3,239 research outputs found
Certification of the Specific Micropore Volume and the Median Micropore Width of Two Microporous Reference Materials According to Draft-DIN 66135-4, BCR-704, BCR-705.
Abstract not availableJRC.D-Institute for Reference Materials and Measurements (Geel
The dynamical Green's function and an exact optical potential for electron-molecule scattering including nuclear dynamics
We derive a rigorous optical potential for electron-molecule scattering
including the effects of nuclear dynamics by extending the common many-body
Green's function approach to optical potentials beyond the fixed-nuclei limit
for molecular targets. Our formalism treats the projectile electron and the
nuclear motion of the target molecule on the same footing whereby the dynamical
optical potential rigorously accounts for the complex many-body nature of the
scattering target. One central result of the present work is that the common
fixed-nuclei optical potential is a valid adiabatic approximation to the
dynamical optical potential even when projectile and nuclear motion are
(nonadiabatically) coupled as long as the scattering energy is well below the
electronic excitation thresholds of the target. For extremely low projectile
velocities, however, when the cross sections are most sensitive to the
scattering potential, we expect the influences of the nuclear dynamics on the
optical potential to become relevant. For these cases, a systematic way to
improve the adiabatic approximation to the dynamical optical potential is
presented that yields non-local operators with respect to the nuclear
coordinates.Comment: 22 pages, no figures, accepted for publ., Phys. Rev.
Drift dependence of optimal trade execution strategies under transient price impact
We give a complete solution to the problem of minimizing the expected
liquidity costs in presence of a general drift when the underlying market
impact model has linear transient price impact with exponential resilience. It
turns out that this problem is well-posed only if the drift is absolutely
continuous. Optimal strategies often do not exist, and when they do, they
depend strongly on the derivative of the drift. Our approach uses elements from
singular stochastic control, even though the problem is essentially
non-Markovian due to the transience of price impact and the lack in Markovian
structure of the underlying price process. As a corollary, we give a complete
solution to the minimization of a certain cost-risk criterion in our setting
Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000
In this paper we present the results of simultaneous observations of the TeV
blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the
Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope
system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively.
The source was monitored from February 2nd to February 16th and from May 3rd to
May 8th, 2000. We discuss in detail the temporal and spectral properties of the
source. Remarkably, the TeV observations of February 7th/8th showed
statistically significant evidence for substantial TeV flux variability on 30
min time scale. We show the results of modeling the data with a time dependent
homogeneous Synchrotron Self-Compton (SSC) model. The X-ray and TeV gamma-ray
emission strengths and energy spectra together with the rapid flux variability
strongly suggest that the emission volume is approaching the observer with a
Doppler factor of 50 or higher. The different flux variability time scales
observed at X-rays and TeV Gamma-rays indicate that a more detailed analysis
will require inhomogeneous models with several emission zones.Comment: Accepted for Publication in ApJ, 21 Pages, 5 Figure
Rejection of the hypothesis that Markarian 501 TeV photons are pure Bose-Einstein condensates
The energy spectrum of the Blazar type galaxy Markarian 501 (Mrk 501) as
measured by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cerenkov telescopes
extends beyond 16 TeV and constitutes the most energetic photons observed from
an extragalactic object. A fraction of the emitted spectrum is possibly
absorbed in interactions with low energy photons of the diffuse extragalactic
infrared radiation, which in turn offers the unique possibility to measure the
diffuse infrared radiation density by TeV spectroscopy. The upper limit on the
density of the extragalactic infrared radiation derived from the TeV
observations imposes constraints on models of galaxy formation and stellar
evolution. One of the recently published ideas to overcome severe absorption of
TeV photons is based upon the assumption that sources like Mrk 501 could
produce Bose-Einstein condensates of coherent photons. The condensates would
have a higher survival probability during the transport in the diffuse
radiation field and could mimic TeV air shower events. The powerful
stereoscopic technique of the HEGRA air Cerenkov telescopes allows to test this
hypothesis by reconstructing the penetration depths of TeV air shower events:
Air showers initiated by Bose-Einstein condensates are expected to reach the
maximum of the shower development in the atmosphere earlier than single photon
events. By comparing the energy-dependent penetration depths of TeV photons
from Mrk 501 with those from the TeV standard-candle Crab Nebula and simulated
air shower events, we can reject the hypothesis that TeV photons from Mrk 501
are pure Bose-Einstein condensates.Comment: 9 pages, 2 figures, published by ApJ Letters, revised version
(simulation results added
The TeV Energy Spectrum of Mkn 501 Measured with the Stereoscopic Telescope System of HEGRA during 1998 and 1999
During 1997, the BL Lac object Mkn 501 went into an extraordinary state of
high X-ray and TeV gamma-ray activity, lasting more than 6 months. In this
paper we report on the TeV emission characteristics of the source in the
subsequent years of 1998 and 1999 as measured with the Stereoscopic Cherenkov
Telescope System of HEGRA (La Palma, Canary Islands). Our observations reveal a
1998-1999 mean emission level at 1 TeV of 1/3 of the flux of the Crab Nebula, a
factor of 10 lower than during the year of 1997. A dataset of 122 observations
hours with the HEGRA telescope system makes it possible to assess for the first
time the Mkn 501 TeV energy spectrum for a mean flux level substantially below
that of the Crab Nebula with reasonable statistical accuracy. Excluding the
data of a strong flare, we find evidence that the 1998--1999 low-flux spectrum
is substantially softer (by 0.44+-0.1(stat) in spectral index) than the 1997
time averaged spectrum. The 500 GeV to 10 TeV energy spectrum can well be
described by a power law model with exponential cutoff: dN/dE ~ E^(-alpha)
exp(-E/E0) with alpha=2.31+-0.22(stat), and E0=5.1 (-2.3+7.8)(stat) TeV. Within
statistical accuracy, also a pure power law model gives an acceptable fit to
the data: dN/dE ~ E^(-Gamma) with Gamma=2.76+-0.08(stat). After presenting the
1998-1999 TeV characteristics of the source we discuss the implications of the
results.Comment: Accepted for publication in The Astrophysical Journal, Part 1, on
August 4th, 200
The unidentified TeV source (TeVJ2032+4130) and surrounding field: Final HEGRA IACT-System results
The unidentified TeV source in Cygnus is now confirmed by follow-up
observations from 2002 with the HEGRA stereoscopic system of Cherenkov
Telescopes. Using all data (1999 to 2002) we confirm this new source as steady
in flux over the four years of data taking, extended with radius 6.2 arcmin
(+-1.2 arcmin (stat) +-0.9 arcmin (sys)) and exhibiting a hard spectrum with
photon index -1.9. It is located in the direction of the dense OB stellar
association, Cygnus OB2. Its integral flux above energies E>1 TeV amounts to
\~5% of the Crab assuming a Gaussian profile for the intrinsic source
morphology. There is no obvious counterpart at radio, optical nor X-ray
energies, leaving TeVJ2032+4130 presently unidentified. Observational
parameters of this source are updated here and some astrophysical discussion is
provided. Also included are upper limits for a number of other interesting
sources in the FoV, including the famous microquasar Cygnus X-3.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
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