of this article, now retracted
(https://journals.aps.org/prx/abstract/10.1103/ PhysRevX.12.041007),
contained errors described in the retraction notice
(https://journals.aps.org/prx/ abstract/10.1103/PhysRevX.13.029904). These
errors have been corrected in the current article, which was re-reviewed and
accepted back at Phys. Rev.
- …