8,441 research outputs found
Pneumatic Construction Applied to Multistory Buildings
Extension of the principles of pneumatic construction to include multistory buildings is discussed by Peter R. Smith, Senior Lecturer in Architectural Science at The University of Sydney (Australia) and Jens G. Pohl, Lecturer, School of Architecture and Building, University of New South Wales
A Model of the EGRET Source at the Galactic Center: Inverse Compton Scattering Within Sgr A East and its Halo
Continuum low-frequency radio observations of the Galactic Center reveal the
presence of two prominent radio sources, Sgr A East and its surrounding Halo,
containing non-thermal particle distributions with power-law indices around
2.5-3.3 and 2.4, respectively. The central 1-2 pc region is also a source of
intense (stellar) UV and (dust-reprocessed) far-IR radiation that bathes these
extended synchrotron-emitting structures. A recent detection of gamma-rays
(2EGJ1746-2852) from within around 1 degree of the Galactic Center by EGRET
onboard the Compton GRO shows that the emission from this environment extends
to very high energies.
We suggest that inverse Compton scatterings between the power-law electrons
inferred from the radio properties of Sgr A East and its Halo, and the UV and
IR photons from the nucleus, may account for the possibly diffuse gamma-ray
source as well. We show that both particle distributions may be contributing to
the gamma-ray emission, though their relevant strength depends on the actual
physical properties (such as the magnetic field intensity) in each source. If
this picture is correct, the high-energy source at the Galactic Center is
extended over several arcminutes, which can be tested with thenext generation
of gamma-ray and hard X-ray missions.Comment: latex, 14 pages, 3 figures (accepted for publication in ApJ
DNA loop statistics and torsional modulus
The modelling of DNA mechanics under external constraints is discussed. Two
analytical models are widely known, but disagree for instance on the value of
the torsional modulus. The origin of this embarassing situation is located in
the concept of writhe. This letter presents a unified model for DNA
establishing a relation between the different approaches. I show that the
writhe created by the loops of DNA is at the origin of the discrepancy. To take
this into account, I propose a new treatment of loop statistics based on
numerical simulations using the most general formula for the writhe, and on
analytic calculations with only one fit parameter. One can then compute the
value of the torsional modulus of DNA without the need of any cut-off.Comment: 8 pages, 1 figure. Accepted by Europhysics Letter
On the formation and decay of a molecular ultracold plasma
Double-resonant photoexcitation of nitric oxide in a molecular beam creates a
dense ensemble of Rydberg states, which evolves to form a plasma of
free electrons trapped in the potential well of an NO spacecharge. The
plasma travels at the velocity of the molecular beam, and, on passing through a
grounded grid, yields an electron time-of-flight signal that gauges the plasma
size and quantity of trapped electrons. This plasma expands at a rate that fits
with an electron temperature as low as 5 K, colder that typically observed for
atomic ultracold plasmas. The recombination of molecular NO cations with
electrons forms neutral molecules excited by more than twice the energy of the
NO chemical bond, and the question arises whether neutral fragmentation plays a
role in shaping the redistribution of energy and particle density that directs
the short-time evolution from Rydberg gas to plasma. To explore this question,
we adapt a coupled rate-equations model established for atomic ultracold
plasmas to describe the energy-grained avalanche of electron-Rydberg and
electron-ion collisions in our system. Adding channels of Rydberg
predissociation and two-body, electron- cation dissociative recombination to
the atomic formalism, we investigate the kinetics by which this relaxation
distributes particle density and energy over Rydberg states, free electrons and
neutral fragments. The results of this investigation suggest some mechanisms by
which molecular fragmentation channels can affect the state of the plasma
Radiation from collision-dominated relativistic pair fireballs
It is generally accepted that gamma-ray bursts (GRBs) are initiated by a
relativistic pair fireball, converting its internal energy into kinetic energy
of a relativistically moving plasmoid and subsequently into radiation. Here, we
investigate the early stages of this evolution, after the pair fireball has
become optically thin to gamma-gamma pair production. We show that for a short
period of time, ~ 0.1 - a few seconds after the initial explosion, the pair
plasmoid evolution might be dominated by collisional processes prior to the
formation of a collisionless shock. We simulate these processes during the
early pair plasmoid evolution and calculate the expected radiative signatures.
We show that the radiation from the collision-dominated pair plasmoid phase
results in a short (~ a few ms) flash of thermal soft X-ray emission, followed
by a transition phase of < 1 s during which the fireball turns Thomson thin,
but its radiation remains dominated by thermal Comptonization, peaking at
around E_pk ~ 100 MeV - a few GeV. While the very early thermal emission could
be associated with the quasi-thermal radiation signatures found in the very
early phases of several bright BATSE GRBs, the predicted subsequent flash of
high-energy emission should be easily detectable with the GLAST satellite.Comment: AASTeX, 25 pages, including 7 figures. Accepted for publication in
Ap
Observability of Forming Planets and their Circumplanetary Disks I. -- Parameter Study for ALMA
We present mock observations of forming planets with ALMA. The possible
detections of circumplanetary disks (CPDs) were investigated around planets of
Saturn, 1, 3, 5, and 10 Jupiter-masses that are placed at 5.2 AU from their
star. The radiative, three dimensional hydrodynamic simulations were then
post-processed with RADMC3D and the ALMA Observation Simulator. We found that
even though the CPDs are too small to be resolved, they are hot due to the
accreting planet in the optically thick limit, therefore the best chance to
detect them with continuum observations in this case is at the shortest ALMA
wavelengths, such as Band 9 (440 microns). Similar fluxes were found in the
case of Saturn and Jupiter-mass planets, as for the 10
gas-giant, due to temperature weighted optical depth effects: when no deep gap
is carved, the planet region is blanketed by the optically thick circumstellar
disk leading to a less efficient cooling there. A test was made for a 52 AU
orbital separation, showed that optically thin CPDs are also detectable in band
7 but they need longer integration times (5hrs). Comparing the gap profiles
of the same simulation at various ALMA bands and the hydro simulation confirmed
that they change significantly, first because the gap is wider at longer
wavelengths due to decreasing optical depth; second, the beam convolution makes
the gap shallower and at least 25% narrower. Therefore, caution has to be made
when estimating planet masses based on ALMA continuum observations of gaps.Comment: Accepted for publication at MNRAS. Typos are corrected since previous
version. 11 pages, 5 tables, 4 figure
COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission
The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy
range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have
been analyzed. During the first two years PKS 0528+134 was most significantly
detected at energies above 3 MeV. During the last year there is only evidence
for the quasar at energies below 3 MeV indicating a spectral change. The
time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well
represented by a power-law shape. Spectra collected from different
observational periods reveal different power-law shapes: a hard state during
flaring observations reported by EGRET, and a soft state otherwise. The
combined simultaneous EGRET and COMPTEL spectra indicate these two spectral
states as well. During low intensisty gamma-ray phases no spectral break is
obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray
flaring phases however, the combined COMPTEL and EGRET data require a spectral
bending at MeV-energies. By fitting broken power-law functions the best-fit
values for the break in photon index range between 0.6 and 1.7, and for the
break energy between ~5 MeV and ~20 MeV. Because the flux values measured by
COMPTEL below 3 MeV in both states are roughly equal, the observations would be
consistent with an additional spectral component showing up during gamma-ray
flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a
high-energy electron-positron population with a low-energy cutoff in their bulk
Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for
gamma-ray flaring phases shows that the major energy release across the entire
electro-magnetic spectrum is measured at MeV-energies.Comment: 10 pages, 8 postscript figures, latex, to appear in: A&A 328, 33
(1997
Practical synthetic strategies towards lipophilic 6-iodotetrahydroquinolines and -dihydroquinolines
The synthesis of novel tetrahydroquinolines (THQ) and dihydroquinolines (DHQ) are reported using three practical, scalable synthetic approaches to access highly lipophilic analogues bearing a 6-iodo substituent, each with a different means of cyclisation. A versatile and stable quinolin-2-one intermediate was identified, which could be reduced to the corresponding THQ with borane reagents, or to the DHQ with diisobutylaluminium hydride via a novel elimination that is more favourable at higher temperatures. Coupling these strongly electron-donating scaffolds to electron-accepting moieties caused the resulting structures to exhibit strong fluorescence
On the Nature of MeV-blazars
Broad-band spectra of the FSRQ (flat-spectrum-radio quasars) detected in the
high energy gamma-ray band imply that there may be two types of such objects:
those with steep gamma-ray spectra, hereafter called MeV-blazars, and those
with flat gamma-ray spectra, GeV-blazars. We demonstrate that this difference
can be explained in the context of the ERC (external-radiation-Compton) model
using the same electron injection function. A satisfactory unification is
reachable, provided that: (a) spectra of GeV-blazars are produced by internal
shocks formed at the distances where cooling of relativistic electrons in a jet
is dominated by Comptonization of broad emission lines, whereas spectra of
MeV-blazars are produced at the distances where cooling of relativistic
electrons is dominated by Comptonization of near-IR radiation from hot dust;
(b) electrons are accelerated via a two step process and their injection
function takes the form of a double power-law, with the break corresponding to
the threshold energy for the diffusive shock acceleration. Direct predictions
of our model are that, on average, variability time scales of the MeV-blazars
should be longer than variability time scales of the GeV-blazars, and that both
types of the blazar phenomenon can appear in the same object.Comment: Accepted for publication in the Astrophysical Journa
Diffuse Gamma-Ray Emission from Starburst Galaxies and M31
We present a search for high energy gamma-ray emission from 9 nearby
starburst galaxies and M31 with the EGRET instrument aboard CGRO. Though the
diffuse gamma-ray emission from starburst galaxies was suspected to be
detectable, we find no emission from NGC 253, M82 nor from the average of all 9
galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the
averaged survey observations is 1.8 x 10-8 ph cm-2 s-1. From a model of the
expected radio and gamma-ray emission, we find that the magnetic field in the
nuclei of these galaxies is > 25 micro Gauss, and the ratio of proton and
electron densities is < 400. The EGRET limits indicate that the rate of massive
star formation in the survey galaxies is only about an order of magnitude
higher than in the Milky Way. The upper limit to the gamma-ray flux above 100
MeV for M31 is 1.6 x 10-8 ph cm-2 s-1. At the distance of M31, the Milky Way
flux would be over twice this value, indicating higher gamma-ray emissivities
in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher
than in M31, our null detection of M31 supports the theory of the supernova
origin of cosmic rays in galaxies.Comment: 17 pages, plus 1 Postscript figure, AAS Latex macros v4.0, accepted
for publication in ApJ Main Journa
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