755 research outputs found
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
Streaming Motions Towards the Supermassive Black Hole in NGC 1097
We have used GMOS-IFU and high resolution HST-ACS observations to map, in
unprecedented detail, the gas velocity field and structure within the 0.7 kpc
circumnuclear ring of the SBb LINER/Seyfert 1 galaxy NGC 1097. We find clear
evidence of radial streaming motions associated with spiral structures leading
to the unresolved (<3.5 parsecs) nucleus, which we interpret as part of the
fueling chain by which gas is transported to the nuclear starburst and
supermassive black hole.Comment: 4 pages, 3 figures using emulateapj. Accepted for publication in
Astrophysical Journal Letters. Download high-resolution version from
http://www.astro.uu.se/~kambiz/DOC/paper-N1097.pd
Reverberation Mapping and the Physics of Active Galactic Nuclei
Reverberation-mapping campaigns have revolutionized our understanding of AGN.
They have allowed the direct determination of the broad-line region size,
enabled mapping of the gas distribution around the central black hole, and are
starting to resolve the continuum source structure. This review describes the
recent and successful campaigns of the International AGN Watch consortium,
outlines the theoretical background of reverberation mapping and the
calculation of transfer functions, and addresses the fundamental difficulties
of such experiments. It shows that such large-scale experiments have resulted
in a ``new BLR'' which is considerably different from the one we knew just ten
years ago. We discuss in some detail the more important new results, including
the luminosity-size-mass relationship for AGN, and suggest ways to proceed in
the near future.Comment: Review article to appear in Astronomical Time Series, Proceedings of
the Wise Observatory 25th Ann. Symposium. 24 pages including 7 figure
The characterization and role of zinc binding in yeast Cox4.
Yeast Cox4 is a zinc binding subunit of cytochrome c oxidase. Cox4 is the only cofactor-containing subunit that is not directly part of the catalytic core of the enzyme located in the mitochondrial inner membrane. The Zn(II) site is shown to be distinct from the bovine ortholog, as it results from the x-ray structure of the entire cytochrome c oxidase in having a single histidyl residue and three conserved cysteines residues in the coordination sphere. Substitutions at the Cys ligand positions result in non-functional Cox4 proteins that fail to lead to cytochrome oxidase assembly. Limited function exists in His-119 mutants when overexpressed. Zn(II) binding in Cox4 is, therefore, important for the stability of the complex. The solution structure of yeast Cox4 elucidated by multidimensional NMR reveals a C-terminal globular domain consisting of two beta sheets analogous to the bovine ortholog except the loop containing the coordinating His in the yeast protein and the fourth Cys in the bovine protein are in different positions in the two structures. The conformation of this loop is dictated by the different sequence position of the fourth coordinating zinc ligand. The Zn(II) ion is buried within the domain, consistent with its role in structural stability. Potential functions of this matrix-facing subunit are discussed
The Earliest Near-infrared Time-series Spectroscopy of a Type Ia Supernova
We present ten medium-resolution, high signal-to-noise ratio near-infrared
(NIR) spectra of SN 2011fe from SpeX on the NASA Infrared Telescope Facility
(IRTF) and Gemini Near-Infrared Spectrograph (GNIRS) on Gemini North, obtained
as part of the Carnegie Supernova Project. This data set constitutes the
earliest time-series NIR spectroscopy of a Type Ia supernova (SN Ia), with the
first spectrum obtained at 2.58 days past the explosion and covering -14.6 to
+17.3 days relative to B-band maximum. C I {\lambda}1.0693 {\mu}m is detected
in SN 2011fe with increasing strength up to maximum light. The delay in the
onset of the NIR C I line demonstrates its potential to be an effective tracer
of unprocessed material. For the first time in a SN Ia, the early rapid decline
of the Mg II {\lambda}1.0927 {\mu}m velocity was observed, and the subsequent
velocity is remarkably constant. The Mg II velocity during this constant phase
locates the inner edge of carbon burning and probes the conditions under which
the transition from deflagration to detonation occurs. We show that the Mg II
velocity does not correlate with the optical light-curve decline rate
{\Delta}m15. The prominent break at ~1.5 {\mu}m is the main source of concern
for NIR k-correction calculations. We demonstrate here that the feature has a
uniform time evolution among SNe Ia, with the flux ratio across the break
strongly correlated with {\Delta}m15. The predictability of the strength and
the onset of this feature suggests that the associated k-correction
uncertainties can be minimized with improved spectral templates.Comment: 14 pages, 13 figures, accepted for publication in Ap
HST FOC spectroscopy of the NLR of NGC 4151. I. Gas kinematics
We present the results from a detailed kinematic analysis of both
ground-based, and Hubble Space Telescope/Faint Object Camera long-slit
spectroscopy at sub-arcsec spatial resolution, of the narrow-line region of NGC
4151. In agreement with previous work, the extended emission gas (R > 4") is
found to be in normal rotation in the galactic plane, a behaviour that we were
able to trace even across the nuclear region, where the gas is strongly
disturbed by the interaction with the radio jet, and connects smoothly with the
large scale rotation defined by the neutral gas emission. The HST data, at
0.029" spatial resolution, allow us for the first time to truly isolate the
kinematic behaviour of the individual clouds in the inner narrow-line region.
We find that, underlying the perturbations introduced by the radio ejecta, the
general velocity field can still be well represented by planar rotation down to
a radius of ~ 0.5" (30 pc), distance at which the rotation curve has its
turnover.
The most striking result that emerges from our analysis is that the galaxy
potential derived fitting the rotation curve changes from a "dark halo" at the
ENLR distances to dominated by the central mass concentration in the NLR, with
an almost Keplerian fall-off in the 1"< R < 4" interval. The observed velocity
of the gas at 0.5" implies a mass of M ~ 10E9 M(sol) within the inner 60 pc.
The presence of a turnover in the rotation curve indicates that this central
mass concentration is extended. The first measured velocity point (outside the
region saturated by the nucleus) would imply an enclosed mass of ~ 5E7 M(sol)
within R ~ 0.15" (10 pc) which represents an upper limit to any nuclear point
mass.Comment: 30 pages (aaspp4.sty), 14 figures. Fig. 1, 2 and 4 available by
anonymous FTP at 143.54.2.51 (cd /pub/winge) as GIF files; or upon request to
[email protected]. Accepted for publication in the Astrophysical Journal
(part 1
Primary structure and spectroscopic studies of Neurospora copper metallothionein.
When Neurospora crassa is grown in the presence of Cu(II) ions, it accumulates the metal with the concomitant synthesis of a low molecular weight copper-binding protein. The molecule binds 6 g-atom of copper per mole protein (Mr = 2200) and shows a striking sequence homology to the zinc- and cadmium-binding vertebrate metallothioneins. Absorption, circular dichroism, and electron paramagnetic resonance spectroscopy of Neurospora metallothionein indicate the copper to be bound to cysteinyl residues as a Cu(I)-thiolate complex of the polymeric mu-thiolate structure [Cu(I)6RS7]-. This metal-binding mode is also in agreement with the unusual luminescence of the protein. Spectral perturbation studies with HgCl2 and p-(chloromercuri)benzoate suggest that the 6 Cu(I)ions are coordinated to the seven cysteinyl residues in the form of a single metal cluster. Neurospora apometallothionein is also capable of binding in vivo group IIB metal ions [Zn(II), Cd(II), and Hg(II)] as well as paramagnetic Co(II) ions with an overall metal-to-protein stoichiometry of 3. The spectroscopic properties of the fully substituted forms are indicative of a distorted tetrahedral coordination. However, metal titration of the apoprotein shows the third metal ion to be differently coordinated than the other two metal ions. This difference can be explained by the presence of only seven cysteine residues in Neurospora metallothionein as opposed to nine cysteine residues in the three-metal cluster of the mammalian metallothioneins
Chemical Abundances in Twelve Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy
High-resolution infrared spectra (R=50,000) have been obtained for twelve
red-giant members of the LMC with the Gemini South 8.3-meter telescope plus
Phoenix spectrometer. Quantitative chemical abundances of carbon-12, carbon-13,
nitrogen-14, and oxygen-16 were derived from molecular lines of CO, CN, and OH,
while sodium, scandium, titanium, and iron abundances were derived from neutral
atomic lines. The LMC giants have masses from about 1 to 4 solar masses and
span a metallicity range from [Fe/H]= -1.1 to -0.3. The program red giants all
show evidence of first dredge-up mixing, with low 12C/13C ratios, and low 12C
correlated with high 14N abundances. Comparisons of the oxygen-to-iron ratios
in the LMC and the Galaxy indicate that the trend of [O/Fe] versus [Fe/H] in
the LMC falls about 0.2 dex below the Galactic trend. Such an offset can be
modeled as due to an overall lower rate of supernovae per unit mass in the LMC
relative to the Galaxy, as well as a slightly lower ratio of supernovae of type
II to supernovae of type Ia.Comment: 25 pages, 6 tables, 10 figures. "In press" in The Astronomical
Journal (scheduled for December 2002
Determination of masses of the central black holes in NGC524 and NGC2549 using Laser Guide Star Adaptive Optics
[abridged] We present observations of NGC524 and NGC2549 with LGS AO obtained
at GEMINI North telescope using the NIFS IFU in the K band. The purpose of
these observations, together with previously obtained observations with the
SAURON IFU, is to determine the masses (Mbh) of the supermassive black holes
(SMBH). The targeted galaxies were chosen to have central light profiles
showing a core (NGC524) and a cusp (NGC2549), to probe the feasibility of using
the galaxy centre as the NGS required for LGS AO. We employ an innovative `open
loop' technique. The data have spatial resolution of 0.23" and 0.17" FWHM,
showing that high quality LGS AO observations of these objects are possible. We
construct axisymmetric three-integral dynamical models which are constrained
with both the NIFS and SAURON data. The best fitting models yield Mbh=(8.3 +2.7
-1.3) x 10^8 Msun for NGC524 and Mbh=(1.4 +0.2 -1.3) x 10^7 Msun for NGC2549
(all errors are at the 3 sigma CL). We demonstrate that the wide-field SAURON
data play a crucial role in the M/L determination increasing the accuracy of
M/L by a factor of at least 5, and constraining the upper limits on Mbh. The
NIFS data are crucial in constraining the lower limits of Mbh and in
combination with the large scale data reducing the uncertainty by a factor of 2
or more. We find that the orbital structure of NGC524 shows significant
tangential anisotropy, while at larger radii both galaxies are consistent with
having almost perfectly oblate velocity ellipsoids. Tangential anisotropy in
NGC524 coincides with the size of SMBH sphere of influence and the core region
in the light profile. We test the accuracy to which Mbh can be measured using
seeings obtained from typical LGS AO observations, and conclude that for a
typical conditions and Mbh the expected uncertainty is of the order of 50%.Comment: 19 pages, 14 figure
- …