27,453 research outputs found
Evidence for GeV emission from the Galactic Center Fountain
The region near the Galactic center may have experienced recurrent episodes
of injection of energy in excess of 10 ergs due to repeated
starbursts involving more than 10 supernovae. This hypothesis can be
tested by measurements of -ray lines produced by the decay of
radioactive isotopes and positron annihilation, or by searches for pulsars
produced during starbursts. Recent OSSE observations of 511 keV emission
extending above the Galactic center led to the suggestion of a starburst driven
fountain from the Galactic center. We present EGRET observations that might
support this picture.Comment: 5 pages, 1 embedded Postscript figure. To appear in the Proceedings
of the Fourth Compton Symposiu
Fluid machines: Expanding the limits, past and future
During the 40 yr period from 1940 to 1980, the capabilities and operating limits of fluid machines were greatly extended. This was due to a research program, carried out to meet the needs of aerospace programs. Some of the events are reviewed. Overall advancements of all machinery components are discussed followed by a detailed examination of technology advancements in axial compressors and pumps. Future technology needs are suggested
Scaling behavior of interactions in a modular quantum system and the existence of local temperature
We consider a quantum system of fixed size consisting of a regular chain of
-level subsystems, where is finite. Forming groups of subsystems
each, we show that the strength of interaction between the groups scales with
. As a consequence, if the total system is in a thermal state with
inverse temperature , a sufficient condition for subgroups of size
to be approximately in a thermal state with the same temperature is , where is the width of the occupied
level spectrum of the total system. These scaling properties indicate on what
scale local temperatures may be meaningfully defined as intensive variables.
This question is particularly relevant for non-equilibrium scenarios such as
heat conduction etc.Comment: 7 pages, accepted for publication in Europhysics Letter
Search for GRB afterglows in the ROSAT all-sky survey
We report on the status of our search for X-ray afterglows of gamma-ray
bursts (GRBs) using the ROSAT all-sky survey data. The number of potential
X-ray afterglow candidates with respect to the expected number of beamed GRBs
allows to constrain the relative beaming angles of GRB emission and afterglow
emission at about 1-5 hrs after the GRB.Comment: 3 pages A&A style, 1 color ps-figure; To appear in A&A Suppl. Series,
Proc. of Rome 1998 GRB workshop, also available from
http://www.aip.de/~jcg/publis.htm
Pre-encounter observations of 951 Gaspra
Photometry and colorimetry of 951 Gaspra were obtained on nine nights during the 1990 opposition. A composite lightcurve constructed using data from eight of those nights yielded a synodic rotational period of 7.04346 +/- 0.00006 hours, a mean absolute V magnitude of 11.8026 +/- 0.0025, and a slope parameter of 0.285 +/- 0.005. The apparent discrepancy can be easily resolved by realizing that their determination is based primarily on data obtained after opposition. Different phase functions pre- and post-opposition are a natural consequence of a changing aspect during an opposition. If the sub-Earth latitude on Gaspra is at a less equatorial aspect after opposition than it was before opposition, then we would expect to see a shallower phase function (corresponding to a larger numerical value of the slope parameter). Adding weight to this hypothesis is the last observation of the opposition, made in May after Gaspra had passed post opposition quadrature, which is displaced toward brighter absolute magnitudes relative to the rest of our data, indicating an even more poleward sub-Earth latitude than earlier in the opposition. Because the orbits of Earth and Gaspra are nearly coplanar, a substantial change in sub-Earth latitude during the opposition would not have been possible unless the obliquity of the asteroid's rotational axis is not small
The use of pure carbon for permanent percutaneous electrical connector systems
Pure carbon was used as an electrode in the clinical application of long-term neuromuscular stimulation, as well as a connector for permanent neuroelectrodes. The history of this material and some examples of the material in use are presented
Strong photon non-linearities and photonic Mott insulators
We show, that photon non-linearities in electromagnetically induced
transparency can be at least one order of magnitude larger than predicted in
all previous approaches. As an application we demonstrate that, in this regime
they give rise to very strong photon - photon interactions which are strong
enough to make an experimental realization of a photonic Mott insulator state
feasible in arrays of coupled ultra high-Q micro-cavities.Comment: minor changes, to appear in Phys. Rev. Let
Nonlinear nanomechanical resonators for quantum optoelectromechanics
We present a scheme for tuning and controlling nano mechanical resonators by
subjecting them to electrostatic gradient fields, provided by nearby tip
electrodes. We show that this approach enables access to a novel regime of
optomechanics, where the intrinsic nonlinearity of the nanoresonator can be
explored. In this regime, one or several laser driven cavity modes coupled to
the nanoresonator and suitably adjusted gradient fields allow to control the
motional state of the nanoresonator at the single phonon level. Some
applications of this platform have been presented previously [New J. Phys. 14,
023042 (2012), Phys. Rev. Lett. 110, 120503 (2013)]. Here, we provide a
detailed description of the corresponding setup and its optomechanical coupling
mechanisms, together with an in-depth analysis of possible sources of damping
or decoherence and a discussion of the readout of the nanoresonator state.Comment: 15 pages, 6 figure
Effect of mass asymmetry on the mass dependence of balance energy
We demonstrate the role of the mass asymmetry on the balance energy (Ebal) by
studying asymmetric reactions throughout the periodic table and over entire
colliding geometry. Our results, which are almost independent of the system
size and as well as of the colliding geometries indicate a sizeable effect of
the asymmetry of the reaction on the balance energy.Comment: Journal of Physics - Conference Series - Online end of March (2011
Probing the evolving massive star population in Orion with kinematic and radioactive tracers
We assemble a census of the most massive stars in Orion, then use stellar
isochrones to estimate their masses and ages, and use these results to
establish the stellar content of Orion's individual OB associations. From this,
our new population synthesis code is utilized to derive the history of the
emission of UV radiation and kinetic energy of the material ejected by the
massive stars, and also follow the ejection of the long-lived radioactive
isotopes 26Al and 60Fe. In order to estimate the precision of our method, we
compare and contrast three distinct representations of the massive stars. We
compare the expected outputs with observations of 26Al gamma-ray signal and the
extent of the Eridanus cavity. We find an integrated kinetic energy emitted by
the massive stars of 1.8(+1.5-0.4)times 10^52 erg. This number is consistent
with the energy thought to be required to create the Eridanus superbubble. We
also find good agreement between our model and the observed 26Al signal,
estimating a mass of 5.8(+2.7-2.5) times 10^-4 Msol of 26Al in the Orion
region. Our population synthesis approach is demonstrated for the Orion region
to reproduce three different kinds of observable outputs from massive stars in
a consistent manner: Kinetic energy as manifested in ISM excavation, ionization
as manifested in free-free emission, and nucleosynthesis ejecta as manifested
in radioactivity gamma-rays. The good match between our model and the
observables does not argue for considerable modifications of mass loss. If
clumping effects turn out to be strong, other processes would need to be
identified to compensate for their impact on massive-star outputs. Our
population synthesis analysis jointly treats kinematic output and the return of
radioactive isotopes, which proves a powerful extension of the methodology that
constrains feedback from massive stars.Comment: Accepted for publication in A&A, 10 page
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