4,309 research outputs found
Exploring the temporally resolved electron density evolution in EUV induced plasmas
We measured for the first time the electron density in an Extreme
Ultra-Violet induced plasma. This is achieved in a low-pressure argon plasma by
using a method called microwave cavity resonance spectroscopy. The measured
electron density just after the EUV pulse is m. This
is in good agreement with a theoretical prediction from photo ionization, which
yields a density of m. After the EUV pulse the density
slightly increase due to electron impact ionization. The plasma (i.e. electron
density) decays in tens of microseconds.Comment: 3 pages, 4 figure
Molecfit: A general tool for telluric absorption correction II. Quantitative evaluation on ESO-VLT X-Shooter spectra
Context: Absorption by molecules in the Earth's atmosphere strongly affects
ground-based astronomical observations. The resulting absorption line strength
and shape depend on the highly variable physical state of the atmosphere, i.e.
pressure, temperature, and mixing ratio of the different molecules involved.
Usually, supplementary observations of so-called telluric standard stars (TSS)
are needed to correct for this effect, which is expensive in terms of telescope
time. We have developed the software package molecfit to provide synthetic
transmission spectra based on parameters obtained by fitting narrow ranges of
the observed spectra of scientific objects. These spectra are calculated by
means of the radiative transfer code LBLRTM and an atmospheric model. In this
way, the telluric absorption correction for suitable objects can be performed
without any additional calibration observations of TSS. Aims: We evaluate the
quality of the telluric absorption correction using molecfit with a set of
archival ESO-VLT X-Shooter visible and near-infrared spectra. Methods: Thanks
to the wavelength coverage from the U to the K band, X-Shooter is well suited
to investigate the quality of the telluric absorption correction with respect
to the observing conditions, the instrumental set-up, input parameters of the
code, the signal-to-noise of the input spectrum, and the atmospheric profiles.
These investigations are based on two figures of merit, I_off and I_res, that
describe the systematic offsets and the remaining small-scale residuals of the
corrections. We also compare the quality of the telluric absorption correction
achieved with moelcfit to the classical method based on a telluric standard
star. (Abridged)Comment: Acc. by A&A; Software available via ESO:
http://www.eso.org/sci/software/pipelines/skytools
Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
Context: The interaction of the light from astronomical objects with the
constituents of the Earth's atmosphere leads to the formation of telluric
absorption lines in ground-based collected spectra. Correcting for these lines,
mostly affecting the red and infrared region of the spectrum, usually relies on
observations of specific stars obtained close in time and airmass to the
science targets, therefore using precious observing time. Aims: We present
molecfit, a tool for correcting for telluric absorption lines based on
synthetic modelling of the Earth's atmospheric transmission. Molecfit is
versatile and can be used with data obtained with various ground-based
telescopes and instruments. Methods: Molecfit combines a publicly available
radiative transfer code, a molecular line database, atmospheric profiles, and
various kernels to model the instrument line spread function. The atmospheric
profiles are created by merging a standard atmospheric profile representative
of a given observatory's climate, of local meteorological data, and of
dynamically retrieved altitude profiles for temperature, pressure, and
humidity. We discuss the various ingredients of the method, its applicability,
and its limitations. We also show examples of telluric line correction on
spectra obtained with a suite of ESO Very Large Telescope (VLT) instruments.
Results: Compared to previous similar tools, molecfit takes the best results
for temperature, pressure, and humidity in the atmosphere above the observatory
into account. As a result, the standard deviation of the residuals after
correction of unsaturated telluric lines is frequently better than 2% of the
continuum. Conclusion: Molecfit is able to accurately model and correct for
telluric lines over a broad range of wavelengths and spectral resolutions.
(Abridged)Comment: 18 pages, 13 figures, 5 tables, accepted for publication in Astronomy
and Astrophysic
The METCRAX II Field Experiment: A Study of Downslope Windstorm-Type Flows in Arizona\u2019s Meteor Crater
The second Meteor Crater Experiment (METCRAX II) was conducted in October 2013 at Arizona\u2019s Meteor Crater. The experiment was designed to investigate nighttime downslope windstorm 12type flows that form regularly above the inner southwest sidewall of the 1.2-km diameter crater as a southwesterly mesoscale katabatic flow cascades over the crater rim. The objective of METCRAX II is to determine the causes of these strong, intermittent, and turbulent inflows that bring warm-air intrusions into the southwest part of the crater. This article provides an overview of the scientific goals of the experiment; summarizes the measurements, the crater topography, and the synoptic meteorology of the study period; and presents initial analysis results
LOFAR early-time search for coherent radio emission from GRB 180706A
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3 limit of 1.7 mJy beam at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.Peer reviewedFinal Accepted Versio
Using sonic anemometer temperature to measure sensible heat flux in strong winds
Sonic anemometers simultaneously measure the turbulent fluctuations of vertical wind (<i>w</i>') and sonic temperature (<i>T</i><sub>s</sub>'), and are commonly used to measure sensible heat flux (<i>H</i>). Our study examines 30-min heat fluxes measured with a Campbell Scientific CSAT3 sonic anemometer above a subalpine forest. We compared <i>H</i> calculated with <i>T</i><sub>s</sub> to <i>H</i> calculated with a co-located thermocouple and found that, for horizontal wind speed (<i>U</i>) less than 8 m s<sup>−1</sup>, the agreement was around ±30 W m<sup>−2</sup>. However, for <i>U</i> &asymp; 8 m s<sup>−1</sup>, the CSAT <i>H</i> had a generally positive deviation from <i>H</i> calculated with the thermocouple, reaching a maximum difference of ≈250 W m<sup>−2</sup> at <i>U</i> ≈ 18 m s<sup>−1</sup>. With version 4 of the CSAT firmware, we found significant underestimation of the speed of sound and thus <i>T</i><sub>s</sub> in high winds (due to a delayed detection of the sonic pulse), which resulted in the large CSAT heat flux errors. Although this <i>T</i><sub>s</sub> error is qualitatively similar to the well-known fundamental correction for the crosswind component, it is quantitatively different and directly related to the firmware estimation of the pulse arrival time. For a CSAT running version 3 of the firmware, there does not appear to be a significant underestimation of <i>T</i><sub>s</sub>; however, a <i>T</i><sub>s</sub> error similar to that of version 4 may occur if the CSAT is sufficiently out of calibration. An empirical correction to the CSAT heat flux that is consistent with our conceptual understanding of the <i>T</i><sub>s</sub> error is presented. Within a broader context, the surface energy balance is used to evaluate the heat flux measurements, and the usefulness of side-by-side instrument comparisons is discussed
Anomalous Chiral Behavior in Quenched Lattice QCD
A study of the chiral behavior of pseudoscalar masses and decay constants is
carried out in quenched lattice QCD with Wilson fermions. Using the modified
quenched approximation (MQA) to cure the exceptional configuration problem,
accurate results are obtained for pion masses as low as 200 MeV. The
anomalous chiral log effect associated with quenched loops is studied
in both the relation between vs. and in the light-mass
behavior of the pseudoscalar and axial vector matrix elements. The size of
these effects agrees quantitatively with a direct measurement of the
hairpin graph, as well as with a measurement of the topological susceptibility,
thus providing several independent and quantitatively consistent determinations
of the quenched chiral log parameter . For with
clover-improved fermions all results are consistent with
.Comment: 51 pages, 20 figures, Late
Transport model analysis of the transverse momentum and rapidity dependence of pion interferometry at SPS energies
Based on the UrQMD transport model, the transverse momentum and the rapidity
dependence of the Hanbury-Brown-Twiss (HBT) radii , , as well
as the cross term at SPS energies are investigated and compared with
the experimental NA49 and CERES data. The rapidity dependence of the ,
, is weak while the is significantly increased at large
rapidities and small transverse momenta. The HBT "life-time" issue (the
phenomenon that the calculated value is larger than
the correspondingly extracted experimental data) is also present at SPS
energies.Comment: 17 pages, 11 figure
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