1,601 research outputs found
Hydromagnetics of advective accretion flows around black holes: Removal of angular momentum by large scale magnetic stresses
We show that the removal of angular momentum is possible in the presence of
large scale magnetic stresses in geometrically thick, advective, sub-Keplerian
accretion flows around black holes in steady-state, in the complete absence of
alpha-viscosity. The efficiency of such an angular momentum transfer could be
equivalent to that of alpha-viscosity with alpha=0.01-0.08. Nevertheless,
required field is well below its equipartition value, leading to a magnetically
stable disk flow. This is essentially important in order to describe the hard
spectral state of the sources, when the flow is non/sub-Keplerian. We show in
our simpler 1.5-dimensional, vertically averaged disk model that larger the
vertical-gradient of azimuthal component of magnetic field, stronger the rate
of angular momentum transfer is, which in turn may lead to a faster rate of
outflowing matter. Finding efficient angular momentum transfer, in black hole
disks, via magnetic stresses alone is very interesting, when the generic origin
of alpha-viscosity is still being explored.Comment: 14 pages including 8 figures; accepted for publication in The
Astrophysical Journa
Accelerating AGN jets to parsec scales using general relativistic MHD simulations
Accreting black holes produce collimated outflows, or jets, that traverse
many orders of magnitude in distance, accelerate to relativistic velocities,
and collimate into tight opening angles. Of these, perhaps the least understood
is jet collimation due to the interaction with the ambient medium. In order to
investigate this interaction, we carried out axisymmetric general relativistic
magnetohydrodynamic simulations of jets produced by a large accretion disc,
spanning over 5 orders of magnitude in time and distance, at an unprecedented
resolution. Supported by such a disc, the jet attains a parabolic shape,
similar to the M87 galaxy jet, and the product of the Lorentz factor and the
jet half-opening angle, , similar to values found from very
long baseline interferometry (VLBI) observations of active galactic nuclei
(AGN) jets; this suggests extended discs in AGN. We find that the interaction
between the jet and the ambient medium leads to the development of pinch
instabilities, which produce significant radial and lateral variability across
the jet by converting magnetic and kinetic energy into heat. Thus pinched
regions in the jet can be detectable as radiating hotspots and may provide an
ideal site for particle acceleration. Pinching also causes gas from the ambient
medium to become squeezed between magnetic field lines in the jet, leading to
enhanced mass-loading of the jet and potentially contributing to the
spine-sheath structure observed in AGN outflows.Comment: 18 pages, 24 figures, submitted to MNRAS, revised version. See our
Youtube channel for accompanying animations:
https://www.youtube.com/playlist?list=PLjldVlE2vDFzHMGn75tgc2Lod0kcTWZd
Extension of topological groupoids and Serre, Hurewicz morphisms
In this paper, we introduce the notion of a topological groupoid extension
and relate it to the already existing notion of a gerbe over a topological
stack. We further study the properties of a gerbe over a Serre, Hurewicz stack
Chern-Weil theory for principal bundles over Lie groupoids and differentiable stacks
Let be a Lie groupoid equipped with a
connection, given by a smooth distribution
transversal to the fibers of the source map. Under the assumption that the
distribution is integrable, we define a version of de Rham
cohomology for the pair , and we study connections
on principal -bundles over in terms of the
associated Atiyah sequence of vector bundles. We also discuss associated
constructions for differentiable stacks. Finally, we develop the corresponding
Chern-Weil theory and describe characteristic classes of principal -bundles
over a pair .Comment: comments are welcom
Energy Extraction from Spinning Stringy Black Holes
We perform the first numerical simulations modeling the inflow and outflow of
magnetized plasma in the Kerr-Sen spacetime, which describes classical spinning
black holes (BHs) in string theory. We find that the Blandford-Znajek (BZ)
mechanism, which is believed to power astrophysical relativistic outflows or
``jets'', is valid even for BHs in an alternate theory of gravity, including
near the extremal limit. The BZ mechanism releases outward
Poynting-flux-dominated plasma as frame-dragging forces magnetic field lines to
twist. However, for nonspinning BHs, where the frame-dragging is absent, we
find an alternate powering mechanism through the release of gravitational
potential energy during accretion. Outflows from non-spinning stringy BHs can
be approximately more powerful as compared to Schwarzschild BHs, due to
their relatively smaller event horizon sizes and, thus, higher curvatures.
Finally, by constructing the first synthetic images of near-extremal non-Kerr
BHs from time-dependent simulations, we find that these can be ruled out by
horizon-scale interferometric images of accreting supermassive BHs.Comment: To be submitted to journal. Comments are welcom
Atiyah sequences and connections on principal bundles over differentiable stacks
We construct and study general connections on Lie groupoids and
differentiable stacks as well as on principal bundles over them using Atiyah
sequences associated to transversal tangential distributions.Comment: comments are welcom
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