8,165 research outputs found
A High Galactic Latitude HI 21cm-line Absorption Survey using the GMRT: II. Results and Interpretation
We have carried out a sensitive high-latitude (|b| > 15deg.) HI 21cm-line
absorption survey towards 102 sources using the GMRT. With a 3-sigma detection
limit in optical depth of ~0.01, this is the most sensitive HI absorption
survey. We detected 126 absorption features most of which also have
corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic
neutral Hydrogen. The histogram of random velocities of the absorption features
is well-fit by two Gaussians centered at V(lsr) ~ 0 km/s with velocity
dispersions of 7.6 +/- 0.3 km/s and 21 +/- 4 km/s respectively. About 20% of
the HI absorption features form the larger velocity dispersion component. The
HI absorption features forming the narrow Gaussian have a mean optical depth of
0.20 +/- 0.19, a mean HI column density of (1.46 +/- 1.03) X 10^{20} cm^{-2},
and a mean spin temperature of 121 +/- 69 K. These HI concentrations can be
identified with the standard HI clouds in the cold neutral medium of the
Galaxy. The HI absorption features forming the wider Gaussian have a mean
optical depth of 0.04 +/- 0.02, a mean HI column density of (4.3 +/- 3.4) X
10^{19} cm^{-2}, and a mean spin temperature of 125 +/- 82 K. The HI column
densities of these fast clouds decrease with their increasing random
velocities. These fast clouds can be identified with a population of clouds
detected so far only in optical absorption and in HI emission lines with a
similar velocity dispersion. This population of fast clouds is likely to be in
the lower Galactic Halo.Comment: 19 pages, 19 figures. Accepted for publication in Journal of
Astrophysics & Astronom
The interstellar clouds of Adams and Blaauw revisited: an HI absorption study - I
This investigation is aimed at clarifying the nature of the interstellar gas
seen in absorption against bright O and B stars. Towards this end we have
obtained for the first time HI absorption spectra towards radio sources very
close to the lines of sight towards 25 bright stars previously studied. In this
paper we describe the selection criteria, the details regarding our
observations, and finally present the absorption spectra. In the accompanying
paper we analyse the results and draw conclusions.Comment: 20 pages, 12 figures, accepted by Journal of Astrophysics and
Astronom
On Schwinger Pair Creation in Gravity and in Closed Superstring Theory
We investigate the Schwinger pair creation process in the context of
gravitational models with the back reaction of the electric field included in
the geometry. The background is also an exact solution of type II superstring
theory, where the electric field arises by Kaluza-Klein reduction. We obtain a
closed formula for the pair creation rate that incorporates the gravitational
back reaction. At weak fields it has the same structure as the general
Schwinger formula, albeit pairs are produced by a combination of Schwinger and
Unruh effect, the latter due to the presence of a Rindler horizon. In four
spacetime dimensions, the rate becomes constant at strong electric fields. For
states with mass of Kaluza-Klein origin, the rate has a power-like dependence
in the electric field, rather than the familiar (non-perturbative) exponential
dependence. We also reproduce the same formula from the string partition
function for winding string states. Finally, we comment on the generalization
to excited string states.Comment: 21 page
Linear and nonlinear optical spectroscopy of a strongly-coupled microdisk-quantum dot system
A fiber taper waveguide is used to perform direct optical spectroscopy of a
microdisk-quantum-dot system, exciting the system through the photonic (light)
channel rather than the excitonic (matter) channel. Strong coupling, the regime
of coherent quantum interactions, is demonstrated through observation of vacuum
Rabi splitting in the transmitted and reflected signals from the cavity. The
fiber coupling method also allows the examination of the system's steady-state
nonlinear properties, where saturation of the cavity-QD response is observed
for less than one intracavity photon.Comment: adjusted references, added minor clarification
A new look at the birthrate of Supernova Remnan
We have reanalysed a homogeneous catalogue of shell-type supernova remnants and we find that the radio data are consistent with a birthrate of one in 22±3 yr. Our approach is based on the secular decrease of surface brightness of the historical remnants whose ages are precisely known. The abovementioned birthrate is significantly higher than most previous estimates which range from one in 50-150 yr, and is consistent with the supernova rate in our galaxy derived from historical observations, as well as with recent estimates of the pulsar birthrate
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