305 research outputs found
Spectral shifts in quasi-stellar objects
Red and blue shift frequency distribution of quasi-stellar objects from nearby galaxie
Perfect magnetohydrodynamics as a field theory
We propose the generally covariant action for the theory of a self-coupled
complex scalar field and electromagnetism which by virtue of constraints is
equivalent, in the regime of long wavelengths, to perfect magnetohydrodynamics
(MHD). We recover from it the Euler equation with Lorentz force, and the
thermodynamic relations for a prefect fluid. The equation of state of the
latter is related to the scalar field's self potential. We introduce 1+3
notation to elucidate the relation between MHD and field variables. In our
approach the requirement that the scalar field be single valued leads to the
quantization of a certain circulation in steps of ; this feature leads,
in the classical limit, to the conservation of that circulation. The
circulation is identical to that in Oron's generalization of Kelvin's
circulation theorem to perfect MHD; we here characterize the new conserved
helicity associated with it. We also demonstrate the existence for MHD of two
Bernoulli-like theorems for each spacetime symmetry of the flow and geometry;
one of these is pertinent to suitably defined potential flow. We exhibit the
conserved quantities explicitly in the case that two symmetries are
simultaneously present, and give examples. Also in this case we exhibit a new
conserved MHD circulation distinct from Oron's, and provide an example.Comment: RevTeX, 16 pages, no figures; clarifications added and typos
corrected; version to be published in Phys. Rev.
Radio Images of 3C 58: Expansion and Motion of its Wisp
New 1.4 GHz VLA observations of the pulsar-powered supernova remnant 3C 58
have resulted in the highest-quality radio images of this object to date. The
images show filamentary structure over the body of the nebula. The present
observations were combined with earlier ones from 1984 and 1991 to investigate
the variability of the radio emission on a variety of time-scales. No
significant changes are seen over a 110 day interval. In particular, the upper
limit on the apparent projected velocity of the wisp is 0.05c. The expansion
rate of the radio nebula was determined between 1984 and 2004, and is
0.014+/-0.003%/year, corresponding to a velocity of 630+/-70 km/s along the
major axis. If 3C 58 is the remnant of SN 1181, it must have been strongly
decelerated, which is unlikely given the absence of emission from the supernova
shell. Alternatively, the low expansion speed and a number of other arguments
suggest that 3C 58 may be several thousand years old and not be the remnant of
SN 1181.Comment: 12 pages; accepted for publication in the Astrophysical Journa
Universal relation between longitudinal and transverse conductivities in quantum Hall effect
We show that any critical transition region between two adjacent Hall
plateaus in either integer or fractional quantum Hall effect is characterized
by a universal semi-circle relationship between the longitudinal and transverse
conductivities, provided the sample is homogeneous and isotropic on a large
scale. This conclusion is demonstrated both for the phase-coherent quantum
transport as well as for the incoherent transport.Comment: REVTEX 3.0, 1 figure, 4 pages. SISSA-08179
A Multi-Frequency Radio Study of Supernova Remnant G292.0+1.8 and its Pulsar Wind Nebula
(Abridged) We present a detailed radio study of the young supernova remnant
(SNR) G292.0+1.8 and its associated pulsar PSR J1124-5916, using the Australia
Telescope Compact Array at observing wavelengths of 20, 13 and 6 cm. We find
that the radio morphology of the source consists of three main components: a
polarized flat-spectrum central core coincident with the pulsar J1124-5916, a
surrounding circular steep-spectrum plateau with sharp outer edges and,
superimposed on the plateau, a series of radial filaments with spectra
significantly flatter than their surroundings. HI absorption argues for a lower
limit on the distance to the system of 6 kpc.
The core clearly corresponds to radio emission from a pulsar wind nebula
powered by PSR J1124-5916, while the plateau represents the surrounding SNR
shell. The plateau's sharp outer rim delineates the SNR's forward shock, while
the thickness of the plateau region demonstrates that the forward and reverse
shocks are well-separated. Assuming a distance of 6 kpc and an age for the
source of 2500 yr, we infer an expansion velocity for the SNR of ~1200 km/s and
an ambient density ~0.9 cm^-3. We interpret the flat-spectrum radial filaments
superimposed on the steeper-spectrum plateau as Rayleigh-Taylor unstable
regions between the forward and reverse shocks of the SNR. The flat radio
spectrum seen for these features results from efficient second-order Fermi
acceleration in strongly amplified magnetic fields.Comment: 11 pages of text, plus 7 embedded EPS figures. Accepted to ApJ. Added
missing units on x-axis of Fig
X-Ray Observations of the supernova remnant G21.5-0.9
We present the analysis of archival X-ray observations of the supernova
remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties,
G21.5-0.9 has been classified as a Crab-like SNR. In their early analysis of
the CHANDRA calibration data, Slane et al. (2000) discovered a
low-surface-brightness, extended emission. They interpreted this component as
the blast wave formed in the supernova (SN) explosion. In this paper, we
present the CHANDRA analysis using a total exposure of ~150 ksec. We also
include ROSAT and ASCA observations. Our analysis indicates that the extended
emission is non-thermal -- a result in agreement with XMM observations. The
entire remnant of radius ~ 2'.5 is best fitted with a power law model with a
photon index steepening away from the center. The total unabsorbed flux in the
0.5-10 keV is 1.1E-10 erg/cm2/s with an 85% contribution from the 40" radius
inner core. Timing analysis of the High-Resolution Camera (HRC) data failed to
detect any pulsations. We put a 16% upper limit on the pulsed fraction. We
derive the physical parameters of the putative pulsar and compare them with
those of other plerions (such as the Crab and 3C 58). G21.5-0.9 remains the
only plerion whose size in X-rays is bigger than in the radio. Deep radio
observations will address this puzzle.Comment: 23 pages including 11 figures and 3 tables; accepted by ApJ June 22,
2001; to appear in Oct 20, 2001 issue of Ap
Self-consistent local-equilibrium model for density profile and distribution of dissipative currents in a Hall bar under strong magnetic fields
Recent spatially resolved measurements of the electrostatic-potential
variation across a Hall bar in strong magnetic fields, which revealed a clear
correlation between current-carrying strips and incompressible strips expected
near the edges of the Hall bar, cannot be understood on the basis of existing
equilibrium theories. To explain these experiments, we generalize the
Thomas-Fermi--Poisson approach for the self-consistent calculation of
electrostatic potential and electron density in {\em total} thermal equilibrium
to a {\em local equilibrium} theory that allows to treat finite gradients of
the electrochemical potential as driving forces of currents in the presence of
dissipation. A conventional conductivity model with small values of the
longitudinal conductivity for integer values of the (local) Landau-level
filling factor shows that, in apparent agreement with experiment, the current
density is localized near incompressible strips, whose location and width in
turn depend on the applied current.Comment: 9 pages, 7 figure
Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
HIRES on the Keck I telescope has been used to measure the first radial
velocities for stars belonging to eleven, heavily-reddened globular clusters in
the direction of the inner Galaxy. The question of kinematic substructuring
among the Galactic globular cluster system is investigated using an updated
catalog of globular cluster distances, metallicities and velocities. It is
found that the population of metal-rich globular clusters shows significant
rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc
of the Galactic center, the measured rotation velocity and line-of-sight
velocity dispersion are similar to those of bulge field stars. We investigate
claims that the metal-rich clusters are associated with the central Galactic
bar by comparing the kinematics of the innermost clusters to that of the atomic
hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich
and metal-poor clusters bears a remarkable similarity to that of the gas,
including the same non-circular motions which have traditionally been
interpreted as evidence for a Galactic bar, or, alternatively, a
non-axisymmetric bulge. However, uncertainties in the existing
three-dimensional Galactocentric positions for most of the clusters do not yet
allow an unambiguous discrimination between the competing scenarios of
membership in a rigidly rotating bar, or in a bulge which is an oblate
isotropic rotator. We conclude that the majority of metal-rich clusters within
the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and
not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available
at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical
Journa
DA495 - an aging pulsar wind nebula
We present a radio continuum study of the pulsar wind nebula (PWN) DA 495
(G65.7+1.2), including images of total intensity and linear polarization from
408 to 10550 MHz based on the Canadian Galactic Plane Survey and observations
with the Effelsberg 100-m Radio Telescope. Removal of flux density
contributions from a superimposed \ion{H}{2} region and from compact
extragalactic sources reveals a break in the spectrum of DA 495 at 1.3 GHz,
with a spectral index below the break and
above it (). The
spectral break is more than three times lower in frequency than the lowest
break detected in any other PWN. The break in the spectrum is likely the result
of synchrotron cooling, and DA 495, at an age of 20,000 yr, may have
evolved from an object similar to the Vela X nebula, with a similarly energetic
pulsar. We find a magnetic field of 1.3 mG inside the nebula. After
correcting for the resulting high internal rotation measure, the magnetic field
structure is quite simple, resembling the inner part of a dipole field
projected onto the plane of the sky, although a toroidal component is likely
also present. The dipole field axis, which should be parallel to the spin axis
of the putative pulsar, lies at an angle of {\sim}50\degr east of the North
Celestial Pole and is pointing away from us towards the south-west. The upper
limit for the radio surface brightness of any shell-type supernova remnant
emission around DA 495 is OAWatt
m Hz sr (assuming a radio spectral index of ), lower than the faintest shell-type remnant known to date.Comment: 25 pages, accepted by Ap
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