9,751 research outputs found
Multi-wavelength Observations of Blazar AO 0235+164 in the 2008-2009 Flaring State
The blazar AO 0235+164 (z = 0.94) has been one of the most active objects observed by Fermi Large Area Telescope (LAT) since its launch in Summer 2008. In addition to the continuous coverage by Fermi, contemporaneous observations were carried out from the radio to γ-ray bands between 2008 September and 2009 February. In this paper, we summarize the rich multi-wavelength data collected during the campaign (including F-GAMMA, GASP-WEBT, Kanata, OVRO, RXTE, SMARTS, Swift, and other instruments), examine the cross-correlation between the light curves measured in the different energy bands, and interpret the resulting spectral energy distributions in the context of well-known blazar emission models. We find that the γ-ray activity is well correlated with a series of near-IR/optical flares, accompanied by an increase in the optical polarization degree. On the other hand, the X-ray light curve shows a distinct 20 day high state of unusually soft spectrum, which does not match the extrapolation of the optical/UV synchrotron spectrum. We tentatively interpret this feature as the bulk Compton emission by cold electrons contained in the jet, which requires an accretion disk corona with an effective covering factor of 19% at a distance of 100 R_g. We model the broadband spectra with a leptonic model with external radiation dominated by the infrared emission from the dusty torus
Split-Stirling-cycle displacer linear-electric drive
The retrofit of a 1/4-W split-Stirling cooler with a linear driven on the displacer was achieved and its performance characterized. The objective of this work was to demonstrate that a small linear motor could be designed to meet the existing envelope specifications of the cooler and that an electric linear drive on the displacer could improve the cooler's reliability and performance. The paper describes the characteristics of this motor and presents cooler test results
Systematic search for high-energy gamma-ray emission from bow shocks of runaway stars
Context. It has been suggested that the bow shocks of runaway stars are
sources of high-energy gamma rays (E > 100 MeV). Theoretical models predicting
high-energy gamma-ray emission from these sources were followed by the first
detection of non-thermal radio emission from the bow shock of BD+43^\deg 3654
and non-thermal X-ray emission from the bow shock of AE Aurigae. Aims. We
perform the first systematic search for MeV and GeV emission from 27 bow shocks
of runaway stars using data collected by the Large Area Telescope (LAT) onboard
the Fermi Gamma-ray Space Telescope (Fermi). Methods. We analysed 57 months of
Fermi-LAT data at the positions of 27 bow shocks of runaway stars extracted
from the Extensive stellar BOw Shock Survey catalogue (E-BOSS). A likelihood
analysis was performed to search for gamma-ray emission that is not compatible
with diffuse background or emission from neighbouring sources and that could be
associated with the bow shocks. Results. None of the bow shock candidates is
detected significantly in the Fermi-LAT energy range. We therefore present
upper limits on the high-energy emission in the energy range from 100 MeV to
300 GeV for 27 bow shocks of runaway stars in four energy bands. For the three
cases where models of the high-energy emission are published we compare our
upper limits to the modelled spectra. Our limits exclude the model predictions
for Zeta Ophiuchi by a factor 5.Comment: 5 pages, 5 figures, 1 table, accepted by A&
Gravitational Collapse in One Dimension
We simulate the evolution of one-dimensional gravitating collisionless
systems from non- equilibrium initial conditions, similar to the conditions
that lead to the formation of dark- matter halos in three dimensions. As in the
case of 3D halo formation we find that initially cold, nearly homogeneous
particle distributions collapse to approach a final equilibrium state with a
universal density profile. At small radii, this attractor exhibits a power-law
behavior in density, {\rho}(x) \propto |x|^(-{\gamma}_crit), {\gamma}_crit
\simeq 0.47, slightly but significantly shallower than the value {\gamma} = 1/2
suggested previously. This state develops from the initial conditions through a
process of phase mixing and violent relaxation. This process preserves the
energy ranks of particles. By warming the initial conditions, we illustrate a
cross-over from this power-law final state to a final state containing a
homogeneous core. We further show that inhomogeneous but cold power-law initial
conditions, with initial exponent {\gamma}_i > {\gamma}_crit, do not evolve
toward the attractor but reach a final state that retains their original
power-law behavior in the interior of the profile, indicating a bifurcation in
the final state as a function of the initial exponent. Our results rely on a
high-fidelity event-driven simulation technique.Comment: 14 Pages, 13 Figures. Submitted to MNRA
Afrika-Seminaar: Die geologie van Afrika in oorsig
Dit is vanselfsprekend dat in ’n artikel soos hierdie die geologie van Afrika slegs in hooftrekke aangedui kan word en dat daar geen sprake van besonderhede kan wees nie. Wat hierop volg kan dus eintlik maar net as’n selektiewe uittreksel uit die publikasies van sekere skrywers oor Afrika beskou word, en met die oog hierop is dit miskien goed om eers kortliks die vernaamste bronne van inligting aan te dui
H2 molecule in strong magnetic fields
The Pauli-Hamiltonian of a molecule with fixed nuclei in a strong constant
magnetic field is asymptotic, in norm-resolvent sense, to an effective
Hamiltonian which has the form of a multi-particle Schr\"odinger operator with
interactions given by one-dimensional \delta-potentials. We study this
effective Hamiltonian in the case of the H2 -molecule and establish existence
of the ground state. We also show that the inter-nuclear equilibrium distance
tends to 0 as the field-strength tends to infinity
Study of ion feedback in multi-GEM structures
We study the feedback of positive ions in triple and quadruple Gas Electron
Multiplier (GEM) detectors. The effects of GEM hole diameter, detector gain,
applied voltages, number of GEMs and other parameters on ion feedback are
investigated in detail. In particular, it was found that the ion feedback is
independent of the gas mixture and the pressure. In the optimized multi-GEM
structure, the ion feedback current can be suppressed down to 0.5% of the anode
current, at a drift field of 0.1 kV/cm and gain of 10^4. A simple model of ion
feedback in multi-GEM structures is suggested. The results obtained are
relevant to the performance of time projection chambers and gas
photomultipliers.Comment: 9 pages, 11 figures. Submitted to Nucl. Instr. and Meth.
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