2,643 research outputs found
Graded Lie algebras of maximal class IV
We describe the isomorphism classes of certain infinite-dimensional graded
Lie algebras of maximal class, generated by an element of weight one and an
element of weight two, over fields of odd characteristic.Comment: 38 pages. See also http://www-math.science.unitn.it/~caranti/ and
http://users.ox.ac.uk/~vlee
Particle-in-cell simulations of rf breakdown
Breakdown voltages of a capacitively coupled radio frequency argon discharge
at 27 MHz are studied. We use a one-dimensional electrostatic PIC code to
investigate the effect of changing the secondary emission properties of the
electrodes on the breakdown voltage, particularly at low pd values. Simulation
results are compared with the available experimental results and a satisfactory
agreement is found.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Presentations for the groups of order for prime
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Local lung hypoxia determines epithelial fate decisions during alveolar regeneration.
After influenza infection, lineage-negative epithelial progenitors (LNEPs) exhibit a binary response to reconstitute epithelial barriers: activating a Notch-dependent ΔNp63/cytokeratin 5 (Krt5) remodelling program or differentiating into alveolar type II cells (AEC2s). Here we show that local lung hypoxia, through hypoxia-inducible factor (HIF1α), drives Notch signalling and Krt5pos basal-like cell expansion. Single-cell transcriptional profiling of human AEC2s from fibrotic lungs revealed a hypoxic subpopulation with activated Notch, suppressed surfactant protein C (SPC), and transdifferentiation toward a Krt5pos basal-like state. Activated murine Krt5pos LNEPs and diseased human AEC2s upregulate strikingly similar core pathways underlying migration and squamous metaplasia. While robust, HIF1α-driven metaplasia is ultimately inferior to AEC2 reconstitution in restoring normal lung function. HIF1α deletion or enhanced Wnt/β-catenin activity in Sox2pos LNEPs blocks Notch and Krt5 activation, instead promoting rapid AEC2 differentiation and migration and improving the quality of alveolar repair
The Similarity of Broad Iron Lines in X-ray Binaries and Active Galactic Nuclei
We have compared the 2001 XMM-Newton spectra of the stellar mass black hole
binary XTE J1650-500 and the active galaxy MGC-6-30-15, focusing on the broad,
excess emission features at ~4--7 keV displayed by both sources. Such features
are frequently observed in both low mass X-ray binaries and active galactic
nuclei. For the former case it is generally accepted that the excess arises due
to iron emission, but there is some controversy over whether their width is
partially enhanced by instrumental processes, and hence also over the intrinsic
broadening mechanism. Meanwhile, in the latter case, the origin of this feature
is still subject to debate; physically motivated reflection and absorption
interpretations are both able to reproduce the observed spectra. In this work
we make use of the contemporaneous BeppoSAX data to demonstrate that the
breadth of the excess observed in XTE J1650-500 is astrophysical rather than
instrumental, and proceed to highlight the similarity of the excesses present
in this source and MGC-6-30-15. Both optically thick accretion discs and
optically thin coronae, which in combination naturally give rise to
relativistically-broadened iron lines when the disc extends close to the black
hole, are commonly observed in both class of object. The simplest solution is
that the broad emission features present arise from a common process, which we
argue must be reflection from the inner regions of an accretion disc around a
rapidly rotating black hole; for XTE J1650-500 we find spin constraints of 0.84
< a* < 0.98 at the 90 per cent confidence level. Other interpretations proposed
for AGN add potentially unnecessary complexities to the theoretical framework
of accretion in strong gravity.Comment: Accepted for publication in MNRAS; 22 pages, 17 figure
Evolution of the X-ray spectrum in the flare model of Active Galactic Nuclei
Nayakshin & Kazanas (2002) have considered the time-dependent illumination of
an accretion disc in Active Galactic Nuclei, in the lamppost model. We extend
their study to the flare model, which postulates the release of a large X-ray
flux above a small region of the accretion disc. A fundamental difference with
the lamppost model is that the region of the disc below the flare is not
illuminated before the onset of the flare.
A few test models show that the spectrum which follows immediately the
increase in continuum flux should display the characteristics of a highly
illuminated but dense gas, i.e. very intense X-ray emission lines and
ionization edges in the soft X-ray range. The behaviour of the iron line is
different in the case of a "moderate" and a ``strong'' flare: for a moderate
flare, the spectrum displays a neutral component of the Fe K line at
6.4 keV, gradually leading to more highly ionized lines. For a strong flare,
the lines are already emitted by FeXXV (around 6.7 keV) after the onset, and
have an equivalent width of several hundreds of eV. We find that the observed
correlations between , , and the X-ray flux, are well accounted by a
combination of flares having not achieved pressure equilibrium, strongly
suggesting that the observed spectrum is dominated by regions in non-pressure
equilibrium, typical of the onset of the flares. Finally a flare being confined
to a small region of the disc, the spectral lines should be narrow (except for
a weak Compton broadening), Doppler shifted, and moving.Comment: 14 pages, 13 figures, accepted in A & A, english corrected versio
Universal spectral shape of high accretion rate AGN
The spectra of quasars and NLS1 galaxies show surprising similarity in their
spectral shape. They seem to scale only with the accretion rate. This is in
contradiction with the simple expectations from the standard disk model which
predicts lower disk temperature for higher black hole mass. Here we consider
two mechanisms modifying the disk spectrum: the irradiation of the outer disk
due to the scattering of the flux by the extended ionized medium (warm absorber
and the development of the warm Comptonizing disk skin under the effect of the
radiation pressure instability. Those two mechanisms seem to lead to a spectrum
which indeed roughly scales, as observed, only with the accretion rate. The
scenario applies only to objects with relatively high luminosity to the
Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 14 pages, 14 figures, 1 table, accepted for publication in A&
Collapsar Accretion and the Gamma-Ray Burst X-Ray Light Curve
We present axisymmetric hydrodynamical simulations of the long-term accretion
of a rotating GRB progenitor star, a "collapsar," onto the central compact
object. The simulations were carried out with the adaptive mesh refinement code
FLASH in two spatial dimensions and with an explicit shear viscosity. The
evolution of the central accretion rate exhibits phases reminiscent of the long
GRB gamma-ray and X-ray light curve, which lends support to the proposal that
the luminosity is modulated by the central accretion rate. After a few tens of
seconds, an accretion shock sweeps outward through the star. The formation and
outward expansion of the accretion shock is accompanied with a sudden and rapid
power-law decline in the central accretion rate Mdot ~ t^{-2.8}, which
resembles the L_X ~ t^{-3} decline observed in the X-ray light curves. The
collapsed, shock-heated stellar envelope settles into a thick, low-mass
equatorial disk embedded within a massive, pressure-supported atmosphere. After
a few hundred seconds, the inflow of low-angular-momentum material in the axial
funnel reverses into an outflow from the surface of the thick disk. Meanwhile,
the rapid decline of the accretion rate slows down, or even settles a in steady
state with Mdot ~ 5x10^{-5} Msun/s, which resembles the "plateau" phase in the
X-ray light curve. While the duration of the "prompt" phase depends on the
resolution in our simulations, we provide an analytical model taking into
account neutrino losses that estimates the duration to be ~20 s. The model
suggests that the steep decline in GRB X-ray light curves is triggered by the
circularization of the infalling stellar envelope at radii where the virial
temperature is below ~10^{10} K, such that neutrino cooling shuts off and an
outward expansion of the accretion shock becomes imminent.Comment: 16 pages, 10 figure
The On-Orbit Performance of the Galaxy Evolution Explorer
We report the first year on-orbit performance results for the Galaxy
Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey
of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter
modified Ritchey-Chretien telescope with a 1.25 degree field of view,
selectable imaging and objective grism spectroscopic modes, and an innovative
optical system with a thin-film multilayer dichroic beam splitter that enables
simultaneous imaging by a pair of photon counting, microchannel plate, delay
line readout detectors. Initial measurements demonstrate that GALEX is
performing well, meeting its requirements for resolution, efficiency,
astrometry, bandpass definition and survey sensitivity.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issu
Search Engine Similarity Analysis: A Combined Content and Rankings Approach
How different are search engines? The search engine wars are a favorite topic
of on-line analysts, as two of the biggest companies in the world, Google and
Microsoft, battle for prevalence of the web search space. Differences in search
engine popularity can be explained by their effectiveness or other factors,
such as familiarity with the most popular first engine, peer imitation, or
force of habit. In this work we present a thorough analysis of the affinity of
the two major search engines, Google and Bing, along with DuckDuckGo, which
goes to great lengths to emphasize its privacy-friendly credentials. To do so,
we collected search results using a comprehensive set of 300 unique queries for
two time periods in 2016 and 2019, and developed a new similarity metric that
leverages both the content and the ranking of search responses. We evaluated
the characteristics of the metric against other metrics and approaches that
have been proposed in the literature, and used it to (1) investigate the
similarities of search engine results, (2) the evolution of their affinity over
time, (3) what aspects of the results influence similarity, and (4) how the
metric differs over different kinds of search services. We found that Google
stands apart, but Bing and DuckDuckGo are largely indistinguishable from each
other.Comment: Shorter version of this paper was accepted in the 21st International
Conference on Web Information Systems Engineering (WISE 2020). The final
authenticated version is available online at
https://doi.org/10.1007/978-3-030-62008-0_
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