501 research outputs found
Collimation of Highly Variable Magnetohydrodynamic Disturbances around a Rotating Black Hole
We have studied non-stationary and non-axisymmetric perturbations of a
magnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assuming that
the magnetic field dominates the plasma accretion, we find that the accretion
suffers a large radial acceleration resulting from the Lorentz force, and
becomes highly variable compared with the electromagnetic field there. In fact,
we further find an interesting perturbed structure of the plasma velocity with
a large peak in some narrow region located slightly inside of the
fast-magnetosonic surface. This is due to the concentrated propagation of the
fluid disturbances in the form of fast-magnetosonic waves along the separatrix
surface. If the fast-magnetosonic speed is smaller in the polar regions than in
the equatorial regions, the critical surface has a prolate shape for radial
poloidal field lines. In this case, only the waves that propagate towards the
equator can escape from the super-fast-magnetosonic region and collimate
polewards as they propagate outwards in the sub-fast-magnetosonic regions. We
further discuss the capabilities of such collimated waves in accelerating
particles due to cyclotron resonance in an electron-positron plasma.Comment: 15 pages, 6 postscript figures, LaTe
The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres
Recent simulations of force-free, degenerate (ffde) black hole magnetospheres
indicate that the fast mode radiated from (or near) the event horizon can
modify the global potential difference in the poloidal direction orthogonal to
the magnetic field, V, in a black hole magnetosphere. There is a fundamental
contradiction in a wave that alters V coming from near the horizon. The
background fields in ffde satisfy the ``ingoing wave condition'' near the
horizon (that arises from the requirement that all matter is ingoing at the
event horizon), yet outgoing waves are radiated from this region in the
simulation. Studying the properties of the waves in the simulations are useful
tools to this end. It is shown that regularity of the stress-energy tensor in a
freely falling frame requires that the outgoing (as viewed globally) waves near
the event horizon are redshifted away and are ineffectual at changing V. It is
also concluded that waves in massless MHD (ffde) are extremely inaccurate
depictions of waves in a tenuous MHD plasma, near the event horizon, as a
consequence black hole gravity. Any analysis based on ffde near the event
horizon is seriously flawed.Comment: 9 pages to appear in ApJ Letter
Electrodynamics of Outer-Gap Accelerator: Formation of Soft Power-law Spectrum Between 100 MeV and 3 GeV
We investigate a stationary pair production cascade in the outer
magnetosphere of a spinning neutron star. The charge depletion due to global
flows of charged particles, causes a large electric field along the magnetic
field lines. Migratory electrons and/or positrons are accelerated by this field
to radiate gamma-rays via curvature and inverse-Compton processes. Some of such
gamma-rays collide with the X-rays to materialize as pairs in the gap. The
replenished charges partially screen the electric field, which is
self-consistently solved together with the energy distribution of particles and
gamma-rays at each point along the field lines. By solving the set of Maxwell
and Boltzmann equations, we demonstrate that an external injection of charged
particles at nearly Goldreich-Julian rate does not quench the gap but shifts
its position and that the particle energy distribution cannot be described by a
power-law. The injected particles are accelerated in the gap and escape from it
with large Lorentz factors. We show that such escaping particles migrating
outside of the gap contribute significantly to the gamma-ray luminosity for
young pulsars and that the soft gamma-ray spectrum between 100 MeV and 3 GeV
observed for the Vela pulsar can be explained by this component. We also
discuss that the luminosity of the gamma-rays emitted by the escaping particles
is naturally proportional to the square root of the spin-down luminosity.Comment: Accepted to Astroph. J, 25 pages, 13 figure
Pair Plasma Dominance in the Parsec-Scale Relativistic Jet of 3C345
We investigate whether a pc-scale jet of 3C345 is dominated by a normal
plasma or an electron-positron plasma. We present a general condition that a
jet component becomes optically thick for synchrotron self-absorption, by
extending the method originally developed by Reynolds et al. The general
condition gives a lower limit of the electron number density, with the aid of
the surface brightness condition, which enables us to compute the magnetic
field density. Comparing the lower limit with another independent constraint
for the electron density that is deduced from the kinetic luminosity, we can
distinguish the matter content. We apply the procedure to the five components
of 3C345 (C2, C3, C4, C5, and C7) of which angular diameters and radio fluxes
at the peak frequencies were obtainable from literature. Evaluating the
representative values of Doppler beaming factors by their equipartition values,
we find that all the five components are likely dominated by an
electron-positron plasma. The conclusion does not depend on the lower cutoff
energy of the power-law distribution of radiating particles.Comment: 17 page
Calorimetry of Active Galactic Nucleus jets: testing plasma composition in Cygnus A
We examine plasma composition of jets in active galactic nuclei through the
comparison of the total pressure () with partial pressures of electrons and
protons in a cocoon. The total pressure is estimated from the analysis of an
expanding cocoon dynamics. We determine the average kinetic energy per particle
for several representative cases of particle energy distribution such as one-
and two-temperature thermal plasmas and non-thermal electrons by evaluating the
dissipation of total kinetic energy of the jet into the internal energy of
cocoon plasma. The number density of the total electrons/positrons ()
in the cocoon is constrained by using the particle supply from hot spots and
the absence of thermal bremsstrahlung emission from radio lobes. By inserting
, and the particle energy of each population into the equation of
state, the number density () and pressure () of protons in the
cocoon can be constrained. Applying this method to Cygnus A, we find that (i)
electron/positron () pairs always dominate in terms of number density,
but that (ii) either an "-supported cocoon (i.e., )"
or "proton-supported one (i.e, )" is possible.Comment: 12 pages, 5 figures, accepted for publication in Ap
On Pair Content and Variability of Sub-Parsec Jets in Quasars
X-ray observations of blazars associated with the OVV (Optically Violently
Variable) quasars put strong constraints on the electron - positron pair
content of radio-loud quasar jets. From those observations, we infer that jets
in quasars contain many more electron - positron pairs than protons, but
dynamically are still dominated by protons. In particular, we show that pure
electron - positron jet models can be excluded, as they overpredict soft X-ray
radiation; likewise, pure proton - electron jets can be excluded, as they
predict too weak nonthermal X-ray radiation. An intermediate case is viable. We
demonstrate that jets which are initially proton-electron ("proto-jets") can be
pair-loaded via interaction with 100 - 300 keV photons produced in hot
accretion disc coronae, likely to exist in active galactic nuclei in general.
If the coronal radiation is powered by magnetic flares, the pair loading is
expected to be non-uniform and non-axisymmetric. Together with radiation drag,
this leads to velocity and density perturbations in a jet and formation of
shocks, where the pairs are accelerated. Such a scenario can explain rapid
(time scale of about a day) variability observed in OVV quasars.Comment: Accepted for publication in the Astrophysical Journa
African American English speaking 2nd graders, verbal-s, and educational achievement: Event related potential and math study findings
A number of influential linguistic analyses hold that African American English (AAE) has no verbal-s, the-s that, for example, turns drink into drinks in more mainstream English varieties.On such accounts, sentences like Mary drinks coffee are ungrammatical in AAE. Previous behavioral studies suggest that in addition to being ungrammatical, AAE speaking children find these sentences cognitively demanding, and that their presence in mathematical reasoning tests can depress scores. Until now, however, no online sentence processing study nor investigation of neurophysiological markers has been done to support these findings. Aimed at addressing this gap in the literature, the auditory ERP experiment described herein revealed two different processes associated with AAE speaking 2nd graders listening to this type of sentence: a morphosyntactic structure building problem, reflected in a bilateral early anterior-central negativity; and an increase in working memory load, indicated by a bilateral late long-lasting anterior-central negativity. Study participants also took an orally administered test of math word problems. Consistent with previous findings, results showed they answered fewer questions correctly when those questions contained verbal-s than when they did not
A New Solution to the Plasma Starved Event Horizon Magnetosphere: Application to the Forked Jet in M87
© 2018 ESO. Reproduced with permission from Astronomy & Astrophysics. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.Very Long Baseline Interferometry observations at 86 GHz reveal an almost hollow jet in M87 with a forked morphology. The detailed analysis presented here indicates that the spectral luminosity of the central spine of the jet in M87 is a few percent of that of the surrounding hollow jet 200-400 μ as from the central black hole. Furthermore, recent jet models indicate that a hollow "tubular" jet can explain a wide range of plausible broadband spectra originating from jetted plasma located within ~30 μ as of the central black hole, including the 230 GHz correlated flux detected by the Event Horizon Telescope. Most importantly, these hollow jets from the inner accretion flow have an intrinsic power capable of energizing the global jet out to kiloparsec scales. Thus motivated, this paper considers new models of the event horizon magnetosphere (EHM) in low luminosity accretion systems. Contrary to some models, the spine is not an invisible powerful jet. It is an intrinsically weak jet. In the new EHM solution, the accreted poloidal magnetic flux is weak and the background photon field is weak. It is shown how this accretion scenario naturally results in the dissipation of the accreted poloidal magnetic flux in the EHM not the accumulation of poloidal flux required for a powerful jet. The new solution indicates less large scale poloidal magnetic flux (and jet power) in the EHM than in the surrounding accretion flow and cannot support significant EHM driven jets.Peer reviewe
X-ray Phase-Resolved Spectroscopy of PSRs B0531+21, B1509-58, and B0540-69 with RXTE
The Rossi X-ray Timing Explorer ({\sl RXTE}) has made hundreds of
observations on three famous young pulsars (PSRs) B0531+21 (Crab), B1509-58,
and B0540-69. Using the archive {\sl RXTE} data, we have studied the
phase-resolved spectral properties of these pulsars in details. The variation
of the X-ray spectrum with phase of PSR B0531+21 is confirmed here much more
precisely and more details are revealed than the previous studies: the spectrum
softens from the beginning of the first pulse, turns to harden right at the
pulse peak and becomes the hardest at the bottom of the bridge, softens
gradually until the second peak, and then softens rapidly. Different from the
previous studies, we found that the spectrum of PSR B1509-58 is significantly
harder in the center of the pulse, which is also in contrast to that of PSR
B0531+21. The variation of the X-ray spectrum of PSR B0540-69 seems similar to
that of PSR B1509-58, but with a lower significance. Using the about 10 years
of data span, we also studied the real time evolution of the spectra of these
pulsars, and no significant evolution has been detected. We have discussed
about the constraints of these results on theoretical models of pulsar X-ray
emission.Comment: 42 pages, 24 figure
Processing load imposed by line breaks in English temporal Wh-questions
Prosody plays an important role in online sentence processing both explicitly and implicitly. It has been shown that prosodically packaging together parts of a sentence that are interpreted together facilitates processing of the sentence. This applies not only to explicit prosody but also implicit prosody. The present work hypothesizes that a line break in a written text induces an implicit prosodic break, which, in turn, should result in a processing bias for interpreting English wh-questions. Two experiments-one self-paced reading study and one questionnaire study-are reported. Both supported the "line break" hypothesis mentioned above. The results of the self-paced reading experiment showed that unambiguous wh-questions were read faster when the location of line breaks (or frame breaks) matched the scope of a wh-phrase (main or embedded clause) than when they did not. The questionnaire tested sentences with an ambiguous wh-phrase, one that could attach either to the main or the embedded clause. These sentences were interpreted as attaching to the main clause more often than to the embedded clause when a line break appeared after the main verb, but not when it appeared after the embedded verb
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