493 research outputs found
The Axisymmetric Pulsar Magnetosphere
We present, for the first time, the structure of the axisymmetric force-free
magnetosphere of an aligned rotating magnetic dipole, in the case in which
there exists a sufficiently large charge density (whose origin we do not
question) to satisfy the ideal MHD condition, , everywhere.
The unique distribution of electric current along the open magnetic field lines
which is required for the solution to be continuous and smooth is obtained
numerically. With the geometry of the field lines thus determined we compute
the dynamics of the associated MHD wind. The main result is that the
relativistic outflow contained in the magnetosphere is not accelerated to the
extremely relativistic energies required for the flow to generate gamma rays.
We expect that our solution will be useful as the starting point for detailed
studies of pulsar magnetospheres under more general conditions, namely when
either the force-free and/or the ideal MHD condition are not
valid in the entire magnetosphere. Based on our solution, we consider that the
most likely positions of such an occurrence are the polar cap, the crossings of
the zero space charge surface by open field lines, and the return current
boundary, but not the light cylinder.Comment: 15 pages AAS Latex, 5 postscript figure
Magnetohydrodynamic jets from different magnetic field configurations
Using axisymmetric MHD simulations we investigate how the overall jet
formation is affected by a variation in the disk magnetic flux profile and/or
the existence of a central stellar magnetosphere. Our simulations evolve from
an initial, hydrostatic equilibrium state in a force-free magnetic field
configuration. We find a unique relation between the collimation degree and the
disk wind magnetization power law exponent. The collimation degree decreases
for steeper disk magnetic field profiles. Highly collimated outflows resulting
from a flat profile tend to be unsteady. We further consider a magnetic field
superposed of a stellar dipole and a disk field in parallel or anti-parallel
alignment. Both stellar and disk wind may evolve in a pair of outflows,
however, a reasonably strong disk wind component is essential for jet
collimation. Strong flares may lead to a sudden change in mass flux by a factor
two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in
Context, held in Rhodes, July 7-12, 200
Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts
We present a detailed numerical study of the dynamics and evolution of
ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under
extreme magnetization conditions. We find that Lorentz factors of up to 3000
are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows
for a wide variation in the flow Lorentz factor. Pending contamination induced
by mass-entrainment, the linear Michel scaling links modulations in the
ultrarelativistic wind to variations in mass accretion in the disk for a given
magnetization. The jet is asymptotically dominated by the toroidal magnetic
field allowing for efficient collimation. We discuss our solutions (jets) in
the context of Gamma ray bursts and describe the relevant features such as the
high variability in the Lorentz factor and how high collimation angles (~ 0-5
degrees), or cylindrical jets, can be achieved. We isolate a jet instability
mechanism we refer to as the "bottle-neck" instability which essentially relies
on a high magnetization and a recollimation of the magnetic flux surfaces. The
instability occurs at large radii where any dissipation of the magnetic energy
into radiation would in principle result in an optically thin emission.Comment: 31 pages, 6 figures. Submitted to ApJ. Higher Quality figures at
http://www.capca.ucalgary.ca/paper
Pain-relief learning in flies, rats, and man: basic research and applied perspectives
Memories relating to a painful, negative event are adaptive and can be stored for a lifetime to support preemptive avoidance, escape, or attack behavior. However, under unfavorable circumstances such memories can become overwhelmingly powerful. They may trigger excessively negative psychological states and uncontrollable avoidance of locations, objects, or social interactions. It is therefore obvious that any process to counteract such effects will be of value. In this context, we stress from a basic-research perspective that painful, negative events are "Janus-faced" in the sense that there are actually two aspects about them that are worth remembering: What made them happen and what made them cease. We review published findings from fruit flies, rats, and man showing that both aspects, respectively related to the onset and the offset of the negative event, induce distinct and oppositely valenced memories: Stimuli experienced before an electric shock acquire negative valence as they signal upcoming punishment, whereas stimuli experienced after an electric shock acquire positive valence because of their association with the relieving cessation of pain. We discuss how memories for such punishment-and relief-learning are organized, how this organization fits into the threat-imminence model of defensive behavior, and what perspectives these considerations offer for applied psychology in the context of trauma, panic, and nonsuicidal self-injury
Locking of the Rotation of Disk-Accreting Magnetized Stars
We investigate the rotational equilibrium state of a disk accreting
magnetized stars using axisymmetric magnetohydrodynamic (MHD) simulations. In
this ``locked'' state, the spin-up torque balances the spin-down torque so that
the net average torque on the star is zero. We investigated two types of
initial conditions, one with a relatively weak stellar magnetic field and a
high coronal density, and the other with a stronger stellar field and a lower
coronal density. We observed that for both initial conditions the rotation of
the star is locked to the rotation of the disk. In the second case, the radial
field lines carry significant angular momentum out of the star. However, this
did not appreciably change the condition for locking of the rotation of the
star. We find that in the equilibrium state the corotation radius is
related to the magnetospheric radius as for
case (1) and for case (2). We estimated periods of
rotation in the equilibrium state for classical T Tauri stars, dwarf novae and
X-ray millisecond pulsars.Comment: 10 pages, 9 figures. Accepted by ApJ, will appear in vol. 634, 2005
December
Can Protostellar Jets Drive Supersonic Turbulence in Molecular Clouds?
Jets and outflows from young stellar objects are proposed candidates to drive
supersonic turbulence in molecular clouds. Here, we present the results from
multi-dimensional jet simulations where we investigate in detail the energy and
momentum deposition from jets into their surrounding environment and quantify
the character of the excited turbulence with velocity probability density
functions. Our study include jet--clump interaction, transient jets, and
magnetised jets. We find that collimated supersonic jets do not excite
supersonic motions far from the vicinity of the jet. Supersonic fluctuations
are damped quickly and do not spread into the parent cloud. Instead subsonic,
non-compressional modes occupy most of the excited volume. This is a generic
feature which can not be fully circumvented by overdense jets or magnetic
fields. Nevertheless, jets are able to leave strong imprints in their cloud
structure and can disrupt dense clumps. Our results question the ability of
collimated jets to sustain supersonic turbulence in molecular clouds.Comment: 33 pages, 18 figures, accepted by ApJ, version with high resolution
figures at:
http://www.ita.uni-heidelberg.de/~banerjee/publications/jet_paper.pd
Aetiology of pneumonia following isolated closed head injury
AbstractPatients undergoing mechanical ventilation (MV) after an isolated closed head injury (ICHI) have often been found to develop hospital-acquired pneumonia (HAP) well before subjects who require MV for different reasons. In a prospective study of patients receiving MV after an ICHI, 38 subjects (out of 65 with clinically suspected HAP) had a bacteriological diagnosis established on the basis of correspondence between cultures made from bronchoalveolar lavage and protected specimen brush (with quantitative thresholds of 104 and 103 cfu mlâ1, respectively). Patients were separated according to the time of onset of HAP, with 20 subjects who developed HAP within 4 days of the start of MV (early onset pneumonia, EOP) and 18 subjects who developed HAP after the fourth day (late onset pneumonia, LOP). In those who had LOP, an expected spectrum of organisms was found, with Gram-negatives (especially Pseudomonas sp.) accounting for the majority of isolates. However, in EOP cases, Gram-positive bacteria (especially Staphylococcus sp. and Streptococcus pneumoniae) were found to largely predominate (P = 0·0000026). This confirms the high incidence of staphylococcal pneumonia in neurosurgery patients, and also provides evidence that the vast majority of such staphylococcal pneumonia are EOP. Unlike most previous reports, the microbiological findings from the present study suggest that a cut-off point of 4 days successfully distinguishes between EOP and LOP. Since these two clinical entities differ significantly in terms of pathogenesis and aetiology, preventive measures and therapeutical protocols have to be tailored accordingly
Global asymptotic solutions for relativistic MHD jets and winds
We consider relativistic, stationary, axisymmetric, polytropic, unconfined,
perfect MHD winds, assuming their five lagrangian first integrals to be known.
The asymptotic structure consists of field-regions bordered by boundary layers
along the polar axis and at null surfaces, such as the equatorial plane, which
have the structure of charged column or sheet pinches supported by plasma or
magnetic poloidal pressure. In each field-region cell, the proper current
(defined here as the ratio of the asymptotic poloidal current to the asymptotic
Lorentz factor) remains constant. Our solution is given in the form of matched
asymptotic solutions separately valid outside and inside the boundary layers.
An Hamilton-Jacobi equation, or equivalently a Grad-Shafranov equation, gives
the asymptotic structure in the field-regions of winds which carry Poynting
flux to infinity. An important consistency relation is found to exist between
axial pressure, axial current and asymptotic Lorentz factor. We similarly
derive WKB-type analytic solutions for winds which are kinetic-energy dominated
at infinity and whose magnetic surfaces focus to paraboloids. The density on
the axis in the polar boundary column is shown to slowly fall off as a negative
power of the logarithm of the distance to the wind source. The geometry of
magnetic surfaces in all parts of the asymptotic domain, including boundary
layers, is explicitly deduced in terms of the first-integrals.Comment: 39 pages, 7 figures, accepted for publication in Ap
The structure of black hole magnetospheres. I. Schwarzschild black holes
We introduce a multipolar scheme for describing the structure of stationary,
axisymmetric, force-free black-hole magnetospheres in the ``3+1'' formalism. We
focus here on Schwarzschild spacetime, giving a complete classification of the
separable solutions of the stream equation. We show a transparent term-by-term
analogy of our solutions with the familiar multipoles of flat-space
electrodynamics. We discuss electrodynamic processes around disk-fed black
holes in which our solutions find natural applications: (a) ``interior''
solutions in studies of the Blandford-Znajek process of extracting the hole's
rotational energy, and of the formation of relativistic jets in active galactic
nuclei and ``microquasars'', and, (b) ``exterior'' solutions in studies of
accretion disk dynamos, disk-driven winds and jets. On the strength of existing
numerical studies, we argue that the poloidal field structures found here are
also expected to hold with good accuracy for rotating black holes, except for
maximum possible rotation rates. We show that the closed-loop exterior
solutions found here are not in contradiction with the Macdonald-Thorne
theorem, since these solutions, which diverge logarithmically on the hole's
horizon , apply only to those regions which exclude .Comment: 6 figures. Accepted for publication by MNRA
On the geometrical origin of periodicity in blazar-type sources
Periodicities in blazar light curves may be related to helical trajectories
in extragalactic radio jets by differential Doppler boosting effects. We
consider ballistic and non-ballistic (i.e., radial) trajectories and discuss
three possible periodic driving mechanisms for the origin of helical jet paths,
namely, orbital motion in a binary black hole system (BBHS), jet precession,
and intrinsic jet rotation. It is shown that precessional-driven ballistic
motion is unlikely to result in observable periods of less than several tens of
years. We demonstrate that for non-ballistic helical motion the observed period
is generally strongly shortened relative to the real physical driving period
because of light-travel time effects. Internal jet rotation may thus account
for observed periods days. Periodicity due to
orbital-driven (non-ballistic) helical motion, on the other hand, is usually
constrained to periods of days, while Newtonian-driven
precession is unlikely to be responsible for periodicity on a timescale days but may well be associated with periods of yr.Comment: 10 pages, ApJ Letters in pres
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