26,588 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
Using radiative energy losses to constrain the magnetisation and magnetic reconnection rate at the base of black hole jets
We calculate the severe radiative energy losses which occur at the base of
black hole jets using a relativistic fluid jet model, including in-situ
acceleration of non-thermal leptons by magnetic reconnection. Our results
demonstrate that including a self-consistent treatment of radiative energy
losses is necessary to perform accurate MHD simulations of powerful jets and
that jet spectra calculated via post-processing are liable to vastly
overestimate the amount of non-thermal emission. If no more than 95% of the
initial total jet power is radiated away by the plasma travels as it travels
along the length of the jet, we can place a lower bound on the magnetisation of
the jet plasma at the base of the jet. For typical powerful jets, we find that
the plasma at the jet base is required to be highly magnetised, with at least
10,000 times more energy contained in magnetic fields than in non-thermal
leptons. Using a simple power-law model of magnetic reconnection, motivated by
simulations of collisionless reconnection, we determine the allowed range of
the large-scale average reconnection rate along the jet, by restricting the
total radiative energy losses incurred and the distance at which the jet first
comes into equipartition. We calculate analytic expressions for the cumulative
radiative energy losses due to synchrotron and inverse-Compton emission along
jets, and derive analytic formulae for the constraint on the initial
magnetisation.Comment: 21 pages, 7 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
A matrix equation arising in statistical filter theory
Solution of Ricatti differential equation arising in statistical filering and optimal control theor
Rarefied-flow aerodynamics
Means for relatively simple and quick procedures are examined for estimating aerodynamic coefficients of lifting reentry vehicles. The methods developed allow aerospace designers not only to evaluate the aerodynamics of specific shapes but also to optimize shapes under given constraints. The analysis was also studied of the effect of thermomolecular flow on pressures measured by an orifice near the nose of a Space Shuttle Orbiter at altitudes above 75 km. It was shown that pressures corrected for thermomolecular flow effect are in good agreement with values predicted by independent theoretical methods. An incidental product was the insight gained about the free molecular thermal accommodation coefficient applicable under 'real' conditions of high speed flow in the Earth's atmosphere. The results are presented as abstracts of referenced papers. One reference paper is presented in its entirety
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
Synchrotron and inverse-Compton emission from blazar jets - III. Compton-dominant blazars
In this paper we develop the extended jet model of Potter & Cotter to model
the simultaneous multi-wavelength spectra of six Compton-dominant blazars. We
include an accelerating parabolic base transitioning to a slowly decelerating
conical jet with a geometry set by observations of M87 and consistent with
simulations and theory. We investigate several jet models and find that the
optically thick to thin synchrotron break in the radio spectrum requires the
jet to first come into equipartition at large distances along the jet (10^5
Schwarzschild radii), consistent with the observed transition from parabolic to
conical in the jet of M87. We confirm this result analytically and calculate
the expected frequency core-shift relations for the models under consideration.
We find that a parabolic jet transitioning to a ballistic conical jet, which
starts in equipartition and becomes more particle dominated at larger
distances, fits the multiwavelength data of the six blazars well, whilst an
adiabatic equipartition conical section requires very large bulk Lorentz
factors to reproduce the Compton-dominance of the blazars.
We find that all these blazars require high power, high bulk Lorentz factor
jets observed close to the line of sight as we expect from the blazar sequence
and consistent with the results from Paper II. The inverse-Compton emission in
our fits is due to inverse-Compton scattering of high-redshift CMB photons at
large distances along the jet due to the high bulk Lorentz factors of the jets.
We postulate a new interpretation of the blazar sequence based on the radius of
the transition region of the jet (where the jet is brightest in synchrotron
emission) scaling linearly with black hole mass.Comment: 13 pages, 5 figures, accepted for publication in MNRA
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