12,453 research outputs found
Where have all the black holes gone?
We have calculated stationary models for accretion disks around super-massive
black holes in galactic nuclei. Our models show that below a critical mass flow
rate of ~3 10**-3 M_Edd advection will dominate the energy budget while above
that rate all the viscously liberated energy is radiated. The radiation
efficiency declines steeply below that critical rate. This leads to a clear
dichotomy between AGN and normal galaxies which is not so much given by
differences in the mass flow rate but by the radiation efficiency. At very low
mass accretion rates below 5 10**-5 M_Edd synchrotron emission and
Bremsstrahlung dominate the SED, while above 2 10**-4 M Edd the inverse Compton
radiation from synchrotron seed photons produce flat to inverted SEDs from the
radio to X-rays. Finally we discuss the implications of these findings for AGN
duty cycles and the long-term AGN evolution.Comment: 7 pages, 5 figures, accepted for publication in A&
Computation of turbulent high speed mixing layers using a two-equation turbulence model
A two-equation turbulence model was extended to be applicable for compressible flows. A compressibility correction based on modelling the dilational terms in the Reynolds stress equations were included in the model. The model is used in conjunction with the SPARK code for the computation of high speed mixing layers. The observed trend of decreasing growth rate with increasing convective Mach number in compressible mixing layers is well predicted by the model. The predictions agree well with the experimental data and the results from a compressible Reynolds stress model. The present model appears to be well suited for the study of compressible free shear flows. Preliminary results obtained for the reacting mixing layers are included
An integrated and modular digital modeling approach for the space station electrical power system development
An electrical power system for the Space Station was designed, developed and built. This system provides for electrical power generation, conditioning, storage, and distribution. The initial configuration uses photovoltaic power generation. The power system control is based on a hierarchical architecture to support the requirements of automation. In the preliminary design and technology development phase of the program, various modeling techniques and software tools were evaluated for the purpose of meeting the Space Station power system modeling requirements. Rocketdyne and LeRC jointly selected the EASY5 simulation software, developed by Boeing Computer Services, as a system level modeling tool. The application of the selected analytical modeling approach to represent the entire power system is described. Typical results of model predictions are also summarized. The equipment modeled includes solar arrays, dc to ac converters, resonant inverters, battery storage system, alternator, transmission line, switch gear, and system level microprocessor controls. During the advanced development phase of this program, several models were developed using this approach
Lifshitz-like systems and AdS null deformations
Following arXiv:1005.3291 [hep-th], we discuss certain lightlike deformations
of in Type IIB string theory sourced by a lightlike dilaton
dual to the N=4 super Yang-Mills theory with a lightlike varying
gauge coupling. We argue that in the case where the -direction is
noncompact, these solutions describe anisotropic 3+1-dim Lifshitz-like systems
with a potential in the -direction generated by the lightlike dilaton. We
then describe solutions of this sort with a linear dilaton. This enables a
detailed calculation of 2-point correlation functions of operators dual to bulk
scalars and helps illustrate the spatial structure of these theories. Following
this, we discuss a nongeometric string construction involving a
compactification along the -direction of this linear dilaton system. We
also point out similar IIB axionic solutions. Similar bulk arguments for
-noncompact can be carried out for deformations of in
M-theory.Comment: Latex, 20pgs, 1 eps fig; v2. references added; v3. minor
clarifications added, to appear in PR
One pion events by atmospheric neutrinos: A three flavor analysis
We study the one-pion events produced via neutral current (NC) and charged
current (CC) interactions by the atmospheric neutrinos. We analyze the ratios
of these events in the framework of oscillations between three neutrino
flavors. The ratios of the CC events induced by to that of the NC
events and a similar ratio defined with help us in distinguishing the
different regions of the neutrino parameter space.Comment: 14 pages, 4 figures (separate postscript files
Self-Similar Force-Free Wind From an Accretion Disk
We consider a self-similar force-free wind flowing out of an infinitely thin
disk located in the equatorial plane. On the disk plane, we assume that the
magnetic stream function scales as , where is the
cylindrical radius. We also assume that the azimuthal velocity in the disk is
constant: , where is a constant. For each choice of the
parameters and , we find an infinite number of solutions that are
physically well-behaved and have fluid velocity throughout the domain
of interest. Among these solutions, we show via physical arguments and
time-dependent numerical simulations that the minimum-torque solution, i.e.,
the solution with the smallest amount of toroidal field, is the one picked by a
real system. For , the Lorentz factor of the outflow increases
along a field line as \gamma \approx M(z/\Rfp)^{(2-\nu)/2} \approx R/R_{\rm
A}, where \Rfp is the radius of the foot-point of the field line on the disk
and R_{\rm A}=\Rfp/M is the cylindrical radius at which the field line
crosses the Alfven surface or the light cylinder. For , the Lorentz
factor follows the same scaling for z/\Rfp < M^{-1/(1-\nu)}, but at larger
distances it grows more slowly: \gamma \approx (z/\Rfp)^{\nu/2}. For either
regime of , the dependence of on shows that the rotation of
the disk plays a strong role in jet acceleration. On the other hand, the
poloidal shape of a field line is given by z/\Rfp \approx
(R/\Rfp)^{2/(2-\nu)} and is independent of . Thus rotation has neither a
collimating nor a decollimating effect on field lines, suggesting that
relativistic astrophysical jets are not collimated by the rotational winding up
of the magnetic field.Comment: 21 pages, 15 figures, accepted to MNRA
Self similar Barkhausen noise in magnetic domain wall motion
A model for domain wall motion in ferromagnets is analyzed. Long-range
magnetic dipolar interactions are shown to give rise to self-similar dynamics
when the external magnetic field is increased adiabatically. The power spectrum
of the resultant Barkhausen noise is of the form , where
can be estimated from the critical exponents for interface
depinning in random media.Comment: 7 pages, RevTex. To appear in Phys. Rev. Let
- …