115 research outputs found
Relativistic Precessing Jets and Cosmological Gamma Ray Bursts
We discuss the possibility that gamma-ray bursts may result from cosmological
relativistic blob emitting neutron star jets that precess past the line of
sight. Beaming reduces the energy requirements, so that the jet emission can
last longer than the observed burst duration. One precession mode maintains a
short duration time scale, while a second keeps the beam from returning to the
line of sight, consistent with the paucity of repeaters. The long life of these
objects reduces the number required for production as compared to short lived
jets. Blobs can account for the time structure of the bursts. Here we focus
largely on kinematic and time scale considerations of beaming, precession, and
blobs--issues which are reasonably independent of the acceleration and jet
collimation mechanisms. We do suggest that large amplitude electro-magnetic
waves could be a source of blob acceleration.Comment: 15 pages, plain TeX, accepted to ApJ
Intrinsic Variability of the Vela Pulsar: Lognormal Statistics and Theoretical Implications
Individual pulses from pulsars have intensity-phase profiles that differ
widely from pulse to pulse, from the average profile, and from phase to phase
within a pulse. Widely accepted explanations for pulsar radio emission and its
time variability do not exist. Here, by analysing data near the peak of the
Vela pulsar's average profile, we show that Vela's variability corresponds to
lognormal field statistics, consistent with the prediction of stochastic growth
theory (SGT) for a purely linear system close to marginal stability. Vela's
variability is therefore a direct manifestation of an SGT state and the field
statistics constrain the emission mechanism to be linear (either direct or
indirect), ruling out nonlinear mechanisms like wave collapse. Field statistics
are thus a powerful, potentially widely applicable tool for understanding
variability and constraining mechanisms and source characteristics of coherent
astrophysical and space emissions.Comment: 7 pages, 4 figures. In press at ApJ Letters - scheduled for December
10 issu
Asymmetric Supernovae, Pulsars, Magnetars, and Gamma-Ray Bursts
We outline the possible physical processes, associated timescales, and
energetics that could lead to the production of pulsars, jets, asymmetric
supernovae, and weak gamma-ray bursts in routine circumstances and to a
magnetar and perhaps stronger gamma-ray burst in more extreme circumstances in
the collapse of the bare core of a massive star. The production of a
LeBlanc-Wilson MHD jet could provide an asymmetric supernova and result in a
weak gamma-ray burst when the jet accelerates down the stellar density gradient
of a hydrogen-poor photosphere. The matter-dominated jet would be formed
promptly, but requires 5 to 10 s to reach the surface of the progenitor of a
Type Ib/c supernova. During this time, the newly-born neutron star could
contract, spin up, and wind up field lines or turn on an alpha-Omega dynamo. In
addition, the light cylinder will contract from a radius large compared to the
Alfven radius to a size comparable to that of the neutron star. This will
disrupt the structure of any organized dipole field and promote the generation
of ultrarelativistic MHD waves (UMHDW) at high density and Large Amplitude
Electromagnetic Waves (LAEMW) at low density. The generation of the these waves
would be delayed by the cooling time of the neutron star about 5 to 10 seconds,
but the propagation time is short so the UMHDW could arrive at the surface at
about the same time as the matter jet. In the density gradient of the star and
the matter jet, the intense flux of UMHDW and LAEMW could drive shocks,
generate pions by proton-proton collision, or create electron/positron pairs
depending on the circumstances. The UMHDW and LAEMW could influence the
dynamics of the explosion and might also tend to flow out the rotation axis to
produce a collimated gamma-ray burst.Comment: 31 pages, LaTeX, revised for referee comments, accepted for ApJ, July
10 issu
On Fueling Gamma-Ray Bursts and Their Afterglows with Pulsars
Cosmological gamma-ray bursts (GRBs) and their afterglows seem to result from
dissipation of bulk energy in relativistic outflows, but their engine has not
been unambiguously identified. The engine could be a young pulsar formed from
accretion induced collapse with a dynamo amplified field. Elsewhere, we suggest
that such a ``Usov type'' strong field pulsar may help explain the bimodal
distribution in GRB durations. Here we discuss possible roles of a pulsar for
the afterglow. We derive the expected bolometric luminosity decay. The
extracted rotational energy could dissipate by shocks or by large amplitude
electromagnetic waves (LAEMW). The simplest LAEMW approach predicts a slower
decay in observed afterglow peak frequency and faster decay in flux than the
simplest blast-wave model, though more complicated models of both can provide
different dependences. LAEMW do not require the rapid magnetic field
amplification demanded of the blast-wave approach because the emission
originates from a nearly fixed radius. Different time dependent behavior of GRB
and post-GRB emission is also predicted. Observational evidence for a pulsar in
a GRB would make some GRB engine models, such as neutron star mergers and black
holes unlikely. Therefore, the question of whether a pulsar is present is an
important one even if it could drive a canonical fireball.Comment: 9 pages LaTeX, submitted to ApJ
New mechanism of pulsar radio emission
It is shown that pulsar radio emission can be generated effectively through a
streaming motion in the polar-cap regions of a pulsar magnetosphere causing
nonresonant growth of waves that can escape directly. As in other beam models,
a relatively low-energy high-density beam is required. The instability
generates quasi-transverse waves in a beam mode at frequencies that can be well
below the resonant frequency. As the waves propagate outward growth continues
until the height at which the wave frequency is equal to the resonant
frequency. Beyond this point the waves escape in a natural plasma mode (L-O
mode). This one-step mechanism is much more efficient than previously widely
considered multi-step mechanisms.Comment: 4 pages, PRL 2002 (in press
A Plane-Symmetric Inhomogeneous Cosmological Model of Perfect Fluid Distribution with Electromagnetic Field I
A plane-symmetric inhomogeneous cosmological model of perfect fluid
distribution with electro-magnetic field is obtained. The source of the
magnetic field is due to an electric current produced along the z-axis.
is the non-vanishing component of electromagnetic field tensor. To get
a deterministic solution, we assume the free gravitational field is Petrov
type-II non-degenerate. The behaviour of the electro-magnetic field tensor
together with some physical aspects of the model are also discussed.Comment: 11 pages, no figur
On the Theory of Relativistic Strong Plasma Waves
The influence of motion of ions and electron temperature on nonlinear
one-dimensional plasma waves with velocity close to the speed of light in
vacuum is investigated. It is shown that although the wavebreaking field weakly
depends on mass of ions, the nonlinear relativistic wavelength essentially
changes. The nonlinearity leads to the increase of the strong plasma
wavelength, while the motion of ions leads to the decrease of the wavelength.
Both hydrodynamic approach and kinetic one, based on Vlasov-Poisson equations,
are used to investigate the relativistic strong plasma waves in a warm plasma.
The existence of relativistic solitons in a thermal plasma is predicted.Comment: 13 pages, 8 figure
Simultaneous Dual Frequency Observations of Giant Pulses from the Crab Pulsar
Simultaneous measurements of giant pulses from the Crab pulsar were taken at
two widely spaced frequencies using the real-time detection of a giant pulse at
1.4 GHz at the Very Large Array to trigger the observation of that same pulse
at 0.6 GHz at a 25-m telescope in Green Bank, WV. Interstellar dispersion of
the signals provided the necessary time to communicate the trigger across the
country via the Internet. About 70% of the pulses are seen at both 1.4 GHz and
0.6 GHz, implying an emission mechanism bandwidth of at least 0.8 GHz at 1 GHz
for pulse structure on time scales of one to ten microseconds.
The arrival times at both frequencies display a jitter of 100 microseconds
within the window defined by the average main pulse profile and are tightly
correlated. This tight correlation places limits on both the emission mechanism
and on frequency dependent propagation within the magnetosphere.
At 1.4 GHz the giant pulses are resolved into several, closely spaced
components. Simultaneous observations at 1.4 GHz and 4.9 GHz show that the
component splitting is frequency independent. We conclude that the multiplicity
of components is intrinsic to the emission from the pulsar, and reject the
hypothesis that this is the result of multiple imaging as the signal propagates
through the perturbed thermal plasma in the surrounding nebula. At both 1.4 GHz
and 0.6 GHz the pulses are characterized by a fast rise time and an exponential
decay time which are correlated. The pulse broadening with its exponential
decay form is most likely the result of multipath propagation in intervening
ionized gas.Comment: LaTeX, 18 pages, 7 figures, accepted for publication in The
Astrophysical Journa
COSMOLOGICAL GAMMA RAY BURSTS AND THE HIGHEST ENERGY COSMIC RAYS
We discuss a scenario in which the highest energy cosmic rays (CR's) and
cosmological -ray bursts (GRB's) have a common origin. This scenario is
consistent with the observed CR flux above , provided that
each burst produces similar energies in -rays and in CR's above
. Protons may be accelerated by Fermi's mechanism to energies
in a dissipative, ultra-relativistic wind, with
luminosity and Lorentz factor high enough to produce a GRB. For a homogeneous
GRB distribution, this scenario predicts an isotropic, time-independent CR
flux.Comment: Phys. Rev. Lett. in press (Received: March 22, 1995; Accepted: May
17, 1995
Corona of Magnetars
We develop a theoretical model that explains the formation of hot coronae
around strongly magnetized neutron stars -- magnetars. The starquakes of a
magnetar shear its external magnetic field, which becomes non-potential and is
threaded by an electric current. Once twisted, the magnetosphere cannot untwist
immediately because of its self-induction. The self-induction electric field
lifts particles from the stellar surface, accelerates them, and initiates
avalanches of pair creation in the magnetosphere. The created plasma corona
maintains the electric current demanded by curl(B) and regulates the
self-induction e.m.f. by screening. This corona persists in dynamic
equilibrium: it is continually lost to the stellar surface on the
light-crossing time of 10^{-4} s and replenished with new particles. In
essence, the twisted magnetosphere acts as an accelerator that converts the
toroidal field energy to particle kinetic energy. Using a direct numerical
experiment, we show that the corona self-organizes quickly (on a millisecond
timescale) into a quasi-steady state, with voltage ~1 GeV along the magnetic
lines. The heating rate of the corona is ~10^{36} erg/s, in agreement with the
observed persistent, high-energy output of magnetars. We deduce that a static
twist that is suddenly implanted into the magnetosphere will decay on a
timescale of 1-10 yrs. The particles accelerated in the corona impact the solid
crust, knock out protons, and regulate the column density of the hydrostatic
atmosphere of the star. The transition layer between the atmosphere and the
corona is the likely source of the observed 100-keV emission from magnetars.
The corona emits curvature radiation and can supply the observed IR-optical
luminosity. (Abridged)Comment: 70 pages, 14 figures, accepted to Ap
- …