515 research outputs found
The Stability of Magnetized Rotating Plasmas with Superthermal Fields
During the last decade it has become evident that the magnetorotational
instability is at the heart of the enhanced angular momentum transport in
weakly magnetized accretion disks around neutron stars and black holes. In this
paper, we investigate the local linear stability of differentially rotating,
magnetized flows and the evolution of the magnetorotational instability beyond
the weak-field limit. We show that, when superthermal toroidal fields are
considered, the effects of both compressibility and magnetic tension forces,
which are related to the curvature of toroidal field lines, should be taken
fully into account. We demonstrate that the presence of a strong toroidal
component in the magnetic field plays a non-trivial role. When strong fields
are considered, the strength of the toroidal magnetic field not only modifies
the growth rates of the unstable modes but also determines which modes are
subject to instabilities. We find that, for rotating configurations with
Keplerian laws, the magnetorotational instability is stabilized at low
wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the
sound speed and the rotational speed. We discuss the significance of our
findings for the stability of cold, magnetically dominated, rotating fluids and
argue that, for these systems, the curvature of toroidal field lines cannot be
neglected even when short wavelength perturbations are considered. We also
comment on the implications of our results for the validity of shearing box
simulations in which superthermal toroidal fields are generated.Comment: 24 pages, 12 figures. Accepted for publication in ApJ. Sections 2 and
5 substantially expanded, added Appendix A and 3 figures with respect to
previous version. Animations are available at
http://www.physics.arizona.edu/~mpessah/research
Conformally flat black hole initial data, with one cylindrical end
We give a complete analytical proof of existence and uniqueness of
extreme-like black hole initial data for Einstein equations, which possess a
cilindrical end, analogous to extreme Kerr, extreme Reissner Nordstrom, and
extreme Bowen-York's initial data. This extends and refines a previous result
\cite{dain-gabach09} to a general case of conformally flat, maximal initial
data with angular momentum, linear momentum and matter.Comment: Minor changes and formula (21) revised according to the published
version in Class. Quantum Grav. (2010). Results unchange
Rich Counter-Examples for Temporal-Epistemic Logic Model Checking
Model checking verifies that a model of a system satisfies a given property,
and otherwise produces a counter-example explaining the violation. The verified
properties are formally expressed in temporal logics. Some temporal logics,
such as CTL, are branching: they allow to express facts about the whole
computation tree of the model, rather than on each single linear computation.
This branching aspect is even more critical when dealing with multi-modal
logics, i.e. logics expressing facts about systems with several transition
relations. A prominent example is CTLK, a logic that reasons about temporal and
epistemic properties of multi-agent systems. In general, model checkers produce
linear counter-examples for failed properties, composed of a single computation
path of the model. But some branching properties are only poorly and partially
explained by a linear counter-example.
This paper proposes richer counter-example structures called tree-like
annotated counter-examples (TLACEs), for properties in Action-Restricted CTL
(ARCTL), an extension of CTL quantifying paths restricted in terms of actions
labeling transitions of the model. These counter-examples have a branching
structure that supports more complete description of property violations.
Elements of these counter-examples are annotated with parts of the property to
give a better understanding of their structure. Visualization and browsing of
these richer counter-examples become a critical issue, as the number of
branches and states can grow exponentially for deeply-nested properties.
This paper formally defines the structure of TLACEs, characterizes adequate
counter-examples w.r.t. models and failed properties, and gives a generation
algorithm for ARCTL properties. It also illustrates the approach with examples
in CTLK, using a reduction of CTLK to ARCTL. The proposed approach has been
implemented, first by extending the NuSMV model checker to generate and export
branching counter-examples, secondly by providing an interactive graphical
interface to visualize and browse them.Comment: In Proceedings IWIGP 2012, arXiv:1202.422
Instability of hyper-compact Kerr-like objects
Viable alternatives to astrophysical black holes include hyper-compact
objects without horizon, such as gravastars, boson stars, wormholes and
superspinars. The authors have recently shown that typical rapidly-spinning
gravastars and boson stars develop a strong instability. That analysis is
extended in this paper to a wide class of horizonless objects with approximate
Kerr-like geometry. A detailed investigation of wormholes and superspinars is
presented, using plausible models and mirror boundary conditions at the
surface. Like gravastars and boson stars, these objects are unstable with very
short instability timescales. This result strengthens previous conclusions that
observed hyper-compact astrophysical objects with large rotation are likely to
be black holes.Comment: 15 pages, 3 figures. To be published in CQ
Turbulent magnetic field amplification from spiral SASI modes in core-collapse supernovae
We describe the initial implementation of magnetohydrodynamics (MHD) in our
astrophysical simulation code \genasis. Then, we present MHD simulations
exploring the capacity of the stationary accretion shock instability (SASI) to
generate magnetic fields by adding a weak magnetic field to an initially
spherically symmetric fluid configuration that models a stalled shock in the
post-bounce supernova environment. Upon perturbation and nonlinear SASI
development, shear flows associated with the spiral SASI mode contributes to a
widespread and turbulent field amplification mechanism. While the SASI may
contribute to neutron star magnetization, these simulations do not show
qualitatively new features in the global evolution of the shock as a result of
SASI-induced magnetic field amplification.Comment: 15 pages, 7 figures, To appear in the Journal of Physics: Conference
Series. Proceedings of the IUPAP Conference on Computational Physics
(CCP2011
Altered maternal profiles in corticotropin-releasing factor receptor 1 deficient mice
BACKGROUND: During lactation, the CNS is less responsive to the anxiogenic neuropeptide, corticotropin-releasing factor (CRF). Further, central injections of CRF inhibit maternal aggression and some maternal behaviors, suggesting decreased CRF neurotransmission during lactation supports maternal behaviors. In this study, we examined the maternal profile of mice missing the CRF receptor 1 (CRFR1). Offspring of knockout (CRFR1-/-) mice were heterozygote to offset possible deleterious effects of low maternal glucocorticoids on pup survival and all mice contained a mixed 50:50 inbred/outbred background to improve overall maternal profiles and fecundity. RESULTS: Relative to littermate wild-type (WT) controls, CRFR1-/- mice exhibited significant deficits in total time nursing, including high arched-back, on each test day. Consistent with decreased nursing, pups of CRFR1-deficient dams weighed significantly less than WT offspring. Licking and grooming of pups was significantly higher in WT mice on postpartum Day 2 and when both test days were averaged, but not on Day 3. Time off nest was higher for CRFR1-/- mice on Day 2, but not on Day 3 or when test days were averaged. Licking and grooming of pups did not differ on Day 2 when this measure was examined as a proportion of time on nest. CRFR1-/- mice showed significantly higher nest building on Day 3 and when tests were averaged. Mean pup number was almost identical between groups and no pup mortality occurred. Maternal aggression was consistently lower in CRFR1-/- mice and in some measures these differences approached, but did not reach significance. Because of high variance, general aggression results are viewed as preliminary. In terms of sites of attacks on intruders, CRFR1-/- mice exhibited significantly fewer attacks to the belly of the intruder on Day 5 and when tests were averaged. Performance on the elevated plus maze was similar between genotypes. Egr-1 expression differences in medial preoptic nucleus and c-Fos expression differences in bed nucleus of stria terminalis between genotype suggest possible sites where loss of gene alters behavioral output. CONCLUSION: Taken together, the results suggest that the presence of an intact CRFR1 receptor supports some aspects of nurturing behavior
Extending the Shakura-Sunyaev approach to a strongly magnetized accretion disc model
We develop a model of thin turbulent accretion discs supported by magnetic
pressure of turbulent magnetic fields. This applies when the turbulent kinetic
and magnetic energy densities are greater than the thermal energy density in
the disc. Whether such discs survive in nature or not remains to be determined,
but here we simply demonstrate that self-consistent solutions exist when the
alpha-prescription for the viscous stress, similar to that of the original
Shakura-Sunyaev model, is used. We show that \alpha \sim 1 for the strongly
magnetized case and we calculate the radial structure and emission spectra from
the disc in the regime when it is optically thick. Strongly magnetized
optically thick discs can apply to the full range of disc radii for objects <
10^{-2} of the Eddington luminosity or for the outer parts of discs in higher
luminosity sources. In the limit that the magnetic pressure is equal to the
thermal or radiation pressure, our strongly magnetized disc model transforms
into the Shakura-Sunyaev model with \alpha=1. Our model produces spectra quite
similar to those of standard Shakura-Sunyaev models. In our comparative study,
we also discovered a small discrepancy in the spectral calculations of Shakura
and Sunyaev (1973).Comment: 27 pages, 11 figures, Astron. Astroph. in press; shortened version
accepted by A&A, all calculations and conclusions are unchange
Black hole mergers: the first light
The coalescence of supermassive black hole binaries occurs via the emission
of gravitational waves, that can impart a substantial recoil to the merged
black hole. We consider the energy dissipation, that results if the recoiling
black hole is surrounded by a thin circumbinary disc. Our results differ
significantly from those of previous investigations. We show analytically that
the dominant source of energy is often potential energy, released as gas in the
outer disc attempts to circularize at smaller radii. Thus, dimensional
estimates, that include only the kinetic energy gained by the disc gas,
underestimate the real energy loss. This underestimate can exceed an order of
magnitude, if the recoil is directed close to the disc plane. We use three
dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional
finite difference simulations to verify our analytic estimates. We also compute
the bolometric light curve, which is found to vary strongly depending upon the
kick angle. A prompt emission signature due to this mechanism may be observable
for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about
1000 km/s. Emission at earlier times can mainly result from the response of the
disc to the loss of mass, as the black holes merge. We derive analytically the
condition for this to happen.Comment: 16 pages, accepted by MNRAS. Animations of the simulations are
available at http://jilawww.colorado.edu/~pja/recoil.htm
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