16 research outputs found
Accretion Disks Around Black Holes: Twenty Five Years Later
We study the progress of the theory of accretion disks around black holes in
last twenty five years and explain why advective disks are the best bet in
explaining varied stationary and non-stationary observations from black hole
candidates. We show also that the recently proposed advection dominated flows
are incorrect.Comment: 30 Latex pages including figures. Kluwer Style files included.
Appearing in `Observational Evidence for Black Holes in the Universe', ed.
Sandip K. Chakrabarti, Kluwer Academic Publishers (DORDRECHT: Holland
Solar Coronal Plumes
Polar plumes are thin long ray-like structures that project beyond the limb of the Sun polar regions, maintaining their identity over distances of several solar radii. Plumes have been first observed in white-light (WL) images of the Sun, but, with the advent of the space era, they have been identified also in X-ray and UV wavelengths (XUV) and, possibly, even in in situ data. This review traces the history of plumes, from the time they have been first imaged, to the complex means by which nowadays we attempt to reconstruct their 3-D structure. Spectroscopic techniques allowed us also to infer the physical parameters of plumes and estimate their electron and kinetic temperatures and their densities. However, perhaps the most interesting problem we need to solve is the role they cover in the solar wind origin and acceleration: Does the solar wind emanate from plumes or from the ambient coronal hole wherein they are embedded? Do plumes have a role in solar wind acceleration and mass loading? Answers to these questions are still somewhat ambiguous and theoretical modeling does not provide definite answers either. Recent data, with an unprecedented high spatial and temporal resolution, provide new information on the fine structure of plumes, their temporal evolution and relationship with other transient phenomena that may shed further light on these elusive features
Interplanetary Scintillation Observations of the Large-Scale Structure of the Solar Wind Using EISCAT
Observation of strong direct-like oscillator strength in the photoluminescence of Si nanoparticles
We have performed time-resolved photoluminescence measurements on suspensions of silicon nanoparticles using near-infrared two-photon femtosecond excitation. Our results for 1 nm particles show wide bandwidth but indicate full conversion to directlike behavior, with a few nanosecond time characteristic, corresponding to oscillator strength comparable to those in direct semiconductors. In addition to fast nanosecond decay, the photoluminescence from 2.85 nm nanoparticle suspension exhibits considerably slower decay, consistent with a transition regime to directlike behavior. The quantum yield is measured to be similar to 0.48, 0.82, and 0.56 for excitation at 254, 310 and 365 nm, respectively, for the blue 1 nm particles, and similar to 0.22, 0.36, and 0.50 for the red 2.85 nm particles. The directlike characteristics are discussed in terms of localization on radiative deep molecularlike Si-Si traps with size-dependent depth