5,002 research outputs found
The Structure of the Cold Neutral ISM on 10-100 Astronomical Unit Scales
We have used the Very Long Baseline Array (VLBA) and the Very Large Array
(VLA) to image Galactic neutral hydrogen in absorption towards four compact
extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA
data by Faison et al (1998) have shown the existence of prominent structures in
the direction of the extragalactic source 3C~138 with scale sizes of 10-20 AU
with changes in HI optical depth in excess of 0.8 0.1. In this paper we
confirm the small scale \hi optical depth variations toward 3C~147 suggested
earlier at a level up to 20 % 5% . The sources 3C~119, 2352+495 and
0831+557 show no significant change in \hi optical depth across the sources
with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently
investigated with the VLBA and VLA, only 3C~138 and 3C~147 show statistically
significant variations in HI opacities.
Deshpande (2000) have attempted to explain the observed small-scale structure
as an extension of the observed power spectrum of structure on parsec size
scales. The predictions of Deshpande (2000) are consistent with the VLBA HI
data observed in the directions of a number of sources, including 3C~147, but
are not consistent with our previous observations of the HI opacity structure
toward 3C~138
Helical motions in the jet of blazar 1156+295
The blazar 1156+295 was observed by VLBA and EVN + MERLIN at 5 GHz in June
1996 and February 1997 respectively. The results show that the jet of the
source has structural oscillations on the milliarcsecond scale and turns
through a large angle to the direction of the arcsecond-scale extension. A
helical jet model can explain most of the observed properties of the radio
structure in 1156+295.Comment: 6 pages, 2 figures, to appear in New Astronomy Reviews (EVN/JIVE
Symposium No. 4, special issue
Pulsar science with the Five hundred metre Aperture Spherical Telescope
With a collecting area of 70 000 m^2, the Five hundred metre Aperture
Spherical Telescope (FAST) will allow for great advances in pulsar astronomy.
We have performed simulations to estimate the number of previously unknown
pulsars FAST will find with its 19-beam or possibly 100-beam receivers for
different survey strategies. With the 19-beam receiver, a total of 5200
previously unknown pulsars could be discovered in the Galactic plane, including
about 460 millisecond pulsars (MSPs). Such a survey would take just over 200
days with eight hours survey time per day. We also estimate that, with about 80
six-hour days, a survey of M31 and M33 could yield 50--100 extra-Galactic
pulsars. A 19-beam receiver would produce just under 500 MB of data per second
and requires about 9 tera-ops to perform the major part of a real time
analysis. We also simulate the logistics of high-precision timing of MSPs with
FAST. Timing of the 50 brightest MSPs to a signal-to-noise of 500 would take
about 24 hours per epoch.Comment: 9 pages, 10 figures; accepted for publication in A&
Scale dependence of cosmological backreaction
Due to the non-commutation of spatial averaging and temporal evolution,
inhomogeneities and anisotropies (cosmic structures) influence the evolution of
the averaged Universe via the cosmological backreaction mechanism. We study the
backreaction effect as a function of averaging scale in a perturbative approach
up to higher orders. We calculate the hierarchy of the critical scales, at
which 10% effects show up from averaging at different orders. The dominant
contribution comes from the averaged spatial curvature, observable up to scales
of 200 Mpc. The cosmic variance of the local Hubble rate is 10% (5%) for
spherical regions of radius 40 (60) Mpc. We compare our result to the one from
Newtonian cosmology and Hubble Space Telescope Key Project data.Comment: 6 pages, 2 figures; v3: substantial modifications, new figure
mixing effects on charmonium and meson decays
We include the meson into the -- mixing formalism
constructed in our previous work, where represents the pseudoscalar
gluball. The mixing angles in this tetramixing matrix are constrained by
theoretical and experimental implications from relevant hadronic processes.
Especially, the angle between and is found to be about
from the measured decay widths of the meson. The pseudoscalar glueball
mass , the pseudoscalar densities and the U(1) anomaly
matrix elements associated with the mixed states are solved from the anomalous
Ward identities. The solution GeV obtained from the
-- mixing is confirmed, while grows to above the pion
mass, and thus increases perturbative QCD predictions for the branching ratios
. We then analyze the -mixing effects on charmonium
magnetic dipole transitions, and on the branching
ratios and CP asymmetries, which further improve the consistency between
theoretical predictions and data. A predominant observation is that the
mixing enhances the perturbative QCD predictions for
by 18%, but does not alter those for . The puzzle due to the
large data is then resolved.Comment: 12 pages, version to appear in PR
A relativistic helical jet in the gamma-ray AGN 1156+295
We present the results of a number of high resolution radio observations of
the AGN 1156+295. These include multi-epoch and multi-frequency VLBI, VSOP,
MERLIN and VLA observations made over a period of 50 months. The 5 GHz MERLIN
images trace a straight jet extending to 2 arcsec at P.A. -18 degrees. Extended
low brightness emission was detected in the MERLIN observation at 1.6 GHz and
the VLA observation at 8.5 GHz with a bend of about 90 degrees at the end of
the 2 arcsecond jet. A region of similar diffuse emission is also seen about 2
arcseconds south of the radio core. The VLBI images of the blazar reveal a
core-jet structure with an oscillating jet on a milli-arcsecond (mas) scale
which aligns with the arcsecond jet at a distance of several tens of
milli-arcseconds from the core. This probably indicates that the orientation of
the jet structure is close to the line of sight, with the northern jet being
relativistically beamed toward us. In this scenario the diffuse emission to the
north and south is not beamed and appears symmetrical. For the northern jet at
the mas scale, proper motions of 13.7 +/-3.5, 10.6 +/- 2.8, and 11.8 +/- 2.8 c
are measured in three distinct components of the jet (q_0=0.5, H_0=65 km /s
/Mpc are used through out this paper). Highly polarised emission is detected on
VLBI scales in the region in which the jet bends sharply to the north-west. The
spectral index distribution of the source shows that the strongest compact
component has a flat spectrum, and the extended jet has a steep spectrum. A
helical trajectory along the surface of a cone was proposed based on the
conservation laws for kinetic energy and momentum to explain the observed
phenomena, which is in a good agreement with the observed results on scales of
1 mas to 1 arcsec.Comment: 19 pages with 18 figures. Accepted for publication in the A&
Scaling in directed dynamical small-world networks with random responses
A dynamical model of small-world network, with directed links which describe
various correlations in social and natural phenomena, is presented. Random
responses of every site to the imput message are introduced to simulate real
systems. The interplay of these ingredients results in collective dynamical
evolution of a spin-like variable S(t) of the whole network. In the present
model, global average spreading length \langel L >_s and average spreading time
_s are found to scale as p^-\alpha ln N with different exponents.
Meanwhile, S behaves in a duple scaling form for N>>N^*: S ~ f(p^-\beta
q^\gamma t'_sc), where p and q are rewiring and external parameters, \alpha,
\beta, \gamma and f(t'_sc) are scaling exponents and universal functions,
respectively. Possible applications of the model are discussed.Comment: 4 pages, 6 Figure
Pulsar Science With The Five Hundred Metre Aperture Spherical Telescope
With a collecting area of 70 000
Magic wavelengths for the np-ns transitions in alkali-metal atoms
Extensive calculations of the electric-dipole matrix elements in alkali-metal
atoms are conducted using the relativistic all-order method. This approach is a
linearized version of the coupled-cluster method, which sums infinite sets of
many-body perturbation theory terms. All allowed transitions between the lowest
ns, np_1/2, np_3/2 states and a large number of excited states are considered
in these calculations and their accuracy is evaluated. The resulting
electric-dipole matrix elements are used for the high-precision calculation of
frequency-dependent polarizabilities of the excited states of alkali-metal
atoms. We find magic wavelengths in alkali-metal atoms for which the ns and
np_1/2 and np_3/2 atomic levels have the same ac Stark shifts, which
facilitates state-insensitive optical cooling and trapping.Comment: 12 pages, 8 figure
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