1,156 research outputs found
Reconnection in pulsar winds
The spin-down power of a pulsar is thought to be carried away in an MHD wind
in which, at least close to the star, the energy transport is dominated by
Poynting flux. The pulsar drives a low-frequency wave in this wind, consisting
of stripes of toroidal magnetic field of alternating polarity, propagating in a
region around the equatorial plane. The current implied by this configuration
falls off more slowly with radius than the number of charged particles
available to carry it, so that the MHD picture must, at some point, fail.
Recently, magnetic reconnection in such a structure has been shown to
accelerate the wind significantly. This reduces the magnetic field in the
comoving frame and, consequently, the required current, enabling the solution
to extend to much larger radius. This scenario is discussed and, for the Crab
Nebula, the range of validity of the MHD solution is compared with the radius
at which the flow appears to terminate. For sufficiently high particle
densities, it is shown that a low frequency entropy wave can propagate out to
the termination point. In this case, the "termination shock" itself must be
responsible for dissipating the wave.Comment: LaTeX 13 pages, 3 figures, typos remove
Thermal emission from bare quark matter surfaces of hot strange stars
We consider the thermal emission of photons and electron-positron pairs from
the bare quark surface of a hot strange star. The radiation of high-energy (>
20 MeV) equilibrium photons prevails at the surface temperature T_S > 5 x
10^{10} K, while below this temperature, 8 x 10^8 < T_S < 5 x 10^{10} K,
emission of electron-positron pairs created by the Coulomb barrier at the quark
surface dominates. The thermal luminosity of a hot strange star in both photons
and pairs is estimated.Comment: 10 pages, 2 figures, ApJLetters, in pres
Magnetic Vortex Resonance in Patterned Ferromagnetic Dots
We report a high-resolution experimental detection of the resonant behavior
of magnetic vortices confined in small disk-shaped ferromagnetic dots. The
samples are magnetically soft Fe-Ni disks of diameter 1.1 and 2.2 um, and
thickness 20 and 40 nm patterned via electron beam lithography onto microwave
co-planar waveguides. The vortex excitation spectra were probed by a vector
network analyzer operating in reflection mode, which records the derivative of
the real and the imaginary impedance as a function of frequency. The spectra
show well-defined resonance peaks in magnetic fields smaller than the
characteristic vortex annihilation field. Resonances at 162 and 272 MHz were
detected for 2.2 and 1.1 um disks with thickness 40 nm, respectively. A
resonance peak at 83 MHz was detected for 20-nm thick, 2-um diameter disks. The
resonance frequencies exhibit weak field dependence, and scale as a function of
the dot geometrical aspect ratio. The measured frequencies are well described
by micromagnetic and analytical calculations that rely only on known properties
of the dots (such as the dot diameter, thickness, saturation magnetization, and
exchange stiffness constant) without any adjustable parameters. We find that
the observed resonance originates from the translational motion of the magnetic
vortex core.Comment: submitted to PRB, 17 pages, 5 Fig
High-Energy Emission From Millisecond Pulsars
The X-ray and gamma-ray spectrum of rotation-powered millisecond pulsars is
investigated in a model for acceleration and pair cascades on open field lines
above the polar caps. Although these pulsars have low surface magnetic fields,
their short periods allow them to have large magnetospheric potential drops,
but the majority do not produce sufficient pairs to completely screen the
accelerating electric field. The accelerating particles maintain high Lorentz
factors and undergo cyclotron resonant absorption of radio emission, that
produces and maintains a large pitch angle, resulting in a strong synchrotron
component. The resulting spectra consist of several distinct components:
curvature radiation from primary electrons dominating from 1 - 100 GeV,
synchrotron radiation from primary and secondary electrons dominating up to
about 100 MeV, and much weaker inverse-Compton radiation from primary electrons
at 0.1 - 1 TeV. We find that the relative size of these components depends on
pulsar period, period derivative, and neutron star mass and radius with the
level of the synchrotron component also depending sensitively on the radio
emission properties. This model is successful in describing the observed X-ray
and gamma-ray spectrum of PSR J0218+4232 as synchrotron radiation, peaking
around 100 MeV and extending up to a turnover around several GeV. The predicted
curvature radiation components from a number of millisecond pulsars, as well as
the collective emission from the millisecond pulsars in globular clusters,
should be detectable with AGILE and GLAST. We also discuss a hidden population
of X-ray-quiet and radio-quiet millisecond pulsars which have evolved below the
pair death line, some of which may be detectable by telescopes sensitive above
1 GeV.Comment: 34 pages, 6 figures, accepted for publication in Astrophysical
Journa
Evolution and stability of a magnetic vortex in small cylindrical ferromagnetic particle under applied field
The energy of a displaced magnetic vortex in a cylindrical particle made of
isotropic ferromagnetic material (magnetic dot) is calculated taking into
account the magnetic dipolar and the exchange interactions. Under the
simplifying assumption of small dot thickness the closed-form expressions for
the dot energy is written in a non-perturbative way as a function of the
coordinate of the vortex center. Then, the process of losing the stability of
the vortex under the influence of the externally applied magnetic field is
considered. The field destabilizing the vortex as well as the field when the
vortex energy is equal to the energy of a uniformly magnetized state are
calculated and presented as a function of dot geometry. The results (containing
no adjustable parameters) are compared to the recent experiment and are in good
agreement.Comment: 4 pages, 2 figures, RevTe
Afterglow Light Curve Modulated by a Highly Magnetized Millisecond Pulsar
We investigate consequences of a continuously energy-injecting central engine
of gamma-ray burst (GRB) afterglow emission, assuming that a highly magnetized
pulsar is left beaming in the core of a GRB progenitor. Beaming and continuous
energy-injection are natural consequences of the pulsar origin of GRB
afterglows. Whereas previous studies have considered continuous
energy-injection from a new-born pulsar to interpret the deviation of afterglow
light curves of GRBs from those with the simple power law behavior, a beaming
effect, which is one of the most important aspects of pulsar emissions, is
ignored in earlier investigations. We explicitly include the beaming effect and
consider a change of the beaming with time due to a dynamical evolution of a
new-born pulsar. We show that the magnitude of the afterglow from this fireball
indeed first decreases with time, subsequently rises, and declines again. One
of the most peculiar optical afterglows light curve of GRB 970508 can be
accounted for by continuous energy injection with beaming due to a highly
magnetized new-born pulsar. We discuss implications on such observational
evidence for a pulsar.Comment: 4 pages, 1 table, submitted to Astronomy and Astrophysics (Letters
Linearly polarized X-ray flares following short gamma-ray bursts
Soft X-ray flares were detected to follow the short-duration gamma-ray burst
GRB 050724. The temporal properties of the flares suggest that they are likely
due to the late time activity of the central engine. We argue that if short
GRBs are generated through compact star mergers, as is supported by the recent
observations, the jet powering the late X-ray flares must be launched via
magnetic processes rather than via neutrino-antineutrino annihilations. As a
result, the X-ray flares following short GRBs are expected to be linearly
polarized. The argument may also apply to the X-ray flares following long GRBs.
Future observations with the upcoming X-ray polarimeters will test this
prediction.Comment: 4 pages (no figure), accepted for publication in ApJL, typos
correcte
Gamma-ray burst early afterglows: reverse shock emission from an arbitrarily magnetized ejecta
Evidence suggests that the gamma-ray burst (GRB) ejecta is likely magnetized,
although the degree of magnetization of the ejecta is unknown. We derive a
rigorous analytical solution for the relativistic 90 degree shocks under the
ideal MHD condition, and use them to study the reverse shock emission
properties of an arbitrarily magnetized ejecta. Contrary to the previous
belief, we find that strong relativistic shocks still exist in the high-sigma
limit. Assuming a constant density of the circumburst medium, we study the
shell-medium interaction in detail and categorize various critical radii for
shell evolution. With typical GRB parameters, a reverse shock exists when sigma
is less than a few tens or a few hundreds. The shell evolution can be still
categorized into the thick and thin shell regimes, but the separation between
the two regime now depends on sigma and the thick shell regime greatly shrinks
at high-sigma. The early optical afterglow lightcurves are calculated for GRBs
with a wide range of the sigma values. We find that the reverse shock emission
level increases steadily with sigma initially, but starts to decline when sigma
becomes larger than unity. In the high-sigma regime the reverse shock peak is
usually broadened due to the separation of the shock crossing radius and the
deceleration radius in the thin shell regime. The early afterglow data and
tight upper limits of known GRBs could be understood within the theoretical
framework developed in this paper, with the inferred sigma value varying in a
wide range.Comment: Updated to match the version to appear in Ap
GaN evaporation and enhanced diffusion of Ar during high-temperature ion implantation
GaN films were implanted with 150 keV Ar+ at temperatures up to 1100 °C to a dose of 3×1015 cm-2. Concentration profiles of Ar were measured by secondary ion mass spectroscopy and depth distributions of ion-induced damage were estimated from Rutherford backscattering/channeling spectra. No redistribution of Ar atoms was detected up to 700 °C. At 1000 °C a deep penetrating diffusion tail and a shift of the Ar peak to the surface were observed. At temperatures higher than 800 °C shift of the damage peak to the surface was also observed. We attributed the shift of the Ar peak and the damage peaks to evaporation of thin layer of GaN during high-temperature implantation and estimated its temperature dependence
Gamma-Ray Burst Afterglows with Energy Injection: Homogeneous Versus Wind External Media
Assuming an adiabatic evolution of a gamma-ray burst (GRB) fireball
interacting with an external medium, we calculate the hydrodynamics of the
fireball with energy injection from a strongly magnetic millisecond pulsar
through magnetic dipole radiation, and obtain the light curve of the optical
afterglow from the fireball by synchrotron radiation. Results are given both
for a homogeneous external medium and for a wind ejected by GRB progenitor. Our
calculations are also available in both ultra-relativistic and non-relativistic
phases. Furthermore, the observed R-band light curve of GRB{000301C} can be
well fitted in our model, which might provide a probe of the properties of GRB
progenitors.Comment: revised version for publication in Chin. Phys. Let
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