23,556 research outputs found
Modelling blazar flaring using a time-dependent fluid jet emission model - an explanation for orphan flares and radio lags
Blazar jets are renowned for their rapid violent variability and
multiwavelength flares, however, the physical processes responsible for these
flares are not well understood. In this paper we develop a time-dependent
inhomogeneous fluid jet emission model for blazars. We model optically thick
radio flares for the first time and show that they are delayed with respect to
the prompt optically thin emission by ~ months to decades, with a lag that
increases with the jet power and observed wavelength. This lag is caused by a
combination of the travel time of the flaring plasma to the optically thin
radio emitting sections of the jet and the slow rise time of the radio flare.
We predict two types of flares: symmetric flares - with the same rise and decay
time, which occur for flares whose duration is shorter than both the radiative
lifetime and the geometric path-length delay timescale; extended flares - whose
luminosity tracks the power of particle acceleration in the flare, which occur
for flares with a duration longer than both the radiative lifetime and
geometric delay. Our model naturally produces orphan X-ray and -ray
flares. These are caused by flares which are only observable above the
quiescent jet emission in a narrow band of frequencies. Our model is able to
successfully fit to the observed multiwavelength flaring spectra and
lightcurves of PKS1502+106 across all wavelengths, using a transient flaring
front located within the broad-line region.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Aerodynamics of 3-dimensional bodies in transitional flow
Based on considerations of fluid dynamic simulation appropriate to hypersonic, viscous flow over blunt-nosed lifting bodies, a method was presented earlier for estimating drag coefficients in the transitional-flow regime. The extension of the same method to prediction of lift coefficients is presented. Correlation of available experimental data by a simulation parameter appropriate for this purpose is the basis for the procedure described. The ease of application of the method makes it useful for preliminary studies which involve a wide variety of three-dimensional vehicle configurations or a range of angles of attack of a given vehicle
Commentary on the 1985 NASA/Vanderbilt Symposium on Future Hypervelocity Flight Requirements
The discussion, started in Semiannual Status Report Number 1, on aerothermal problems of hypervelocity flight and experiments that may lead to significant improvements in analytical/computational predictive methods, continues. The commentary is based on presentations made by speakers at a symposium on this subject held in December 1985. Symposium participants focused on the serious deficiencies that exist in knowledge of real-gas, nonequilibrium thermochemical-kinetic processes, catalytic processes, surface and shock slip, gas/surface interaction, boundary layer transition, and vortical leeside flows under hypervelocity conditions. Programs of laboratory research and computations leading toward in-flight experiments were recommended. Feasibility of appropriate measurement techniques for the flight environment was assessed and problems for study in that area identified. A synopsis of the oral presentations is given
Uncovering the physics behind the blazar sequence using a realistic model for jet emission
Blazar spectra are one of the most important windows into the physical
processes occurring along jets. The spectrum, composed from the different
emitting regions along the jet, allows us to constrain the physical conditions
in the jet. I present my work modelling blazar spectra using an extended
inhomogeneous jet model with an accelerating, magnetically dominated, parabolic
base transitioning to a slowly decelerating, conical section motivated by
observations, simulations and theory. We set the inner geometry of our
multi-zone model using observations of the jet in M87 which transitions from
parabolic to conical at 10^5 Schwarzschild radii. This model is able to
reproduce quiescent blazar spectra very well across all wavelengths (including
radio observations) for a sample of 42 BL Lacs and FSRQs.
Using this inhomogeneous model we are able to constrain the location at which
the synchrotron emission is brightest in these jets by fitting to the optically
thick to thin synchrotron break. We find that the radius of the jet at which
the synchrotron emission is brightest (where the jet first approaches
equipartition) scales approximately linearly with the jet power. We also find a
correlation between the length of the accelerating, parabolic section of the
jet and the maximum bulk Lorentz factor. In agreement with previous work we
find that BL Lacs are low power blazars whereas FSRQs are high power blazars.
Together with our simple jet power-radius relation this leads us to a deeper
understanding of the physics underlying the blazar sequence.Comment: 5 pages, 5 figures, to appear in "The Innermost Regions of
Relativistic Jets and Their Magnetic Fields" conference proceedings; includes
minor change
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Dynamic holographic interferometry using a Bi12SiO20 photorefractive crystal and monomode optical fibres
Dynamic holographic interferometry using polarization preserving opticai fibres as light guides and incorporating a photorefractive Bi12SiO20 (BSO) crystal as the recording medium is described. An experimental investigation of the
recording of time average holograms through the diffusion process (employing anisotropic self-diffraction) and the drift process (application of d.c. and a.c. electric fields across the crystal) is also described. The holographic interferometer was optimised to produce holograms with a high diffraction efficiency and a high signal-to-noise ratio. Results are presented on optimising parameters such as the writing beam angle and writing beam intensity ratio. The advantages that can be gained by deploying this holographic interferometer in an industrial environment, where the laser light is guided
to the location of the object by means of monomode fibres and images are stored within a photorefractive crystal is described. The holographic interferometer is capable of producing time average and double exposure interferograms of vibrating and deformed objects which can be displayed in real time
A simulation study of Large Area Crop Inventory Experiment (LACIE) technology
The author has identified the following significant results. The LACIE performance predictor (LPP) was used to replicate LACIE phase 2 for a 15 year period, using accuracy assessment results for phase 2 error components. Results indicated that the (LPP) simulated the LACIE phase 2 procedures reasonably well. For the 15 year simulation, only 7 of the 15 production estimates were within 10 percent of the true production. The simulations indicated that the acreage estimator, based on CAMS phase 2 procedures, has a negative bias. This bias was too large to support the 90/90 criterion with the CV observed and simulated for the phase 2 production estimator. Results of this simulation study validate the theory that the acreage variance estimator in LACIE was conservative
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