32,339 research outputs found
On the time variability of gamma-ray sources: A numerical analysis of variability indices
We present a Monte Carlo analysis of the recently introduced variability
indices (Tompkins 1999) and (Zhang et al. 2000 & Torres et al. 2001)
for -ray sources. We explore different variability criteria and prove
that these two indices, despite the very different approaches used to compute
them, are statistically correlated (5 to 7). This conclusion is
maintained also for the subset of AGNs and high latitude ( deg)
sources, whereas the correlation is lowered for the low latitude ones, where
the influence of the diffuse galactic emission background is strong.Comment: Small changes to match published version in Astronomische Nachrichten
(2001). Paper accepted in July 200
Antiresonances as precursors of decoherence
We show that, in presence of a complex spectrum, antiresonances act as a
precursor for dephasing enabling the crossover to a fully decoherent transport
even within a unitary Hamiltonian description. This general scenario is
illustrated here by focusing on a quantum dot coupled to a chaotic cavity
containing a finite, but large, number of states using a Hamiltonian
formulation. For weak coupling to a chaotic cavity with a sufficiently dense
spectrum, the ensuing complex structure of resonances and antiresonances leads
to phase randomization under coarse graining in energy. Such phase
instabilities and coarse graining are the ingredients for a mechanism producing
decoherence and thus irreversibility. For the present simple model one finds a
conductance that coincides with the one obtained by adding a ficticious voltage
probe within the Landauer-Buettiker picture. This sheds new light on how the
microscopic mechanisms that produce phase fluctuations induce decoherence.Comment: 7 pages, 2 figures, to appear in Europhys. Let
The Transition to College Process in PR-CETP Scholars
This article describes a study about the experiences of a group of students during the transition from high school to college. The students are future teachers who evidenced a high level of academic achievement in high school and received merit scholarships from the Puerto Rico Collaborative for Excellence in Teacher Preparation (PR-CETP). Two groups of students were compared: those who sustained a high GPA during their freshman year, and those who did not and, therefore, no longer qualified for the scholarship. The study was carried out through focused interviews with eight students, from three universities, four of whom maintained the scholarship and four who did not. Findings indicate that the main problems encountered were academic and social, and that the students received support from their families during the entire process. Regarding formal support, they pointed out that they felt highly satisfied with the services provided by PR-CETP and the universities, but they also pointed out (particularly those who lost the scholarship) that they needed additional services from the universities. They suggested, for example, better tutoring, and social activities among the scholars. The interviewed students, in general, consider that they faced the transition successfully since most of them described their academic, emotional, and social status as satisfactory at the time of the interviews
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