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Institution

University of Maryland, Baltimore County

EducationBaltimore, Maryland, United States
About: University of Maryland, Baltimore County is a education organization based out in Baltimore, Maryland, United States. It is known for research contribution in the topics: Population & Aerosol. The organization has 8749 authors who have published 20843 publications receiving 795706 citations. The organization is also known as: UMBC.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors reported the detection of a narrow Fe K? emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating.
Abstract: We report the detection of a narrow Fe K? emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In the galaxy frame we measure a center energy of 6.402 keV, a FWHM of 4515 km s-1, an intensity of 3.6 ? 10-5 photons cm-2 s-1, with an equivalent width of 133 eV (errors are 90% confidence for one parameter). The line is only marginally resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ~Fe XVIII are not ruled out. We cannot constrain the detailed dynamics but, assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light month from the central X-ray source. We cannot rule out some contribution to the narrow Fe K? line from a putative, parsec-scale, obscuring torus that is postulated to be a key component of active galactic nuclei (AGNs) unification models. The continuum intensity during the Chandra observation was a factor ~2 less than typical historical levels. If the X-ray continuum was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe K? intensity had not yet responded to such a change, then the predicted line intensity and equivalent width for an origin in the BLR is within the 90% measurement errors. Anisotropic X-ray continuum illumination of the BLR and/or additional line emission from a torus structure would improve the agreement with observation. Two out of three archival ASCA data sets are consistent with the narrow line being present with the same intensity as in the Chandra observation. However, there is strong evidence that the narrow-line intensity varied and was unusually low during one of the ASCA campaigns. In general, inclusion of the narrow line to model the overall broad Fe K? line profile in terms of a rotating disk plus black-hole model can have a non-negligible effect on the disk line intensity and variability properties. Variability of the broad disk line in NGC 5548 is difficult to reconcile with the expectations of the simple disk model, even when the narrow-line component is accounted for. It will be critical to ascertain the importance of a similar nondisk Fe K? line in other Seyfert 1 galaxies. Future monitoring of the narrow Fe K? component with large collecting area and high spectral resolution will enable reverberation mapping the BLR region, complementary to similar studies using the optical/UV lines, and therefore provide independent constraints on the black-hole mass.

173 citations

Book ChapterDOI
20 Oct 2003
TL;DR: This paper proposes several security-related ontologies that are designed to represent well-known security concepts that are used to describe the security requirements and capabilities of web services providers and requesting agents.
Abstract: In the next generation of the Internet semantic annotations will enable software agents to extract and interpret web content more quickly than it is possible with current techniques. The focus of this paper is to develop security annotations for web services that are represented in DAML-S and used by agents. We propose several security-related ontologies that are designed to represent well-known security concepts. These ontologies are used to describe the security requirements and capabilities of web services providers and requesting agents. A reasoning engine decides whether agents and web service have comparable security characteristics. Our prototypical implementation uses the Java Theorem Prover from Stanford for deciding the degree to which the requirements and capabilities match based on our matching algorithm. The security reasoner is integrated with the Semantic Matchmaker from CMU giving it the ability to provide security brokering between agents and services.

173 citations

Journal ArticleDOI
TL;DR: The experiments constitute the first direct demonstration that melanopsin forms a photopigment capable of activating a G-protein, but its spectral properties are not consistent with the action spectrum for circadian entrainment.
Abstract: Recently, melanopsin has emerged as the leading candidate for the elusive photopigment of the mammalian circadian system. This novel opsin-like protein is expressed in retinal ganglion cells that form the retinohypothalamic tract, a neuronal connection between the retina and the suprachiasmatic nucleus. These hypothalamic structures contain the circadian pacemaker, which generates daily rhythms in physiology and behavior. In mammals, proper synchronization of these rhythms to the environmental light-dark cycle requires retinal input. Surprisingly, rod and cone photoreceptors are not required. Instead, the melanopsin-containing ganglion cells are intrinsically sensitive to light, perhaps responding via a melanopsin-based signaling pathway. To test this hypothesis, we have characterized melanopsin following heterologous expression in COS cells. We found that melanopsin absorbed maximally at 424 nm after reconstitution with 11-cis-retinal. Furthermore, melanopsin activated the photoreceptor G-protein, transducin, in a light-dependent manner. In agreement with the measured absorbance spectrum, melanopsin was most efficiently excited by blue light (420-440 nm). In contrast, published action spectra suggest that the photopigment underlying the intrinsic light sensitivity of SCN-projecting RGCs has an absorption maximum near 484 nm. In summary, our experiments constitute the first direct demonstration that melanopsin forms a photopigment capable of activating a G-protein, but its spectral properties are not consistent with the action spectrum for circadian entrainment.

173 citations

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that electrons are energized in two distinct regions: a low-energy population (less than or equal to a few kiloelectronvolts) that arises in a diffusion region where particles are demagnetized and the magnetic topology changes, and a high-energy component (approaching 100 keV) that results from betatron acceleration within dipolarization fronts that sweep towards the inner magnetosphere far from the diffusion region.
Abstract: Magnetic reconnection in magnetized plasmas represents a change in magnetic field topology and is associated with a concomitant energization of charged particles that results from a conversion of magnetic energy into particle energy. In Earth’s magnetosphere this process is associated with the entry of the solar wind into the magnetosphere and with the initiation of auroral substorms. Using data from the THEMIS mission, together with global and test particle simulations, we demonstrate that electrons are energized in two distinct regions: a low-energy population (less than or equal to a few kiloelectronvolts) that arises in a diffusion region where particles are demagnetized and the magnetic topology changes, and a high-energy component (approaching 100 keV) that results from betatron acceleration within dipolarization fronts that sweep towards the inner magnetosphere far from the diffusion region. Thus, the observed particle energization is associated with both magnetic reconnection and with betatron acceleration associated with macroscopic flows.

173 citations

Journal ArticleDOI
01 Jan 1991
TL;DR: Finite state, finite action, stochastic games over an infinite time horizon, algorithms for the computation of minimax optimal stationary strategies in the zerosum case, and of Nash equilibria in stationary Strategies in the nonzerosum case are surveyed.
Abstract: We consider finite state, finite action, stochastic games over an infinite time horizon. We survey algorithms for the computation of minimax optimal stationary strategies in the zerosum case, and of Nash equilibria in stationary strategies in the nonzerosum case. We also survey those theoretical results that pave the way towards future development of algorithms.

173 citations


Authors

Showing all 8862 results

NameH-indexPapersCitations
Robert C. Gallo14582568212
Paul T. Costa13340688454
Igor V. Moskalenko13254258182
James Chiang12930860268
Alex K.-Y. Jen12892161811
Alan R. Shuldiner12055771737
Richard N. Zare120120167880
Vince D. Calhoun117123462205
Rita R. Colwell11578155229
Kendall N. Houk11299754877
Elliot K. Fishman112133549298
Yoram J. Kaufman11126359238
Paulo Artaxo10745444346
Braxton D. Mitchell10255849599
Sushil Jajodia10166435556
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Performance
Metrics
No. of papers from the Institution in previous years
YearPapers
202371
2022165
20211,065
20201,091
2019989
2018929