1,104 research outputs found
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
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
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
Environment and Energy Injection Effects in GRB Afterglows
In a recent paper (Dai & Lu 1999), we have proposed a simple model in which
the steepening in the light curve of the R-band afterglow of the gamma-ray
burst (GRB) 990123 is caused by the adiabatic shock which has evolved from an
ultrarelativistic phase to a nonrelativistic phase in a dense medium. We find
that such a model is quite consistent with observations if the medium density
is about . Here we discuss this model in more
details. In particular, we investigate the effects of synchrotron self
absorption and energy injection. A shock in a dense medium becomes
nonrelativistic rapidly after a short relativistic phase. The afterglow from
the shock at the nonrelativistic stage decays more rapidly than at the
relativistic stage. Since some models for GRB energy sources predict that a
strongly magnetic millisecond pulsar may be born during the formation of GRB,
we discuss the effect of such a pulsar on the evolution of the nonrelativistic
shock through magnetic dipole radiation. We find that after the energy which
the shock obtains from the pulsar is much more than the initial energy of the
shock, the afterglow decay will flatten significantly. When the pulsar energy
input effect disappears, the decay will steepen again. These features are in
excellent agreement with the afterglows of GRB 980519, GRB 990510 and GRB
980326. Furthermore, our model fits very well all the observational data of GRB
980519 including the last two detections.Comment: 21 pages, LaTeX, accepted for publication in ApJ, one paragraph adde
Coulomb crystals in the magnetic field
The body-centered cubic Coulomb crystal of ions in the presence of a uniform
magnetic field is studied using the rigid electron background approximation.
The phonon mode spectra are calculated for a wide range of magnetic field
strengths and for several orientations of the field in the crystal. The phonon
spectra are used to calculate the phonon contribution to the crystal energy,
entropy, specific heat, Debye-Waller factor of ions, and the rms ion
displacements from the lattice nodes for a broad range of densities,
temperatures, chemical compositions, and magnetic fields. Strong magnetic field
dramatically alters the properties of quantum crystals. The phonon specific
heat increases by many orders of magnitude. The ion displacements from their
equilibrium positions become strongly anisotropic. The results can be relevant
for dusty plasmas, ion plasmas in Penning traps, and especially for the crust
of magnetars (neutron stars with superstrong magnetic fields G). The effect of the magnetic field on ion displacements in a
strongly magnetized neutron star crust can suppress the nuclear reaction rates
and make them extremely sensitive to the magnetic field direction.Comment: 25 pages, 8 figures; accepted to Phys. Rev.
The Compact Central Object in the Supernova Remnant G266.2-1.2
We observed the compact central object CXOU J085201.4--461753 in the
supernova remnant G266.2--1.2 (RX J0852.0--4622) with the Chandra ACIS detector
in timing mode. The spectrum of this object can be described by a blackbody
model with the temperature kT=404 eV and radius of the emitting region R=0.28
km, at a distance of 1 kpc. Power-law and thermal plasma models do not fit the
source spectrum. The spectrum shows a marginally significant feature at 1.68
keV. Search for periodicity yields two candidate periods, about 301 ms and 33
ms, both significant at a 2.1 sigma level; the corresponding pulsed fractions
are 13% and 9%, respectively. We find no evidence for long-term variability of
the source flux, nor do we find extended emission around the central object. We
suggest that CXOU J085201.4--461753 is similar to CXOU J232327.9+584842, the
central source of the supernova remnant Cas A. It could be either a neutron
star with a low or regular magnetic field, slowly accreting from a fossil disk,
or, more likely, an isolated neutron star with a superstrong magnetic field. In
either case, a conservative upper limit on surface temperature of a 10 km
radius neutron star is about 90 eV, which suggests accelerated cooling for a
reasonable age of a few thousand years.Comment: Accepted to ApJ, 13 pages, 1 figur
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
Reconnection in a striped pulsar wind
It is generally thought that most of the spin-down power of a pulsar is
carried away in an MHD wind dominated by Poynting flux. In the case of an
oblique rotator, a significant part of this energy can be considered to be in a
low-frequency wave, consisting of stripes of toroidal magnetic field of
alternating polarity, propagating in a region around the equatorial plane.
Magnetic reconnection in such a structure has been proposed as a mechanism for
transforming the Poynting flux into particle energy in the pulsar wind. We have
re-examined this process and conclude that the wind accelerates significantly
in the course of reconnection. This dilates the timescale over which the
reconnection process operates, so that the wind requires a much larger distance
than was previously thought in order to convert the Poynting flux to particle
flux. In the case of the Crab, the wind is still Poynting-dominated at the
radius at which a standing shock is inferred from observation. An estimate of
the radius of the termination shock for other pulsars implies that all except
the milli-second pulsars have Poynting-flux dominated winds all the way out to
the shock front.Comment: 23 pages, 2 figures. Accepted for publication in Ap
Dissipation in Poynting-flux Dominated Flows: the Sigma-Problem of the Crab Pulsar Wind
Flows in which energy is transported predominantly as Poynting flux are
thought to occur in pulsars, gamma-ray bursts and relativistic jets from
compact objects. The fluctuating component of the magnetic field in such a flow
can in principle be dissipated by magnetic reconnection, and used to accelerate
the flow. We investigate how rapidly this transition can take place, by
implementing into a global MHD model, that uses a thermodynamic description of
the plasma, explicit, physically motivated prescriptions for the dissipation
rate: a lower limit on this rate is given by limiting the maximum drift speed
of the current carriers to that of light, an upper limit follows from demanding
that the dissipation zone expand only subsonically in the comoving frame and a
further prescription is obtained by assuming that the expansion speed is
limited by the growth rate of the relativistic tearing mode. In each case,
solutions are presented which give the Lorentz factor of a spherical wind
containing a transverse, oscillating magnetic field component as a function of
radius. In the case of the Crab pulsar, we find that the Poynting flux can be
dissipated before the wind reaches the inner edge of the Nebula if the pulsar
emits electron positron pairs at a rate >1.E40 per second, thus providing a
possible solution to the sigma-problem.Comment: Accepted for publication in Ap
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