3,564 research outputs found
Radiation Damping in the Photoionization of Fe^{14+}
A theoretical investigation of photoabsorption and photoionization of
Fe^{14+} extending beyond an earlier frame transformation R-matrix
implementation is performed using a fully-correlated, Breit-Pauli R-matrix
formulation including both fine-structure splitting of strongly-bound
resonances and radiation damping. The radiation damping of
resonances gives rise to a resonant photoionization cross section that is
significantly lower than the total photoabsorption cross section. Furthermore,
the radiation-damped photoionization cross section is found to be in good
agreement with recent experimental results once a global shift in energy of
eV is applied. These findings have important implications.
Firstly, the presently available synchrotron experimental data are applicable
only to photoionization processes and not to photoabsorption; the latter is
required in opacity calculations. Secondly, our computed cross section, for
which the L-shell ionization threshold is aligned with the NIST value, shows a
series of Rydberg resonances that are uniformly 3-4 eV
higher in energy than the corresponding experimental profiles, indicating that
the L-shell threshold energy values currently recommended by NIST are likely in
error.Comment: 4 pages, 1 figures, and 2 table
Comprehensive transient-state study for CARMENES-NIR high thermal stability
CARMENES has been proposed as a next-generation instrument for the 3.5m Calar
Alto Telescope. Its objective is finding habitable exoplanets around M dwarfs
through radial velocity measurements (m/s level) in the near-infrared.
Consequently, the NIR spectrograph is highly constraint regarding
thermal/mechanical requirements. As a first approach, the thermal stability has
been limited to \pm 0.01K (within year period) over a working temperature of
243K. This can be achieved by means of several temperature-controlled rooms.
The options considered to minimise the complexity of the thermal design are
here presented, as well as the transient-state thermal analyses realised to
make the best choice
Upper limits on neutrino masses from the 2dFGRS and WMAP: the role of priors
Solar, atmospheric, and reactor neutrino experiments have confirmed neutrino
oscillations, implying that neutrinos have non-zero mass, but without pinning
down their absolute masses. While it is established that the effect of
neutrinos on the evolution of cosmic structure is small, the upper limits
derived from large-scale structure data could help significantly to constrain
the absolute scale of the neutrino masses. In a recent paper the 2dF Galaxy
Redshift Survey (2dFGRS) team provided an upper limit m_nu,tot < 2.2 eV, i.e.
approximately 0.7 eV for each of the three neutrino flavours, or phrased in
terms of their contributioin to the matter density, Omega_nu/Omega_m < 0.16.
Here we discuss this analysis in greater detail, considering issues of assumed
'priors' like the matter density Omega_m and the bias of the galaxy
distribution with respect the dark matter distribution. As the suppression of
the power spectrum depends on the ratio Omega_nu/Omega_m, we find that the
out-of- fashion Mixed Dark Matter Model, with Omega_nu=0.2, Omega_m=1 and no
cosmological constant, fits the 2dFGRS power spectrum and the CMB data
reasonably well, but only for a Hubble constant H_0<50 km/s/Mpc. As a
consequence, excluding low values of the Hubble constant, e.g. with the HST Key
Project is important in order to get a strong constraint on the neutrino
masses. We also comment on the improved limit by the WMAP team, and point out
that the main neutrino signature comes from the 2dFGRS and the Lyman alpha
forest.Comment: 24 pages, 12 figures Minor changes to matched version published in
JCA
Adaptive Importance Sampling in General Mixture Classes
In this paper, we propose an adaptive algorithm that iteratively updates both
the weights and component parameters of a mixture importance sampling density
so as to optimise the importance sampling performances, as measured by an
entropy criterion. The method is shown to be applicable to a wide class of
importance sampling densities, which includes in particular mixtures of
multivariate Student t distributions. The performances of the proposed scheme
are studied on both artificial and real examples, highlighting in particular
the benefit of a novel Rao-Blackwellisation device which can be easily
incorporated in the updating scheme.Comment: Removed misleading comment in Section
Reduced model for H-mode sustainment in unfavorable drift configuration in ASDEX Upgrade
A recently developed reduced model of H-mode sustainment based on
interchange-drift-Alfv\'en turbulence description in the vicinity of the
separatrix matching experimental observations in ASDEX Upgrade has been
extended to experiments with the unfavorable drift. The combination
with the theory of the magnetic-shear-induced Reynolds stress offers a
possibility to quantitatively explain the phenomena. The extension of the
Reynolds stress estimate in the reduced model via the magnetic shear
contribution is able to reproduce the strong asymmetry in the access conditions
depending on the ion drift orientation in agreement with
experimental observations. The Reynolds stress profile asymmetry predicted by
the magnetic shear model is further extended by comparison with GRILLIX and
GENE-X simulations matched with comparable experiments in realistic X-point
geometry. The predictions of the radial electric field well depth and its
difference between the favorable and unfavorable configurations at the same
heating power from the extended model also show consistency with experimental
measurements.Comment: Submitted to Nuclear Fusio
The Planck-LFI flight model composite waveguides
The Low Frequency Instrument on board the PLANCK satellite is designed to
give the most accurate map ever of the CMB anisotropy of the whole sky over a
broad frequency band spanning 27 to 77 GHz. It is made of an array of 22
pseudo-correlation radiometers, composed of 11 actively cooled (20 K) Front End
Modules (FEMs), and 11 Back End Modules (BEMs) at 300K. The connection between
the two parts is made with rectangular Wave Guides. Considerations of different
nature (thermal, electromagnetic and mechanical), imposed stringent
requirements on the WGs characteristics and drove their design. From the
thermal point of view, the WG should guarantee good insulation between the FEM
and the BEM sections to avoid overloading the cryocooler. On the other hand it
is essential that the signals do not undergo excessive attenuation through the
WG. Finally, given the different positions of the FEM modules behind the focal
surface and the mechanical constraints given by the surrounding structures,
different mechanical designs were necessary. A composite configuration of
Stainless Steel and Copper was selected to satisfy all the requirements. Given
the complex shape and the considerable length (about 1.5-2 m), manufacturing
and testing the WGs was a challenge. This work deals with the development of
the LFI WGs, including the choice of the final configuration and of the
fabrication process. It also describes the testing procedure adopted to fully
characterize these components from the electromagnetic point of view and the
space qualification process they underwent. Results obtained during the test
campaign are reported and compared with the stringent requirements. The
performance of the LFI WGs is in line with requirements, and the WGs were
successfully space qualified.Comment: this paper is part of the Prelaunch status LFI papers published on
JINST: http://www.iop.org/EJ/journal/-page=extra.proc5/jins
Numerical simulation of time delay interferometry for eLISA/NGO
eLISA/NGO is a new gravitational wave detection proposal with arm length of
10^6 km and one interferometer down-scaled from LISA. Just like LISA and
ASTROD-GW, in order to attain the requisite sensitivity for eLISA/NGO, laser
frequency noise must be suppressed to below the secondary noises such as the
optical path noise, acceleration noise etc. In previous papers, we have
performed the numerical simulation of the time delay interferometry (TDI) for
LISA and ASTROD-GW with one arm dysfunctional by using the CGC 2.7 ephemeris.
The results are well below their respective limits which the laser frequency
noise is required to be suppressed. In this paper, we follow the same procedure
to simulate the time delay interferometry numerically. To do this, we work out
a set of 1000-day optimized mission orbits of the eLISA/NGO spacecraft starting
at January 1st, 2021 using the CGC 2.7 ephemeris framework. We then use the
numerical method to calculate the residual optical path differences in the
second-generation TDI solutions as in our previous papers. The maximum path
length difference, for all configurations calculated, is below 13 mm (43 ps).
It is well below the limit which the laser frequency noise is required to be
suppressed for eLISA/NGO. We compare and discuss the resulting differences due
to the different arm lengths for various mission proposals -- eLISA/NGO, an
NGO-LISA-type mission with a nominal arm length of 2 x 10^6 km, LISA and
ASTROD-GW.Comment: 17 pages, 13 figures, 3 tables, minor changes in description to match
the accepted version of Classical and Quantum Gravity. arXiv admin note: text
overlap with arXiv:1102.496
Late-Time X-ray Flares during GRB Afterglows: Extended Internal Engine Activity
Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. These flares were sometimes intense, rapid, and late
relative to the nominal prompt phase. The most intense of these flares was
observed by XRT with a flux >500 the afterglow. This burst then surprised
observers by flaring again after >10000 s. The intense flare can be most easily
understood within the context of the standard fireball model, if the internal
engine that powers the prompt GRB emission is still active at late times.
Recent observations indicate that X-ray flares are detected in ~1/3 of XRT
detected afterglows. By studying the properties of the varieties of flares
(such as rise/fall time, onset time, spectral variability, etc.) and relating
them to overall burst properties, models of flare production and the GRB
internal engine can be constrained.Comment: To appear in the proceedings of the 16th Annual October Astrophysics
Conference in Maryland "Gamma Ray Bursts in the Swift Era
Dry-season length and runoff control annual variability in stream DOC dynamics in a small, shallowgroundwater-dominated agricultural watershed
International audienceAs a phenomenon integrating climate conditions and hydrological control of the connection betweenstreams and terrestrial dissolved organic carbon (DOC) sources, groundwater dynamics controlpatterns of stream DOC characteristics (concentrations and fluxes). Influence of intra-annualvariations in groundwater level, discharge and climatic factors on DOC concentrations and fluxeswere assessed over 13 years at the headwater watershed of Kervidy-Naizin (5 km²) in westernFrance. Four seasonal periods were delineated within each year according to groundwaterfluctuations (A: rewetting, B: high flow, C: recession, and D: drought). Annual and seasonal baseflow vs stormflow DOC concentrations were defined based on daily hydrograph readings. Highinter-annual variability of annual DOC fluxes (5.4-39.5 kg.ha-1.yr-1) indicates that several years ofdata are required to encompass variations in water flux to evaluate the actual DOC export capacity ofa watershed. Inter-annual variability of mean annual DOC concentrations was much lower (4.9-7.5mg C.l-1), with concentrations decreasing within each year from ca. 9.2 mg C.l-1 in A to ca. 3.0 mgC.l-1 in C. This indicates an intra-annual pattern of stream DOC concentrations controlled by DOCsource characteristics and groundwater dynamics very similar across years. Partial least squareregressions combined with multiple linear regressions showed that the dry season characteristics(length and drawdown) determine the mean annual DOC concentration while annual runoffdetermines the annual flux. Antagonistic mechanisms of production-accumulation and dilution depletioncombined with an unlimited DOC supply from riparian wetland soils can mitigate theresponse of stream concentrations to global changes and climatic variations
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