2,882 research outputs found
Simulating terahertz quantum cascade lasers: Trends from samples from different labs
We present a systematic comparison of the results from our non-equilibrium
Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum
cascade lasers previously published in the literature. Employing identical
material and simulation parameters for all samples, we observe that
discrepancies between measured and calculated peak currents are similar for
samples from a given group. This suggests that the differences between
experiment and theory are partly due to a lacking reproducibility for devices
fabricated at different laboratories. Varying the interface roughness height
for different devices, we find that the peak current under lasing operation
hardly changes, so that differences in interface quality appear not to be the
sole reason for the lacking reproducibility.Comment: 9 pages, 6 figures; section VI with 2 figures added in v2; accepted
for publication in J. Appl. Phy
Superlattice gain in positive differential conductivity region
We analyze theoretically a superlattice structure proposed by A. Andronov et
al. [JETP Lett 102, 207 (2015)] to give Terahertz gain for an operation point
with positive differential conductivity. Here we confirm the existence of gain
and show that an optimized structure displays gain above 20 cm at low
temperatures, so that lasing may be observable. Comparing a variety of
simulations, this gain is found to be strongly affected by elastic scattering.
It is shown that the dephasing modifies the nature of the relevant states, so
that the common analysis based on Wannier-Stark states is not reliable for a
quantitative description of the gain in structures with extremely diagonal
transitions.Comment: 4 pages, 5 figure
Feeding Versus Feedback in AGNs from Near-Infrared IFU Observations: The Case of Mrk79
We have mapped the gaseous kinematics and the emission-line flux
distributions and ratios from the inner ~680pc radius of the Seyfert 1 galaxy
Mrk79, using two-dimensional (2D) near-IR J- and Kl-band spectra obtained with
the Gemini instrument NIFS at a spatial resolution of ~100pc and velocity
resolution of ~40km/s. The molecular hydrogen flux distribution presents two
spiral arms extending by ~700pc, one to the north and another to the south of
the nucleus, with an excitation indicating heating by X-rays from the central
source. The low velocity dispersion (sigma~50km/s) and rotation pattern
supports a location of the H2 gas in the disk of the galaxy. Blueshifts
observed along the spiral arm in the far side of the galaxy and redshifts in
the spiral arm in the near side, suggest that the spiral arms are feeding
channels of H2 to the inner 200pc. From channel maps along the H2 l2.1218um
emission-line profile we estimate a mass inflow rate of ~4E-3 M_Sun/year, which
is one order of magnitude smaller than the mass accretion rate necessary to
power the AGN of Mrk79. The emission from the ionized gas (traced by Pabeta and
[FeII]l1.2570um emission lines) is correlated with the radio jet and with the
narrow-band [OIII] flux distribution. Its kinematics shows both rotation and
outflows to the north and south of the nucleus. The ionized gas mass outflow
rate through a cross section with radius ~320pc located at a distance of ~455pc
from the nucleus is 3.5 MSun/year, which is much larger than the AGN mass
accretion rate, indicating that most of the outflowing gas originates in the
interstellar medium surrounding the galaxy nucleus, which is pushed away by a
nuclear jet.Comment: Accepted by MNRA
Nonlinear response of quantum cascade structures
The gain spectrum of a terahertz quantum cascade laser is analysed by a non
equilibrium Green's functions approach. Higher harmonics of the response
function were retrievable, providing a way to approach nonlinear phenomena in
quantum cascade lasers theoretically. Gain is simulated under operation
conditions and results are presented both for linear response and strong laser
fields. An iterative way of reconstructing the field strength inside the laser
cavity at lasing conditions is described using a measured value of the level of
the losses of the studied system. Comparison with recent experimental data from
time-domain-spectroscopy indicates that the experimental situation is beyond
linear response.Comment: 4 pages, 3 figures included in text, to appear in Applied Physics
Letter
A sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide NLRs -- Quasar light echos?
We report the discovery of Seyfert-2 galaxies in SDSS-DR8 with galaxy-wide,
ultra-luminous narrow-line regions (NLRs) at redshifts z=0.2-0.6. With a space
density of 4.4 per cubic Gpc at z~0.3, these "Green Beans" (GBs) are amongst
the rarest objects in the Universe. We are witnessing an exceptional and/or
short-lived phenomenon in the life cycle of AGN. The main focus of this paper
is on a detailed analysis of the GB prototype galaxy J2240-0927 (z=0.326). Its
NLR extends over 26x44 kpc and is surrounded by an extended narrow-line region
(ENLR). With a total [OIII]5008 luminosity of (5.7+/-0.9)x10e43 erg/s, this is
one of the most luminous NLR known around any type-2 galaxy. Using VLT/XSHOOTER
we show that the NLR is powered by an AGN, and we derive resolved extinction,
density and ionization maps. Gas kinematics is disturbed on a global scale, and
high velocity outflows are absent or faint. This NLR is unlike any other NLR or
extended emission line region (EELR) known. Spectroscopy with Gemini/GMOS
reveals extended, high luminosity [OIII] emission also in other GBs. WISE
24micron luminosities are 5-50 times lower than predicted by the [OIII] fluxes,
suggesting that the NLRs reflect earlier, very active quasar states that have
strongly subsided in less than a galaxies' light crossing time. These light
echos are about 100 times more luminous than any other such light echo known to
date. X-ray data are needed for photo-ionization modeling and to verify the
light echos.Comment: This is an update of our previous submission (1211.7098). Apart from
layout it is identical to the publication in ApJ. Compared to v1 (1211.7098)
type settings and language errors have been corrected. We also added 4 more
redshifts, confirming 3 sources (objects #002, #017, #21) and rejecting one
(#020, low-z HII region
Temporary formworks as torsional bracing system for steel-concrete composite bridges during concreting of the deck
A critical stage for the construction of steel-concrete composite bridges occurs during casting of the bridge deck, when the wet concrete has still not hardened. The entire construction load is then taken by the non-composite steel sections. Bracing may be needed to make the slender girders rigid enough to resist lateral torsional buckling during this phase. If temporary formwork could be attached and be shown to work as torsional bracing; material could be saved and the construction phase could be safer. Within this thesis some of the commonly used temporary formworks are described and analyzed to see if some of them could work as discrete torsional bracing. Findings from this investigation were that the often used formwork system CUPLOK was easy and suitable to be modified and attached to the girders as discrete torsional bracing. With the modified CUPLOK system attached, three different systems were numerically analyzed using the finite element program Abaqus. One with formwork attached to a laboratory test beam set-up with dimensions according to Mehri and two on real bridges with trapezoidal respective I-girder cross section. Findings from analyzes were that with the modified CUPLOK system attached, the stiffness were increased dramatically on the slender I-girder system and also, but relatively less, on the less slender real bridge I-girder system. No stiffening effect was shown on the specific trapezoidal cross section analyzed herein. Finally the findings are discussed and further research proposed
Polycyclic Aromatic Hydrocarbon in the Central Region of the Seyfert 2 Galaxy NGC1808
We present mid infrared (MIR) spectra of the Seyfert 2 (Sy 2) galaxy NGC
1808, obtained with the Gemini's Thermal-Region Camera Spectrograph (T-ReCS) at
a spatial resolution of 26 pc. The high spatial resolution allowed us to detect
bright polycyclic aromatic hydrocarbons (PAHs) emissions at 8.6micron and
11.3micron in the galaxy centre (26 pc) up to a radius of 70 pc from the
nucleus. The spectra also present [Ne ii]12.8micron ionic lines, and H2
S(2)12.27micron molecular gas line. We found that the PAHs profiles are similar
to Peeters's A class, with the line peak shifted towards the blue. The
differences in the PAH line profiles also suggests that the molecules in the
region located 26 pc NE of the nucleus are more in the neutral than in the
ionised state, while at 26 pc SW of the nucleus, the molecules are mainly in
ionised state. After removal of the underlying galaxy contribution, the nuclear
spectrum can be represented by a Nenkova's clumpy torus model, indicating that
the nucleus of NGC 1808 hosts a dusty toroidal structure with an angular cloud
distribution of sigma = 70degree, observer's view angle i = 90degree, and an
outer radius of R0 = 0.55 pc. The derived column density along the line of
sight is NH = 1.5 x 10^24 cm-2, which is sufficient to block the hard radiation
from the active nucleus, and would explain the presence of PAH molecules near
to the NGC 1808's active nucleus.Comment: Accepted by MNRAS 2012 December
Magnetic latitude effects in the solar wind
The Weber-Davis model of the solar wind is generalized to include the effects of latitude. The principal assumptions of high electrical conductivity, rotational symmetry, the polytropic relation between pressure and density, and a flow-alined field in a system rotating with the sun, are retained. An approximate solution to the resulting equations for spherical boundary conditions at the base of the corona indicates a small component of latitudinal flow toward the solar poles at large distances from the sun as result of latitudinal magnetic forces
Extended Gas in Seyfert Galaxies: Near Infrared Observations of NGC 2110 and Circinus
We present results of near--IR long-slit spectroscopy in the J and K bands of
the Seyfert 2 galaxies NGC 2110 and Circinus, investigating the gaseous
distribution, excitation, reddening and kinematics. In NGC 2110, the emission
line ratio [FeII]/Pa beta increases towards the nucleus (to ~ 7). The nuclear
[Fe II]1.257 (microns) and Pa beta lines are broader (FWHM ~ 500 km/s) than the
H2 (2.121) line (FWHM ~ 300 km/s). Both these results suggest that shocks,
driven by the radio jet, are an important source of excitation of [Fe II]. The
H2 excitation appears to be dominated by X-rays from the nucleus. In Circinus,
both [FeII]/Pa beta and H2/Br gamma decrease from ~ 2 at 4 arcsec from the
nucleus to nuclear values of ~ 0.6 and ~ 1, respectively, suggesting that the
starburst dominates the nuclear excitation, while the AGN dominates the
excitation further out (r > 2 arcsec). For both galaxies, the gaseous
kinematics are consistent with circular rotation in the plane of the disk. Our
rotation curves suggest that the nucleus (identified with the peak of the IR
continuum) is displaced from the kinematic centre of the galaxies. This effect
has been observed previously in NGC 2110 based on the kinematics of optical
emission lines, but the displacement is smaller in the infrared, suggesting the
effect is related to obscuration. The continuum J-K colours of the nuclear
region indicate a red stellar population in NGC 2110 and a reddened young
stellar population in Circinus. Right at the nucleus of both galaxies, the
colours are redder, apparently a result of hot dust emission from the inner
edge of a circumnuclear torus.Comment: 11 pages, 14 figures, accepted for publication in MNRA
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