5,488 research outputs found
The Influence of Magnetic Field Geometry on the Evolution of Black Hole Accretion Flows: Similar Disks, Drastically Different Jets
Because the magneto-rotational instability is capable of exponentially
amplifying weak preexisting magnetic fields, it might be hoped that the
character of the magnetic field in accretion disks is independent of the nature
of the seed field. However, the divergence-free nature of magnetic fields in
highly conducting fluids ensures that their large-scale topology is preserved,
no matter how greatly the field intensity is changed. By performing global
two-dimensional and three-dimensional general relativistic magnetohydrodynamic
disk simulations with several different topologies for the initial magnetic
field, we explore the degree to which the character of the flows around black
holes depends on the initial topology. We find that while the qualitative
properties of the accretion flow are nearly independent of field topology,
jet-launching is very sensitive to it: a sense of vertical field consistent for
at least an inner disk inflow time is essential to the support of strong jets.Comment: 42 pages; 17 figures; Accepted for publication in ApJ (some new
discussion and 2 new figures
Magnetically Driven Jets in the Kerr Metric
We compute a series of three-dimensional general relativistic
magnetohydrodynamic simulations of accretion flows in the Kerr metric to
investigate the properties of the unbound outflows that result. The overall
strength of these outflows increases sharply with increasing black hole
rotation rate, but a number of generic features are found in all cases. The
mass in the outflow is concentrated in a hollow cone whose opening angle is
largely determined by the effective potential for matter orbiting with angular
momentum comparable to that of the innermost stable circular orbit. The
dominant force accelerating the matter outward comes from the pressure of the
accretion disk's corona. The principal element that shapes the outflow is
therefore the centrifugal barrier preventing accreting matter from coming close
to the rotation axis. Inside the centrifugal barrier, the cone contains very
little matter and is dominated by electromagnetic fields that rotate at a rate
tied closely to the rotation of the black hole. These fields carry an
outward-going Poynting flux whose immediate energy source is the rotating
spacetime of the Kerr black hole. When the spin parameter a/M of the black hole
exceeds ~0.9, the energy carried to infinity by these outflows can be
comparable to the nominal radiative efficiency predicted in the Novikov-Thorne
model. Similarly, the expelled angular momentum can be comparable to that
accreted by the black hole. Both the inner electromagnetic part and the outer
matter part can contribute in significant fashion to the energy and angular
momentum of the outflow.Comment: 43 pages 12 figures To Appear in the Astrophysical Journal replaced
figure 3c with correct imag
Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows
We have carried out fully relativistic numerical simulations of accretion
disks in the Kerr metric. In this paper we focus on the unbound outflows that
emerge self-consistently from the accretion flow. These outflows are found in
the axial funnel region and consist of two components: a hot, fast, tenuous
outflow in the axial funnel proper, and a colder, slower, denser jet along the
funnel wall. Although a rotating black hole is not required to produce these
unbound outflows, their strength is enhanced by black hole spin. The
funnel-wall jet is excluded from the axial funnel due to elevated angular
momentum, and is also pressure-confined by a magnetized corona. The tenuous
funnel outflow accounts for a significant fraction of the energy transported to
large distances in the higher-spin simulations. We compare the outflows
observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte
Mapping the cellular electrophysiology of rat sympathetic preganglionic neurones to their roles in cardiorespiratory reflex integration:A whole cell recording study in situ
Sympathetic preganglionic neurones (SPNs) convey sympathetic activity flowing from the CNS to the periphery to reach the target organs. Although previous in vivo and in vitro cell recording studies have explored their electrophysiological characteristics, it has not been possible to relate these characteristics to their roles in cardiorespiratory reflex integration. We used the working heartâbrainstem preparation to make whole cell patch clamp recordings from T3â4 SPNs (n = 98). These SPNs were classified by their distinct responses to activation of the peripheral chemoreflex, diving response and arterial baroreflex, allowing the discrimination of muscle vasoconstrictor-like (MVC(like), 39%) from cutaneous vasoconstrictor-like (CVC(like), 28%) SPNs. The MVC(like) SPNs have higher baseline firing frequencies (2.52 ± 0.33 Hz vs. CVC(like) 1.34 ± 0.17 Hz, P = 0.007). The CVC(like) have longer after-hyperpolarisations (314 ± 36 ms vs. MVC(like) 191 ± 13 ms, P < 0.001) and lower input resistance (346 ± 49  MΩ vs. MVC(like) 496 ± 41 MΩ, P < 0.05). MVC(like) firing was respiratory-modulated with peak discharge in the late inspiratory/early expiratory phase and this activity was generated by both a tonic and respiratory-modulated barrage of synaptic events that were blocked by intrathecal kynurenate. In contrast, the activity of CVC(like) SPNs was underpinned by rhythmical membrane potential oscillations suggestive of gap junctional coupling. Thus, we have related the intrinsic electrophysiological properties of two classes of SPNs in situ to their roles in cardiorespiratory reflex integration and have shown that they deploy different cellular mechanisms that are likely to influence how they integrate and shape the distinctive sympathetic outputs
UV Imaging Polarimetry of the peculiar Seyfert 2 galaxy Mrk 477
We present the results of UV imaging polarimetry of the Seyfert 2 galaxy Mrk
477 taken by the Faint Object Camera onboard the Hubble Space Telescope (HST).
From a previous HST UV image (lambda ~ 2180A), Mrk 477 has been known to have
a pointlike bright UV hotspot in the central region, peculiar among nearby
Seyfert 2 galaxies. There are also claims of UV/optical variability, unusual
for a Seyfert 2 galaxy. Our data show that there is an off-nuclear scattering
region ~ 0."6 (~ 500 pc) NE from the hotspot. The data, after the subtraction
of the instrumental effect due to this bright hotspot region, might indicate
that the scattered light is also detected in the central 0."2 radius region and
is extended to a very wide angle. The hotspot location is consistent with the
symmetry center of the PA pattern, which represents the location of the hidden
nucleus, but our data do not provide a strong upper limit to the distance
between the symmetry center and the hotspot. We have obtained high spatial
resolution color map of the continuum which shows that the nuclear spiral arm
of 0."4 scale (~ 300pc) is significantly bluer than the off-nuclear mirror and
the hotspot region. The nature of the hotspot is briefly discussed.Comment: To appear in Ap
High-Field Superconductivity at an Electronic Topological Transition in URhGe
The emergence of superconductivity at high magnetic fields in URhGe is
regarded as a paradigm for new state formation approaching a quantum critical
point. Until now, a divergence of the quasiparticle mass at the metamagnetic
transition was considered essential for superconductivity to survive at
magnetic fields above 30 tesla. Here we report the observation of quantum
oscillations in URhGe revealing a tiny pocket of heavy quasiparticles that
shrinks continuously with increasing magnetic field, and finally disappears at
a topological Fermi surface transition close to or at the metamagnetic field.
The quasiparticle mass decreases and remains finite, implying that the Fermi
velocity vanishes due to the collapse of the Fermi wavevector. This offers a
novel explanation for the re-emergence of superconductivity at extreme magnetic
fields and makes URhGe the first proven example of a material where magnetic
field-tuning of the Fermi surface, rather than quantum criticality alone,
governs quantum phase formation.Comment: A revised version has been accepted for publication in Nature Physic
The carotid body as a putative therapeutic target for the treatment of neurogenic hypertension
Quantitative insertion-site sequencing (QIseq) for high throughput phenotyping of transposon mutants
Genetic screening using random transposon insertions has been a powerful tool for uncovering biology in prokaryotes, where whole-genome saturating screens have been performed in multiple organisms. In eukaryotes, such screens have proven more problematic, in part because of the lack of a sensitive and robust system for identifying transposon insertion sites. We here describe quantitative insertion-site sequencing, or QIseq, which uses custom library preparation and Illumina sequencing technology and is able to identify insertion sites from both the 5' and 3' ends of the transposon, providing an inbuilt level of validation. The approach was developed using piggyBac mutants in the human malaria parasite Plasmodium falciparum but should be applicable to many other eukaryotic genomes. QIseq proved accurate, confirming known sites in >100 mutants, and sensitive, identifying and monitoring sites over a >10,000-fold dynamic range of sequence counts. Applying QIseq to uncloned parasites shortly after transfections revealed multiple insertions in mixed populations and suggests that >4000 independent mutants could be generated from relatively modest scales of transfection, providing a clear pathway to genome-scale screens in P. falciparum QIseq was also used to monitor the growth of pools of previously cloned mutants and reproducibly differentiated between deleterious and neutral mutations in competitive growth. Among the mutants with fitness defects was a mutant with a piggyBac insertion immediately upstream of the kelch protein K13 gene associated with artemisinin resistance, implying mutants in this gene may have competitive fitness costs. QIseq has the potential to enable the scale-up of piggyBac-mediated genetics across multiple eukaryotic systems
The thermodynamics of perchlorates. I. Heat capacity of ND4ClO4 from 7 to 345 K and the analysis of heat capacities and related data of NH4ClO4 and ND4ClO4
The heat capacity of the orthorhombic salt: deuterated ammonium perchlorate, ND4 ClO4 , was measured from 7 to 345 K using adiabatic calorimetry. The heat capacity against temperature curve is smooth, continuous and without anomaly. Values of the standard molar thermodynamic quantities are presented up to 340 K. The heat capacities of ND4 ClO4 and NH4 ClO4 have been analyzed. The contributions to the vibrational heat capacity from the external optical modes of NH+4 or ND+4, ClOâ4 and libration from the external modes of ClOâ4 along with those of vibration from the internal optical modes of NH+4 or ND+4 and ClOâ4, and the acoustic lattice modes for these ions have been calculated. The difference between the experimental and calculated heat capacity, called the residual heat capacity, equals the contribution from ammonium ion rotation and the thermal expansion of the lattice. With recent thermal expansion data, the correction from constant stress to constant strain has been applied and the derived rotational heat capacities of the NH+4 and ND+4 are determined to be in qualitative agreement with those derived from various rotational models.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70290/2/JCPSA6-91-1-399-1.pd
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