890 research outputs found
Boosting jet power in black hole spacetimes
The extraction of rotational energy from a spinning black hole via the
Blandford-Znajek mechanism has long been understood as an important component
in models to explain energetic jets from compact astrophysical sources. Here we
show more generally that the kinetic energy of the black hole, both rotational
and translational, can be tapped, thereby producing even more luminous jets
powered by the interaction of the black hole with its surrounding plasma. We
study the resulting Poynting jet that arises from single boosted black holes
and binary black hole systems. In the latter case, we find that increasing the
orbital angular momenta of the system and/or the spins of the individual black
holes results in an enhanced Poynting flux.Comment: 7 pages, 5 figure
Relativistic MHD with Adaptive Mesh Refinement
This paper presents a new computer code to solve the general relativistic
magnetohydrodynamics (GRMHD) equations using distributed parallel adaptive mesh
refinement (AMR). The fluid equations are solved using a finite difference
Convex ENO method (CENO) in 3+1 dimensions, and the AMR is Berger-Oliger.
Hyperbolic divergence cleaning is used to control the
constraint. We present results from three flat space tests, and examine the
accretion of a fluid onto a Schwarzschild black hole, reproducing the Michel
solution. The AMR simulations substantially improve performance while
reproducing the resolution equivalent unigrid simulation results. Finally, we
discuss strong scaling results for parallel unigrid and AMR runs.Comment: 24 pages, 14 figures, 3 table
Critical Phenomena in Neutron Stars I: Linearly Unstable Nonrotating Models
We consider the evolution in full general relativity of a family of linearly
unstable isolated spherical neutron stars under the effects of very small,
perturbations as induced by the truncation error. Using a simple ideal-fluid
equation of state we find that this system exhibits a type-I critical
behaviour, thus confirming the conclusions reached by Liebling et al. [1] for
rotating magnetized stars. Exploiting the relative simplicity of our system, we
are able carry out a more in-depth study providing solid evidences of the
criticality of this phenomenon and also to give a simple interpretation of the
putative critical solution as a spherical solution with the unstable mode being
the fundamental F-mode. Hence for any choice of the polytropic constant, the
critical solution will distinguish the set of subcritical models migrating to
the stable branch of the models of equilibrium from the set of subcritical
models collapsing to a black hole. Finally, we study how the dynamics changes
when the numerically perturbation is replaced by a finite-size, resolution
independent velocity perturbation and show that in such cases a nearly-critical
solution can be changed into either a sub or supercritical. The work reported
here also lays the basis for the analysis carried in a companion paper, where
the critical behaviour in the the head-on collision of two neutron stars is
instead considered [2].Comment: 15 pages, 9 figure
Evolutions of Magnetized and Rotating Neutron Stars
We study the evolution of magnetized and rigidly rotating neutron stars
within a fully general relativistic implementation of ideal
magnetohydrodynamics with no assumed symmetries in three spatial dimensions.
The stars are modeled as rotating, magnetized polytropic stars and we examine
diverse scenarios to study their dynamics and stability properties. In
particular we concentrate on the stability of the stars and possible critical
behavior. In addition to their intrinsic physical significance, we use these
evolutions as further tests of our implementation which incorporates new
developments to handle magnetized systems.Comment: 12 pages, 8 figure
A joint Chandra and Swift view of the 2015 X-Ray dust-scattering echo of V404 Cygni
We present a combined analysis of the Chandra and Swift observations of the 2015 X-ray echo of V404 Cygni. Using a stacking analysis, we identify eight separate rings in the echo. We reconstruct the soft X-ray light curve of the 2015 June outburst using the high-resolution Chandra images and cross-correlations of the radial intensity profiles, indicating that about 70% of the outburst fluence occurred during the bright flare at the end of the outburst on MJD 57199.8. By deconvolving the intensity profiles with the reconstructed outburst light curve, we show that the rings correspond to eight separate dust concentrations with precise distance determinations. We further show that the column density of the clouds varies significantly across the field of view, with the centroid of most of the clouds shifted toward the Galactic plane, relative to the position of V404 Cyg, invalidating the assumption of uniform cloud column typically made in attempts to constrain dust properties from light echoes. We present a new XSPEC spectral dust-scattering model that calculates the differential dust-scattering cross section for a range of commonly used dust distributions and compositions and use it to jointly fit the entire set of Swift echo data. We find that a standard Mathis-Rumpl-Nordsieck model provides an adequate fit to the ensemble of echo data. The fit is improved by allowing steeper dust distributions, and models with simple silicate and graphite grains are preferred over models with more complex composition. © 2016. The American Astronomical Society. All rights reserved
The Disk Wind in the Neutron Star Low-mass X-Ray Binary GX 13+1
We present the analysis of seven \emph{Chandra} High Energy Transmission
Grating Spectrometer and six simultaneous \emph{RXTE} Proportional Counter
Array observations of the persistent neutron star (NS) low-mass X-ray binary GX
13+1 on its normal and horizontal branches. Across nearly 10 years, GX 13+1 is
consistently found to be accreting at \% Eddington, and all observations
exhibit multiple narrow, blueshifted absorption features, the signature of a
disk wind, despite the association of normal and horizontal branches with jet
activity. A single absorber with standard abundances cannot account for all
seven major disk wind features, indicating that multiple absorption zones may
be present. Two or three absorbers can produce all of the absorption features
at their observed broadened widths and reveal that multiple kinematic
components produce the accretion disk wind signature. Assuming the most ionized
absorber reflects the physical conditions closest to the NS, we estimate a wind
launching radius of cm, for an electron density of
cm. This is consistent with the Compton radius and also with a thermally
driven wind. Because of the source's high Eddington fraction, radiation
pressure likely facilitates the wind launching.Comment: 16 pages, 8 figures, 6 table
Entanglement, avoided crossings and quantum chaos in an Ising model with a tilted magnetic field
We study a one-dimensional Ising model with a magnetic field and show that
tilting the field induces a transition to quantum chaos. We explore the
stationary states of this Hamiltonian to show the intimate connection between
entanglement and avoided crossings. In general entanglement gets exchanged
between the states undergoing an avoided crossing with an overall enhancement
of multipartite entanglement at the closest point of approach, simultaneously
accompanied by diminishing two-body entanglement as measured by concurrence. We
find that both for stationary as well as nonstationary states, nonintegrability
leads to a destruction of two-body correlations and distributes entanglement
more globally.Comment: Corrections in two figure captions and one new reference. To appear
in Phys. Rev.
Magnetized Neutron-Star Mergers and Gravitational-Wave Signals
We investigate the influence of magnetic fields upon the dynamics of, and resulting gravitational waves from, a binary neutron-star merger in full general relativity coupled to ideal magnetohydrodynamics. We consider two merger scenarios: one where the stars have aligned poloidal magnetic fields and one without. Both mergers result in a strongly differentially rotating object. In comparison to the nonmagnetized scenario, the aligned magnetic fields delay the full merger of the stars. During and after merger we observe phenomena driven by the magnetic field, including Kelvin-Helmholtz instabilities in shear layers, winding of the field lines, and transition from poloidal to toroidal magnetic fields. These effects not only mediate the production of electromagnetic radiation, but also can have a strong influence on the gravitational waves. Thus, there are promising prospects for studying such systems with both types of waves
Who the hell was that? Stories, bodies and actions in the world
This article explores a two-way relationship between stories and the experiential actions of bodies in the world. Through an autoethnographic approach, the article presents a series of interlinked story fragments in an effort to show and evoke a feel for the ways in which stories, bodies, and actions influence and shape each other over time. It offers some reflections on the experiences the stories portray from the perspective of a social constructionist conception of narrative theory and suggest that while stories exert a powerful influence on the actions of our bodies, our bodies intrude on or âtalk backâ to this process because bodies have an existence beyond stories
Collapse and black hole formation in magnetized, differentially rotating neutron stars
The capacity to model magnetohydrodynamical (MHD) flows in dynamical,
strongly curved spacetimes significantly extends the reach of numerical
relativity in addressing many problems at the forefront of theoretical
astrophysics. We have developed and tested an evolution code for the coupled
Einstein-Maxwell-MHD equations which combines a BSSN solver with a high
resolution shock capturing scheme. As one application, we evolve magnetized,
differentially rotating neutron stars under the influence of a small seed
magnetic field. Of particular significance is the behavior found for
hypermassive neutron stars (HMNSs), which have rest masses greater the mass
limit allowed by uniform rotation for a given equation of state. The remnant of
a binary neutron star merger is likely to be a HMNS. We find that magnetic
braking and the magnetorotational instability lead to the collapse of HMNSs and
the formation of rotating black holes surrounded by massive, hot accretion tori
and collimated magnetic field lines. Such tori radiate strongly in neutrinos,
and the resulting neutrino-antineutrino annihilation (possibly in concert with
energy extraction by MHD effects) could provide enough energy to power
short-hard gamma-ray bursts. To explore the range of outcomes, we also evolve
differentially rotating neutron stars with lower masses and angular momenta
than the HMNS models. Instead of collapsing, the non-hypermassive models form
nearly uniformly rotating central objects which, in cases with significant
angular momentum, are surrounded by massive tori.Comment: Submitted to a special issue of Classical and Quantum Gravity based
around the New Frontiers in Numerical Relativity meeting at the Albert
Einstein Institute, Potsdam, July 17-21, 200
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