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RISS 인기검색어
- 검색키워드
- 저자 : Senturk, Gunes Demet (검색결과 2 건)
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Senturk, Gunes Demet Columbia University 2013 해외박사(DDOD)
Blazars are active galactic nuclei with a relativistic jet directed towards the observer's line of sight. Characterization of the non-thermal continuum emission originating from the blazar jet is currently an essential question in high-energy astrophysics. A blazar spectral energy distribution (SED) has a typical double-peaked shape in the flux vs. energy representation. The low-energy component of the SED is well-studied and thought to be due to synchrotron emission from relativistic electrons. The high-energy component, on the other hand, is still not completely understood and the emission in this part of the blazar spectrum can extend to energies as high as tera electron volts in some objects. This portion of the electromagnetic spectrum is referred to as the very-high-energy (VHE or TeV, E > 0.1 TeV) regime. At the time of this writing, more than half a hundred blazars have been detected to emit TeV gamma rays, representing the high energy extreme of these objects and constituting a population of its own. Most of these TeV blazars have also been detected in the high-energy (HE or GeV, 0.1 GeV < E < 0.1 TeV) gamma-ray range. In this work, we report on our discovery of the TeV emission from the blazar RBS 0413 and perform a detailed data analysis on this source, including contemporaneous multi-wavelength observations to characterize the broad-band SED and test various emission models for the high-energy component. Further, we extend our focus on the high-energy component to all archival TeV-detected blazars and study their spectral properties in the framework of GeV and TeV gamma-ray observations. To do this, we assemble for the first time the GeV and TeV spectra of a complete sample of TeV-detected blazars available in the archive to date. In the Appendix we present an analysis method for improved observations of large zenith angle targets with VERITAS.
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Stochastic models of workforce agility in production systems
Gel, Esma Senturk Northwestern University 1999 해외박사(DDOD)
We study analytical models of flexible workers with full and partial cross-training to gain insights into the logistical benefits that workforce agility offers in several different production environments. We model the case of full cross-training in flowlines with general demand and service processes. Under the assumption of collaborative work, we prove that expedite policy, under which all workers work together on the furthest downstream job, minimizes cycle time of all jobs. For the noncollaborative work environment the analogous pick-and-run policy, is shown to be highly effective but not necessarily optimal. Finally, our insights from the demand-constrained environment are extended to a capacity-constrained environment operating under a Constant-Work-In-Process (CONWIP) protocol. We consider partial cross-training systems with high and low skilled workers. We characterize the optimal worksharing policy for two worker CONWIP systems with general processing times, which leads us to the “fixed-before-shared” principle. This principle states that a flexible worker should first process tasks he/she is uniquely qualified for before helping others with shared tasks. We extend this principle to more general cases and note its applicability to various real world systems. Overall, our analyses verify that systems with partial cross-training can substantially outperform systems with no cross-training. In some cases, partial cross-training is virtually as good as full cross-training. We consider the problem of optimal worksharing between two workers each of who processes a fixed task in addition to a shared task. We formulate the problem as a Markov Decision Process (MDP) model to compute optimal policies and provide a benchmark for evaluating threshold policy heuristics. We observe that workforce agility can substitute for WIP and partial cross-training can achieve most of the benefits of full cross-training through dynamic line balancing. We study the system architecture factors that effect the potential logistical benefits worksharing offers and find that three such factors are significant: preemption, granularity and variability. The ability to preempt the shared task makes the system more robust against variability and hence, improves the performance of worksharing. However, significant opportunity exists in nonpreemptive systems in which worksharing achieves capacity balancing.
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