652 research outputs found
Perfect preferential orientation of nitrogen-vacancy defects in a synthetic diamond sample
We show that the orientation of nitrogen-vacancy (NV) defects in diamond can
be efficiently controlled through chemical vapor deposition (CVD) growth on a
(111)-oriented diamond substrate. More precisely, we demonstrate that
spontaneously generated NV defects are oriented with a ~ 97 % probability along
the [111] axis, corresponding to the most appealing orientation among the four
possible crystallographic axes. Such a nearly perfect preferential orientation
is explained by analyzing the diamond growth mechanism on a (111)-oriented
substrate and could be extended to other types of defects. This work is a
significant step towards the design of optimized diamond samples for quantum
information and sensing applications.Comment: 6 pages, 4 figure
Ultrasensitive gene regulation by positive feedback loops in nucleosome modification
Eukaryotic transcription involves the synergistic interaction of many different proteins. However, the question remains how eukaryotic promoters achieve ultrasensitive or threshold responses to changes in the concentration or activity of a single transcription factor (TF). We show theoretically that by recruiting a histone-modifying enzyme, a TF binding non-cooperatively to a single site can change the balance between opposing positive feedback loops in histone modification to produce a large change in gene expression in response to a small change in concentration of the TF. This mechanism can also generate bistable promoter responses, allowing a gene to be on in some cells and off in others, despite the cells being in identical conditions. In addition, the system provides a simple means by which the activities of many TFs could be integrated at a promoter
Validation of CARE kV automated tube voltage selection for PET-CT:PET quantification and CT radiation dose reduction in phantoms
CARTE: An Observation Station to Regulate Activity in a Learning Context
This chapter discusses the introduction of a new concept called "regulation" into a use model, which is part of a theoretical observation model called trace-based system (TBS). This concept defines a retroaction mechanism in an observation station. We present the results of experiments, in a learning context, with a prototype observation station called Collection, activity Analysis and Regulation based on Traces Enriched (CARTE)
Stellar velocity dispersion in NLS1 galaxies
Several authors recently explored the Black-Hole mass (M_BH) vs. stellar
velocity dispersion (sigma_*) relationship for Narrow Line Seyfert 1 galaxies
(NLS1s). Their results are more or less in agreement and seem to indicate that
NLS1s fill the region below the fit obtained by Tremaine et al. (2002), showing
a range of sigma_* similar to that of Seyfert 1 galaxies, and a lower M_BH.
Until now the [O III] width has been used in place of the stellar velocity
dispersion, but some indications begin to arise against the effectiveness of
the gaseous kinematics in representing the bulge potential at least in NLS1s.
Bian & Zhao (2004) stressed the urgency to produce true sigma_* measurements.
Here we present new stellar velocity dispersions obtained through direct
measurements of the Ca II absorption triplet (~ 8550 A) in the nuclei of 8 NLS1
galaxies. The resulting sigma_* values and a comparison with sigma_[O III]
confirm our suspects (see Botte et al. 2004) that [O III] typically
overestimates the stellar velocity dispersion and demonstrate that NLS1s follow
the M_BH-sigma_* relation as Seyfert 1, quasars and non-active galaxies.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Exploring the parent population of beamed NLS1s: from the black hole to the jet
The aim of this work is to understand the nature of the parent population of
beamed narrow-line Seyfert 1 galaxies (NLS1s), by studying the physical
properties of three parent candidates samples: steep-spectrum radio-loud NLS1s,
radio-quiet NLS1s and disk-hosted radio-galaxies. In particular, we focused on
the black hole mass and Eddington ratio distribution and on the interactions
between the jet and the narrow-line region.Comment: 6 pages, 2 figures, to appear in Proceedings of High Energy Phenomena
in Relativistic Outflows (HEPRO) V, Workshop Series of the Argentinian
Astronomical Societ
Symbiotic Stars in OGLE Data I. Large Magellanic Cloud Systems
Symbiotic stars are long-orbital-period interacting-binaries characterized by
extended emission over the whole electromagnetic range and by complex
photometric and spectroscopic variability. In this paper, the first of a
series, we present OGLE light curves of all the confirmed symbiotic stars in
the Large Magellanic Cloud, with one exception. By careful visual inspection
and combined time-series analysis techniques, we investigate for the first time
in a systematic way the photometric properties of these astrophysical objects,
trying in particular to distinguish the nature of the cool component (e.g.,
Semi-Regular Variable vs. OGLE Small-Amplitude Red Giant), to provide its
first-order pulsational ephemerides, and to link all this information with the
physical parameters of the binary system as a whole. Among the most interesting
results, there is the discovery of a 20-year-long steady fading of Sanduleak's
star, a peculiar symbiotic star known to produce the largest stellar jet ever
discovered. We discuss by means of direct examples the crucial need for
long-term multi-band observations to get a real understanding of symbiotic and
other interacting binary stars. We eventually introduce BOMBOLO, a multi-band
simultaneous imager for the SOAR 4m Telescope, whose design and construction we
are currently leading.Comment: 16 pages, 4 Tables, 12 Figures. Accepted for publication in MNRA
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