130 research outputs found
On the origin of the March 5, 1979 gamma ray transient: A vibrating neutron star in the Large Magellanic Cloud
It is proposed that a vibrating neutron star in the Large Magellanic Cloud is the source of the March 5 transient. Neutron star vibrations transport energy rapidly to the surface, heat the atmosphere by wave dissipation, and decay by gravitational radiation reaction. The electromagnetic emission arises from e(+)-e(-) pairs which cool and annihilate in the strong magnetic field of the neutron star. The field also confines the pairs, and this allows the production of the redshifted annihilation feature observed in the data. The redshift implies a gravitational radiation damping time which agrees with the 0.15 second duration of the impulsive phase of the event. Thus, the March 5 transient may be both the first detection of a vibrating neutron star and indirect evidence for gravitational radiation
Does Pulsar B1757--24 Have a Fallback Disk?
Radio pulsars are thought to spin-down primarily due to torque from magnetic
dipole radiation (MDR) emitted by the time-varying stellar magnetic field as
the star rotates. This assumption yields a `characteristic age' for a pulsar
which has generally been assumed to be comparable to the actual age. Recent
observational limits on the proper motion of pulsar B1757-24, however, revealed
that the actual age (>39 kyr) of this pulsar is much greater than its MDR
characteristic age (16 kyr) - calling into question the assumption of pure MDR
spin-down for this and other pulsars. To explore the possible cause of this
discrepancy, we consider a scenario in which the pulsar acquired an accretion
disk from supernova ejecta, and the subsequent spin-down occurred under the
combined action of MDR and accretion torques. A simplified model of the
accretion torque involving a constant mass inflow rate at the pulsar
magnetosphere can explain the age and period derivative of the pulsar for
reasonable values of the pulsar magnetic field and inflow rate. We discuss
testable predictions of this model.Comment: Accepted by ApJ Letters. 15 pages with 1 figur
Cosmic Ray Propagation: Nonlinear Diffusion Parallel and Perpendicular to Mean Magnetic Field
We consider the propagation of cosmic rays in turbulent magnetic fields. We
use the models of magnetohydrodynamic turbulence that were tested in numerical
simulations, in which the turbulence is injected on large scale and cascades to
small scales. Our attention is focused on the models of the strong turbulence,
but we also briefly discuss the effects that the weak turbulence and the slab
Alfv\'enic perturbations can have. The latter are likely to emerge as a result
of instabilities with in the cosmic ray fluid itself, e.g., beaming and
gyroresonance instabilities of cosmic rays. To describe the interaction of
cosmic rays with magnetic perturbations we develop a non-linear formalism that
extends the ordinary Quasi-Linear Theory (QLT) that is routinely used for the
purpose. This allows us to avoid the usual problem of 90 degree scattering and
enable our computation of the mean free path of cosmic rays. We apply the
formalism to the cosmic ray propagation in the galactic halo and in the Warm
Ionized medium (WIM). In addition, we address the issue of the transport of
cosmic rays perpendicular to the mean magnetic field and show that the issue of
cosmic ray subdiffusion (i.e., propagation with retracing the trajectories
backwards, which slows down the diffusion) is only important for restricted
cases when the ambient turbulence is far from what numerical simulations
suggest to us. As a result, this work provides formalism that can be applied
for calculating cosmic ray propagation in a wide variety of circumstances.Comment: minor changes, accepted to Ap
Light Element Evolution and Cosmic Ray Energetics
Using cosmic-ray energetics as a discriminator, we investigate evolutionary
models of LiBeB. We employ a Monte Carlo code which incorporates the delayed
mixing into the ISM both of the synthesized Fe, due to its incorporation into
high velocity dust grains, and of the cosmic-ray produced LiBeB, due to the
transport of the cosmic rays. We normalize the LiBeB production to the integral
energy imparted to cosmic rays per supernova. Models in which the cosmic rays
are accelerated mainly out of the average ISM significantly under predict the
measured Be abundance of the early Galaxy, the increase in [O/Fe] with
decreasing [Fe/H] notwithstanding. We suggest that this increase could be due
to the delayed mixing of the Fe. But, if the cosmic-ray metals are accelerated
out of supernova ejecta enriched superbubbles, the measured Be abundances are
consistent with a cosmic-ray acceleration efficiency that is in very good
agreement with the current epoch data. We also find that neither the above
cosmic-ray origin models nor a model employing low energy cosmic rays
originating from the supernovae of only very massive progenitors can account
for the Li data at values of [Fe/H] below 2.Comment: latex 19 pages, 2 tables, 10 eps figures, uses aastex.cls natbib.sty
Submitted to the Astrophysical Journa
Rossi X-Ray Timing Explorer Guest Investigator Program
Rossi X-ray Timing Explorer (RXTE) observations of the bright supernova remnant Cas A have revealed a hard power law component above 10 keV in addition to two thermal components inferred from ASCA measurements of the many line centroids from low-Z elements. The power law can be shown to be consistent with synchrotron emission from radio to hard x-rays by electrons of up to 4 x 10(exp 13) eV. Measurement of the 1157 keV line by CGRO (Compton Gamma Ray Observatory) from SC-44 in the chain of decay of Ti-44 predicts that the two Ti-44 lines at 68 and 78 keV should appear at the CGRO intensity. RXTE has placed upper limits on such lines that are marginally consistent with the CGRO measurement. Implications of these results on sites for cosmic ray acceleration and nucleosynthesis are discussed
Possible Evidence For Axino Dark Matter In The Galactic Bulge
Recently, the SPI spectrometer on the INTEGRAL satellite observed strong 511
keV line emission from the galactic bulge. Although the angular distribution
(spherically symmetric with width of \sim 9 degree) of this emission is
difficult to account for with traditional astrophysical scenarios, light dark
matter particles could account for the observation. In this letter, we consider
the possibility that decaying axinos in an R-parity violating model of
supersymmetry may be the source of this emission. We find that \sim 1-300 MeV
axinos with R-parity violating couplings can naturally produce the observed
emission.Comment: 4 pages, 1 figure. Version accepted by Physical Review
Heliospheric Transport of Neutron-Decay Protons
We report on new simulations of the transport of energetic protons
originating from the decay of energetic neutrons produced in solar flares.
Because the neutrons are fast-moving but insensitive to the solar wind magnetic
field, the decay protons are produced over a wide region of space, and they
should be detectable by current instruments over a broad range of longitudes
for many hours after a sufficiently large gamma-ray flare. Spacecraft closer to
the Sun are expected to see orders-of magnitude higher intensities than those
at the Earth-Sun distance. The current solar cycle should present an excellent
opportunity to observe neutron-decay protons with multiple spacecraft over
different heliographic longitudes and distances from the Sun.Comment: 12 pages, 4 figures, to be published in special issue of Solar
Physic
On the Origin of Cosmic Magnetic Fields
We review the literature concerning how the cosmic magnetic fields pervading
nearly all galaxies actually got started. some observational evidence involves
the chemical abundance of the light elements Be and B, while another one is
based on strong magnetic fields seen in high red shift galaxies. Seed fields,
whose strength is of order 10^{-20} gauss, easily sprung up in the era
preceding galaxy formation. Several mechanisms are proposed to amplify these
seed fields to microgauss strengths. The standard mechanism is the Alpha-Omega
dynamo theory. It has a major difficulty that makes unlikely to provide the
sole origin. The difficulty is rooted in the fact that the total flux is
constant. This implies that flux must be removed from the galactic discs. This
requires that the field and flux be separated, for otherwise interstellar mass
must be removed from the deep galactic gravitational and then their strength
increased by the alpha omega theory.Comment: 90 pages and 6 figures; accepted for publication in Reports of
Progress in Physics as an invited revie
Evidence of X-ray Synchrotron Emission from Electrons Accelerated to 40 TeV in the Supernova Remnant Cassiopeia A
We present the 2-60 keV spectrum of the supernova remnant Cassiopeia A
measured using the Proportional Counter Array and the High Energy X-ray Timing
Experiment on the Rossi X-ray Timing Explorer satellite. In addition to the
previously reported strong emission-line features produced by thermal plasmas,
the broad-band spectrum has a high-energy "tail" that extends to energies at
least as high as 120 keV. This tail may be described by a broken power law that
has photon indices of 1.8 +0.5/-0.6 and 3.04 +0.15/-0.13 and a break energy of
15.9 +0.3/-0.4 keV. We argue that the high-energy component, which dominates
the spectrum above about 10 keV, is produced by synchrotron radiation from
electrons that have energies up to at least 40 TeV. This conclusion supports
the hypothesis that Galactic cosmic rays are accelerated predominantly in
supernova remnants.Comment: 10 pages of text, 3 figures, accepted for Astrophys. J. Letter
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