4,640 research outputs found
Particle Acceleration and the Formation of Relativistic Outflows in Viscous Accretion Disks with Shocks
In this Letter, we present a new self-consistent theory for the production of
the relativistic outflows observed from radio-loud black hole candidates and
active galaxies as a result of particle acceleration in hot, viscous accretion
disks containing standing, centrifugally-supported isothermal shocks. This is
the first work to obtain the structure of such disks for a relatively large
value of the Shakura-Sunyaev viscosity parameter (), and to
consider the implications of the shock for the acceleration of relativistic
particles in viscous disks. In our approach, the hydrodynamics and the particle
acceleration are coupled and the solutions are obtained self-consistently based
on a rigorous mathematical method. We find that particle acceleration in the
vicinity of the shock can provide enough energy to power the observed
relativistic jet in M87.Comment: published in ApJ
Global Slim Accretion Disk Solutions Revisited
We show that there exists a maximal possible accretion rate, beyond which
global slim disk solutions cannot be constructed because in the vertical
direction the gravitational force would be unable to balance the pressure force
to gather the accreted matter. The principle for this restriction is the same
as that for the Eddington luminosity and the corresponding critical accretion
rate, which were derived for spherical accretion by considering the same force
balance in the radial direction. If the assumption of hydrostatic equilibrium
is waived and vertical motion is included, this restriction may become even
more serious as the value of the maximal possible accretion rate becomes
smaller. Previous understanding in the literature that global slim disk
solutions could stand for any large accretion rates is due to the
overestimation of the vertical gravitational force by using an approximate
potential. For accretion flows with large accretion rates at large radii,
outflows seem unavoidable in order for the accretion flow to reduce the
accretion rate and follow a global solution till the central black hole.Comment: Accepted by Ap
Revisiting the Thermal Stability of Radiation-dominated Thin Disks
The standard thin disk model predicts that when the accretion rate is over a
small fraction of the Eddington rate, which corresponds to L \ga 0.06
L_{Edd}, the inner region of the disk is radiation-pressure-dominated and
thermally unstable. However, observations of the high/soft state of black hole
X-ray binaries with luminosity well within this regime (0.01L_{Edd} \la L \la
0.5L_{Edd}) indicate that the disk has very little variability, i.e., quite
stable. Recent radiation magnetohydrodynamic simulations of a vertically
stratified shearing box have confirmed the absence of the thermal instability.
In this paper, we revisit the thermal stability by linear analysis, taking into
account the role of magnetic field in the accretion flow. By assuming that the
field responses negatively to a positive temperature perturbation, we find that
the threshold of accretion rate above which the disk becomes thermally unstable
increases significantly compared with the case of not considering the role of
magnetic field. This accounts for the stability of the observed sources with
high luminosities. Our model also presents a possible explanation as to why
only GRS 1915+105 seems to show thermally unstable behavior. This peculiar
source holds the highest accretion rate (or luminosity) among the known high
state sources, which is well above the accretion rate threshold of the
instability.Comment: 13 pages, 2 figures, accepted by Ap
Dynamics of Poly(Vinyl Acetate)-d₃ on Silica
Poly(vinyl acetate) (PVAc) is an important polymer in applications because of both its bulk and surface characteristics. Its chain architecture gives it a low Tg and, generally, good qualities for processing and applications, which include paints, adhesives, thin films and surface coatings. In this study, we investigate the surface dynamics of PVAc absorbed onto silica with deuterium nuclear magnetic resonance (2H NMR). For dynamics studies, 2H NMR is an excellent technique because it uses an innocuous probe that can report on correlation times (τc) from approximately 10-8 s to 10 s. We report the use of two-dimensional exchange NMR (2D-X) and a side-chain methyl-d3 probe to investigate surface dynamics with τ c s in the range of 10-6 to over 1 s
An analytic relation for the thickness of accretion flows
We take the vertical distribution of the radial and azimuthal velocity into
account in spherical coordinates, and find that the analytic relation
c_{s0}/(v_K \Theta) = [(\gamma -1)/(2\gamma)]^{1/2} is valid for both
geometrically thin and thick accretion flows, where c_{s0} is the sound speed
on the equatorial plane, v_K is the Keplerian velocity, \Theta is the
half-opening angle of the flow, and \gamma is the adiabatic index.Comment: 4 pages, 2 figures, accepted by Science in China Series
Dynamical Structure of Viscous Accretion Disks with Shocks
We develop and discuss global accretion solutions for viscous ADAF disks
containing centrifugally supported isothermal shock waves. The fact that such
shocks can exist at all in ADAF disks is a new result. Interestingly, we find
that isothermal shocks can form even when the level of viscous dissipation is
relatively high. In order to better understand this phenomenon, we explore all
possible combinations of the fundamental flow parameters, such as specific
energy, specific angular momentum, and viscosity, to obtain the complete family
of global solutions. This procedure allows us to identify the region of the
parameter space where isothermal shocks can exist in viscous ADAF disks. The
allowed region is maximized in the inviscid case, and it shrinks as the level
of viscous dissipation increases. Adopting the canonical value gamma=1.5 for
the ratio of specific heats, we find that the shock region disappears
completely when the Shakura-Sunyaev viscosity parameter alpha exceeds the
critical value ~0.27. This establishes for the first time that steady ADAF
disks containing shocks can exist even for relatively high levels of viscous
dissipation. If an isothermal shock is present in the disk, it would have
important implications for the acceleration of energetic particles that can
escape to power the relativistic jets commonly observed around underfed,
radio-loud black holes. In two specific applications, we confirm that the
kinetic luminosity lost from the disk at the isothermal shock location is
sufficient to power the observed relativistic outflows in M87 and Sgr A*.Comment: accepted by Ap
Controlling “chemical nose” biosensor characteristics by modulating gold nanoparticle shape and concentration
Verma, M. S., Chen, P. Z., Jones, L., & Gu, F. X. (2015). Controlling “chemical nose” biosensor characteristics by modulating gold nanoparticle shape and concentration. Sensing and Bio-Sensing Research, 5, 13–18. https://doi.org/10.1016/j.sbsr.2015.04.007Conventional lock-and-key biosensors often only detect a single pathogen because they incorporate biomolecules with high specificity. “Chemical nose” biosensors are overcoming this limitation and identifying multiple pathogens simultaneously by obtaining a unique set of responses for each pathogen of interest, but the number of pathogens that can be distinguished is limited by the number of responses obtained. Herein, we use a gold nanoparticle-based “chemical nose” to show that changing the shapes of nanoparticles can increase the number of responses available for analysis and expand the types of bacteria that can be identified. Using four shapes of nanoparticles (nanospheres, nanostars, nanocubes, and nanorods), we demonstrate that each shape provides a unique set of responses in the presence of different bacteria, which can be exploited for enhanced specificity of the biosensor. Additionally, the concentration of nanoparticles controls the detection limit of the biosensor, where a lower concentration provides better detection limit. Thus, here we lay a foundation for designing “chemical nose” biosensors and controlling their characteristics using gold nanoparticle morphology and concentration
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