2,219 research outputs found
Transcriptional Termination Modulated by Nucleotides Outside the Characterized Gene End Sequence of Respiratory Syncytial Virus
AbstractThe genes of respiratory syncytial (RS) virus are transcribed sequentially by the viral RNA polymerase from a single 3′-proximal promoter. Polyadenylation and termination are directed by a sequence at the end of each gene, after which the polymerase crosses an intergenic region and reinitiates at the start sequence of the next gene. The 10 viral genes have different gene end sequences and different termination efficiencies, which allow for regulation of gene expression, since termination of each gene is required for initiation of the downstream gene. RNA sequences within the previously characterized 13 nucleotide gene end, including a conserved sequence 3′-UCAAU-5′ and a tract of U residues, are important for termination. In this study, two additional sequence elements outside of the 13 nucleotide gene end were found to modulate termination efficiency: the A residue upstream of the 3′-UCAAU-5′ sequence, and the first nucleotide of the intergenic region when it follows a U4 tract
Long-term source monitoring with BATSE
The uncollimated Burst and Transient Source Experiment (BATSE) large area detectors (LADs) are well suited to nearly continuous monitoring of the stronger hard x-ray sources, and time series analysis for pulsars. An overview of the analysis techniques presently being applied to the data are discussed, including representative observations of the Crab Nebula, Crab pulsar, and summaries of the sources detected to data. Results of a search for variability in the Crab Pulsar pulse profile are presented
Hyperfine interaction induced decoherence of electron spins in quantum dots
We investigate in detail, using both analytical and numerical tools, the
decoherence of electron spins in quantum dots (QDs) coupled to a bath of
nuclear spins in magnetic fields or with various initial bath polarizations,
focusing on the longitudinal relaxation in low and moderate field/polarization
regimes. An increase of the initial polarization of nuclear spin bath has the
same effect on the decoherence process as an increase of the external magnetic
field, namely, the decoherence dynamics changes from smooth decay to damped
oscillations. This change can be observed experimentally for a single QD and
for a double-QD setup. Our results indicate that substantial increase of the
decoherence time requires very large bath polarizations, and the use of other
methods (dynamical decoupling or control of the nuclear spins distribution) may
be more practical for suppressing decoherence of QD-based qubits.Comment: Rev. Tex, 5 pages, 3 eps color figures, submitted to Phys. Rev.
Effect of ligand substitution on the exchange interactions in {Mn12}-type single-molecule magnets
We investigate how the ligand substitution affects the intra-molecular spin
exchange interactions, studying a prototypal family of single-molecule magnets
comprising dodecanuclear cluster molecules [Mn12O12(COOR)16]. We identify a
simple scheme based on accumulated Pauling electronegativity numbers (a.e.n.)
of the carboxylate ligand groups (R). The redistribution of the electron
density, controlled by a.e.n. of a ligand, changes the degree of hybridization
between 3d electrons of manganese and 2p electrons of oxygen atoms, thus
changing the exchange interactions. This scheme, despite its conceptual
simplicity, provides a strong correlation with the exchange energies associated
with carboxylate bridges, and is confirmed by the electronic structure
calculations taking into account the Coulomb correlations in magnetic
molecules.Comment: 18 pages, 1 table, 4 figures. Accepted to "Inorganic Chemistry
Occultation analysis of BATSE data: Operational aspects
The Burst and Transient Source Experiment (BATSE) large area detectors are being used to monitor hard x-ray/gamma ray sources on a daily basis for evidence of transient behavior. Flux measurements are performed using a simple earth occultation technique. Daily searches are also being performed to detect occultation steps of sources which are not being routinely monitored. Topics concerning the operational aspects of the occultation measurements are presented. Preliminary spectral results are also presented for several of the brighter sources
The Burst and Transient Source Experiment Earth Occultation Technique
An Earth orbiting detector sensitive to gamma ray photons will see step-like
occultation features in its counting rate when a gamma ray point source crosses
the Earth's limb. This is due to the change in atmospheric attenuation of the
gamma rays along the line of sight. In an uncollimated detector, these
occultation features can be used to locate and monitor astrophysical sources
provided their signals can be individually separated from the detector
background. We show that the Earth occultation technique applied to the Burst
and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory
(CGRO) is a viable and flexible all-sky monitor in the low energy gamma ray and
hard X-ray energy range (20 keV - 1 MeV). The method is an alternative to more
sophisticated photon imaging devices for astronomy, and can serve well as a
cost-effective science capability for monitoring the high energy sky.
Here we describe the Earth occultation technique for locating new sources and
for measuring source intensity and spectra without the use of complex
background models. Examples of transform imaging, step searches, spectra, and
light curves are presented. Systematic uncertainties due to source confusion,
detector response, and contamination from rapid background fluctuations are
discussed and analyzed for their effect on intensity measurements. A sky
location-dependent average systematic error is derived as a function of
galactic coordinates. The sensitivity of the technique is derived as a function
of incident photon energy and also as a function of angle between the source
and the normal to the detector entrance window. Occultations of the Crab Nebula
by the Moon are used to calibrate Earth occultation flux measurements
independent of possible atmospheric scattering effects.Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical
Journal Supplement
Magnetic ordering in EuRh2As2 studied by x-ray resonant magnetic scattering
Element-specific x-ray resonant magnetic scattering investigations were
performed to determine the magnetic structure of Eu in EuRh2As2. In the
temperature range from 46 K down to 6 K, an incommensurate antiferromagnetic
(ICM)structure with a temperature dependent propagation vector (0 0 0.9)
coexists with a commensurate antiferromagnetic (CM) structure.
Angular-dependent measurements of the magnetic intensity indicate that the
magnetic moments lie in the tetragonal basal plane and are ferromagnetically
aligned within the a-b plane for both magnetic structures. The ICM structure is
a spiral-like magnetic structure with a turn angle of 162 deg between adjacent
Eu planes. In the CM structure, this angle is 180 deg. These results are
consistent with band-structure calculations which indicate a strong sensitivity
of the magnetic configuration on the Eu valence.Comment: 5 pages, 5 figures (technical problem with abstract corrected, no
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