8,427 research outputs found
Nonthermal THz to TeV Emission from Stellar Wind Shocks in the Galactic Center
The central parsec of the Galaxy contains dozens of massive stars with a
cumulative mass loss rate of ~ 10^{-3} solar masses per year. Shocks among
these stellar winds produce the hot plasma that pervades the central part of
the galaxy. We argue that these stellar wind shocks also efficiently accelerate
electrons and protons to relativistic energies. The relativistic electrons
inverse Compton scatter the ambient ultraviolet and far infrared radiation
field, producing high energy gamma-rays with a roughly constant luminosity from
\~ GeV to ~ 10 TeV. This can account for the TeV source seen by HESS in the
Galactic Center. Our model predicts a GLAST counterpart to the HESS source with
a luminosity of ~ 10^{35} ergs/s and cooling break at ~ 4 GeV. Synchrotron
radiation from the same relativistic electrons should produce detectable
emission at lower energies, with a surface brightness ~ 10^{32} B^2_{-3}
ergs/s/arcsec^2 from ~ THz to ~ keV, where B_{-3} is the magnetic field
strength in units of mG. The observed level of diffuse thermal X-ray emission
in the central parsec requires B < 300 micro-G in our models. Future detection
of the diffuse synchrotron background in the central parsec can directly
constrain the magnetic field strength, providing an important boundary
condition for models of accretion onto Sgr A*.Comment: submitted to ApJ Letter
Thin-disk laser pump schemes for large number of passes and moderate pump source quality
Novel thin-disk laser pump layouts are proposed yielding an increased number
of passes for a given pump module size and pump source quality. These novel
layouts result from a general scheme which bases on merging two simpler pump
optics arrangements. Some peculiar examples can be realized by adapting
standard commercially available pump optics simply by intro ducing an
additional mirror-pair. More pump passes yield better efficiency, opening the
way for usage of active materials with low absorption. In a standard multi-pass
pump design, scaling of the number of beam passes brings ab out an increase of
the overall size of the optical arrangement or an increase of the pump source
quality requirements. Such increases are minimized in our scheme, making them
eligible for industrial applicationsComment: 16 pages, 9 figure
Demonstrating the feasibility of standardized application program interfaces that will allow mobile/portable terminals to receive services combining UMTS and DVB-T
Crucial to the commercial exploitation of any service combining UMTS and DVB-T is the availability of standardized API’s adapted to the hybrid UMTS and DVB-T network and to the technical limitations of mobile/portable terminals. This paper describes work carried out in the European Commission Framework Program 5 (FP5) project CONFLUENT to demonstrate the feasibility of such Application Program Interfaces (API’s) by enabling the reception of a Multimedia Home Platform (MHP) based application transmitted over DVB-T on five different terminals with parts of the service running on a mobile phone
The effect of the displacement damage on the Charge Collection Efficiency in Silicon Drift Detectors for the LOFT satellite
The technology of Silicon Drift Detectors (SDDs) has been selected for the
two instruments aboard the Large Observatory For X-ray Timing (LOFT) space
mission. LOFT underwent a three year long assessment phase as candidate for the
M3 launch opportunity within the "Cosmic Vision 2015 -- 2025" long-term science
plan of the European Space Agency. During the LOFT assessment phase, we studied
the displacement damage produced in the SDDs by the protons trapped in the
Earth's magnetosphere. In a previous paper we discussed the effects of the Non
Ionising Energy Losses from protons on the SDD leakage current. In this paper
we report the measurement of the variation of Charge Collection Efficiency
produced by displacement damage caused by protons and the comparison with the
expected damage in orbit.Comment: 17 pages, 7 figures. Accepted for publication by Journal of
Instrumentatio
Compact 20-pass thin-disk amplifier insensitive to thermal lensing
We present a multi-pass amplifier which passively compensates for distortions
of the spherical phase front occurring in the active medium. The design is
based on the Fourier transform propagation which makes the output beam
parameters insensitive to variation of thermal lens effects in the active
medium. The realized system allows for 20 reflections on the active medium and
delivers a small signal gain of 30 with M = 1.16. Its novel geometry
combining Fourier transform propagations with 4f-imaging stages as well as a
compact array of adjustable mirrors allows for a layout with a footprint of 400
mm x 1000 mm.Comment: 7 pages, 6 figure
Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks
Sari, Piran, and Narayan have derived analytic formulas to model the spectra
from gamma-ray burst blast waves that are energized by sweeping up material
from the surrounding medium. We extend these expressions to apply to general
radiative regimes and to include the effects of synchrotron self-absorption.
Electron energy losses due to the synchrotron self-Compton process are also
treated in a very approximate way. The calculated spectra are compared with
detailed numerical simulation results. We find that the spectral and temporal
breaks from the detailed numerical simulation are much smoother than the
analytic formulas imply, and that the discrepancies between the analytic and
numerical results are greatest near the breaks and endpoints of the synchrotron
spectra. The expressions are most accurate (within a factor of ~ 3) in the
optical/X-ray regime during the afterglow phase, and are more accurate when
epsilon_e, the fraction of swept-up particle energy that is transferred to the
electrons, is <~ 0.1. The analytic results provide at best order-of-magnitude
accuracy in the self-absorbed radio/infrared regime, and give poor fits to the
self-Compton spectra due to complications from Klein-Nishina effects and
photon-photon opacity.Comment: 16 pages, 7 figures, ApJ, in press, 537, July 1, 2000. Minor changes
in response to referee report, corrected figure
A current driven instability in parallel, relativistic shocks
Recently, Bell has reanalysed the problem of wave excitation by cosmic rays
propagating in the pre-cursor region of a supernova remnant shock front. He
pointed out a strong, non-resonant, current-driven instability that had been
overlooked in the kinetic treatments, and suggested that it is responsible for
substantial amplification of the ambient magnetic field. Magnetic field
amplification is also an important issue in the problem of the formation and
structure of relativistic shock fronts, particularly in relation to models of
gamma-ray bursts. We have therefore generalised the linear analysis to apply to
this case, assuming a relativistic background plasma and a monoenergetic,
unidirectional incoming proton beam. We find essentially the same non-resonant
instability noticed by Bell, and show that also under GRB conditions, it grows
much faster than the resonant waves. We quantify the extent to which thermal
effects in the background plasma limit the maximum growth rate.Comment: 8 pages, 1 figur
Tackling the Monday Morning Quarterback: Applications of Hindsight Bias in Decision-Making Settings
Extant research has focused largely on what causes hindsight distortion. In contrast, this work examines applied aspects related to the bias in decision-making environments. A conceptual framework is provided and recent real–world examples are presented to outline how decision makers—and those who observe them—show hindsight effects. Then, both negative and positive consequences of the bias are outlined. Strategies are presented to reduce negative effects that occur when decision makers show hindsight distortion. Finally, because it is often not possible to avoid or to correct others\u27 hindsight–tainted evaluations, suggestions for coping with the bias are discussed
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