525 research outputs found
Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks
We study the long term evolution of magnetic fields generated by an initially
unmagnetized collisionless relativistic shock. Our 2D particle-in-cell
numerical simulations show that downstream of such a Weibel-mediated shock,
particle distributions are approximately isotropic, relativistic Maxwellians,
and the magnetic turbulence is highly intermittent spatially, nonpropagating,
and decaying. Using linear kinetic theory, we find a simple analytic form for
these damping rates. Our theory predicts that overall magnetic energy decays
like with , which compares favorably with
simulations, but predicts overly rapid damping of short wavelength modes.
Magnetic trapping of particles within the magnetic structures may be the origin
of this discrepancy. We conclude that initially unmagnetized relativistic
shocks in electron-positron plasmas are unable to form persistent downstream
magnetic fields. These results put interesting constraints on synchrotron
models for the prompt and afterglow emission from GRBs.Comment: 4 pages, 3 figures, contributed talk at the workshop: High Energy
Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007;
Downsampled version for arXiv. Full resolution version available at
http://astro.berkeley.edu/~pchang/proceedings.pd
Experimentally Observed Instability of a Laminar Ekman Flow in a Rotating Basin
In studying the axi-symmetric flow induced by source-sink distributions in a rotating cylindrical
basin in the absence of radial barriers, a highly organized pattern of instability has been
observed in the laminar Ekman layer along the bottom of the basin. The instability manifests
itself in the form of almost perfectly concentric cylindrical sheets or curtains of water which
rise as sharply defined vertical jets from the Ekman layer and penetrate the entire depth of
fluid. A less sharply defined downward motion between the curtains completes the circulation
celis thus developed. At some maximum critical radius, the curtains usually disappear, and the
flow at larger radii is a stable, laminar Ekman flow. Quantitative observations of ring spacing
and critical radius are reported for experiments in which angular velocity, flow rate, viscosity
and total depth of water were varied over experimentally available ranges
Magnetar Driven Bubbles and the Origin of Collimated Outflows from GRBs
We model the interaction between the wind from a newly formed rapidly
rotating magnetar and the surrounding progenitor. In the first few seconds
after core collapse the magnetar inflates a bubble of plasma and magnetic
fields behind the supernova shock, which expands asymmetrically because of the
pinching effect of the toroidal magnetic field, as in PWNe, even if the host
star is spherically symmetric. The degree of asymmetry depends on the ratio of
the magnetic energy to the total energy in the bubble. We assume that the wind
by newly formed magnetars inflating these bubbles is more magnetized than for
PWNe. We show that for a magnetic to total power supplied by the central
magnetar the bubble expands relatively spherically while for values
greater than 0.3, most of the pressure in the bubble is exerted close to the
rotation axis, driving a collimated outflow out through the host star. This can
account for the collimation inferred from observations of long-duration
gamma-ray bursts (GRBs). Given that the wind magnetization increases in time,
we thus suggest that the magnetar-driven bubble initially expands relatively
spherically (enhancing the energy of the associated supernova) while at late
times it becomes progressivelymore collimated (producing the GRB). Similar
processes may operate in more modestly rotating neutron stars to produce
asymmetric supernovae and lower energy transients such as X-ray flashes.Comment: Proceeding of the conference "Astrophysics of Compact Objects", 1-7
July, Huangshan, Chin
Time Variability in the X-ray Nebula Powered by Pulsar B1509-58
We use new and archival Chandra and ROSAT data to study the time variability
of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509-58
on timescales of one week to twelve years. There is variability in the size,
number, and brightness of compact knots appearing within 20" of the pulsar,
with at least one knot showing a possible outflow velocity of ~0.6c (assuming a
distance to the source of 5.2 kpc). The transient nature of these knots may
indicate that they are produced by turbulence in the flows surrounding the
pulsar. A previously identified prominent jet extending 12 pc to the southeast
of the pulsar increased in brightness by 30% over 9 years; apparent outflow of
material along this jet is observed with a velocity of ~0.5c. However, outflow
alone cannot account for the changes in the jet on such short timescales.
Magnetohydrodynamic sausage or kink instabilities are feasible explanations for
the jet variability with timescale of ~1.3-2 years. An arc structure, located
30"-45" north of the pulsar, shows transverse structural variations and appears
to have moved inward with a velocity of ~0.03c over three years. The overall
structure and brightness of the diffuse PWN exterior to this arc and excluding
the jet has remained the same over the twelve year span. The photon indices of
the diffuse PWN and possibly the jet steepen with increasing radius, likely
indicating synchrotron cooling at X-ray energies.Comment: accepted to ApJ, 14 pages, 8 figure
Pair-production multiplicities in rotation-powered pulsars
We discuss the creation of electron-positron cascades in the context of
pulsar polar cap acceleration models and derive several useful analytic and
semi-analytic results for the spatial extent and energy response of the
cascade. Instead of Monte Carlo simulations, we use an integro- differential
equation which describes the development of the cascade energy spectrum in one
space dimension quite well, when it is compared to existing Monte Carlo models.
We reduce this full equation to a single integral equation, from which we can
derive useful results, such as the energy loss between successive generations
of photons and the spectral index of the response. We find that a simple
analytic formula represents the pair cascade multiplicity quite well, provided
that the magnetic field is below 10^12 Gauss, and that an only slightly more
complex formula matches the numerically-calculated cascade at all other field
strengths. Using these results, we find that cascades triggered by gamma rays
emitted through inverse Compton scattering of thermal photons from the neutron
star's surface, both resonant and non-resonant, are important for the dynamics
of the polar cap region in many pulsars.Comment: to appear in ApJ; 19 pages, 18 figure
Thermal emission from low-field neutron stars
We present a new grid of LTE model atmospheres for weakly magnetic
(B<=10e10G) neutron stars, using opacity and equation of state data from the
OPAL project and employing a fully frequency- and angle-dependent radiation
transfer. We discuss the differences from earlier models, including a
comparison with a detailed NLTE calculation. We suggest heating of the outer
layers of the neutron star atmosphere as an explanation for the featureless
X-ray spectra of RX J1856.5-3754 and RX J0720.4-3125 recently observed with
Chandra and XMM.Comment: 8 pages A&A(5)-Latex, 6 Figures, A&A in press. The model spectra
presented here are available as XSPEC tables at
http://www.astro.soton.ac.uk/~btg/outgoing/nsspec
Polarization Sweeps in Rotation Powered Pulsars
We re-examine the characteristic polarization angle sweep of rotation-powered
pulsars and calculate the expected deviations from this sweep caused by
aberrational effects and by polar-cap current flow. We find that in addition to
the previously known phase shift of the entire sweep by , aberration shifts the polarization angle itself by . Similarly, current flow above the polar cap
shifts the polarization sweep by , potentially providing a method of directly measuring the magnitude of
the current. The competition between these two effects produces a potentially
observable signature in the polarization angle sweep. Although these effects
may appear similar to orthogonal mode shifts, they are an independent
phenomenon with distinct observational characteristics.Comment: 23 pages, 8 figures; accepted by Ap
Magnetic Confinement, MHD Waves, and Smooth Line Profiles in AGN
In this paper, we show that if the broad line region clouds are in
approximate energy equipartition between the magnetic field and gravity, as
hypothesized by Rees, there will be a significant effect on the shape and
smoothness of broad emission line profiles in active galactic nuclei. Line
widths of contributing clouds or flow elements are much wider than their
thermal widths, due to the presence of non-dissipative MHD waves, and their
collective contribution produce emission line profiles broader and smoother
than would be expected if a magnetic field were not present. As an
illustration, a simple model of isotropically emitting clouds, normally
distributed in velocity, is used to show that smoothness can be achieved for
less than 80,000 clouds and may even be as low as a few hundred. We conclude
that magnetic confinement has far reaching consequences for observing and
modeling active galactic nuclei.Comment: to appear in MNRA
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