1,076 research outputs found
Mapping land cover from satellite images: A basic, low cost approach
Simple, inexpensive methodologies developed for mapping general land cover and land use categories from LANDSAT images are reported. One methodology, a stepwise, interpretive, direct tracing technique was developed through working with university students from different disciplines with no previous experience in satellite image interpretation. The technique results in maps that are very accurate in relation to actual land cover and relative to the small investment in skill, time, and money needed to produce the products
Dispersion enhancement and damping by buoyancy driven flows in 2D networks of capillaries
The influence of a small relative density difference on the displacement of
two miscible liquids is studied experimentally in transparent 2D networks of
micro channels. Both stable displacements in which the denser fluid enters at
the bottom of the cell and displaces the lighter one and unstable displacements
in which the lighter fluid is injected at the bottom and displaces the denser
one are realized. Except at the lowest mean flow velocity U, the average
of the relative concentration satisfies a convection-dispersion
equation. The dispersion coefficient is studied as function of the relative
magnitude of fluid velocity and of the velocity of buoyancy driven fluid
motion. A model is suggested and its applicability to previous results obtained
in 3D media is discussed
Prospects for Probing the Spacetime of Sgr A* with Pulsars
The discovery of radio pulsars in compact orbits around Sgr A* would allow an
unprecedented and detailed investigation of the spacetime of the supermassive
black hole. This paper shows that pulsar timing, including that of a single
pulsar, has the potential to provide novel tests of general relativity, in
particular its cosmic censorship conjecture and no-hair theorem for rotating
black holes. These experiments can be performed by timing observations with 100
micro-second precision, achievable with the Square Kilometre Array for a normal
pulsar at frequency above 15 GHz. Based on the standard pulsar timing
technique, we develop a method that allows the determination of the mass, spin,
and quadrupole moment of Sgr A*, and provides a consistent covariance analysis
of the measurement errors. Furthermore, we test this method in detailed mock
data simulations. It seems likely that only for orbital periods below ~0.3 yr
is there the possibility of having negligible external perturbations. For such
orbits we expect a ~10^-3 test of the frame dragging and a ~10^-2 test of the
no-hair theorem within 5 years, if Sgr A* is spinning rapidly. Our method is
also capable of identifying perturbations caused by distributed mass around Sgr
A*, thus providing high confidence in these gravity tests. Our analysis is not
affected by uncertainties in our knowledge of the distance to the Galactic
center, R0. A combination of pulsar timing with the astrometric results of
stellar orbits would greatly improve the measurement precision of R0.Comment: 12 pages, 10 Figures, accepted for publication in Ap
A non trivial extension of the two-dimensional Ising model: the d-dimensional "molecular" model
A recently proposed molecular model is discussed as a non-trivial extension
of the Ising model. For d=2 the two models are shown to be equivalent, while
for d>2 the molecular model describes a peculiar second order transition from
an isotropic high temperature phase to a low-dimensional anisotropic low
temperature state. The general mean field analysis is compared with the results
achieved by a variational Migdal-Kadanoff real space renormalization group
method and by standard Monte Carlo sampling for d=3. By finite size scaling the
critical exponent has been found to be 0.44\pm 0.02 thus establishing that the
molecular model does not belong to the universality class of the Ising model
for d>2.Comment: 25 pages, 5 figure
Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene
For most optoelectronic applications of graphene a thorough understanding of
the processes that govern energy relaxation of photoexcited carriers is
essential. The ultrafast energy relaxation in graphene occurs through two
competing pathways: carrier-carrier scattering -- creating an elevated carrier
temperature -- and optical phonon emission. At present, it is not clear what
determines the dominating relaxation pathway. Here we reach a unifying picture
of the ultrafast energy relaxation by investigating the terahertz
photoconductivity, while varying the Fermi energy, photon energy, and fluence
over a wide range. We find that sufficiently low fluence ( 4
J/cm) in conjunction with sufficiently high Fermi energy (
0.1 eV) gives rise to energy relaxation that is dominated by carrier-carrier
scattering, which leads to efficient carrier heating. Upon increasing the
fluence or decreasing the Fermi energy, the carrier heating efficiency
decreases, presumably due to energy relaxation that becomes increasingly
dominated by phonon emission. Carrier heating through carrier-carrier
scattering accounts for the negative photoconductivity for doped graphene
observed at terahertz frequencies. We present a simple model that reproduces
the data for a wide range of Fermi levels and excitation energies, and allows
us to qualitatively assess how the branching ratio between the two distinct
relaxation pathways depends on excitation fluence and Fermi energy.Comment: Nano Letters 201
Three-body Faddeev Calculation for 11Li with Separable Potentials
The halo nucleus Li is treated as a three-body system consisting of an
inert core of Li plus two valence neutrons. The Faddeev equations are
solved using separable potentials to describe the two-body interactions,
corresponding in the n-Li subsystem to a p resonance plus a
virtual s-wave state. The experimental Li energy is taken as input and
the Li transverse momentum distribution in Li is studied.Comment: 6 pages, RevTeX, 1 figur
Two approaches to testing general relativity in the strong-field regime
Observations of compact objects in the electromagnetic spectrum and the
detection of gravitational waves from them can lead to quantitative tests of
the theory of general relativity in the strong-field regime following two very
different approaches. In the first approach, the general relativistic field
equations are modified at a fundamental level and the magnitudes of the
potential deviations are constrained by comparison with observations. In the
second approach, the exterior spacetimes of compact objects are parametrized in
a phenomenological way, the various parameters are measured observationally,
and the results are finally compared against the general relativistic
predictions. In this article, I discuss the current status of both approaches,
focusing on the lessons learned from a large number of recent investigations.Comment: To appear in the proceedings of the conference New Developments in
Gravit
Strong Gravitational Lensing of Quasi-Kerr Compact Object with Arbitrary Quadrupole Moments
We study the strong gravitational lensing on the equatorial plane of a
quasi-Kerr compact object with arbitrary quadrupole moments which can be used
to model the super-massive central object of the galaxy. We find that, when the
quadrupolar correction parameter takes the positive (negative) value, the
photon-sphere radius , the minimum impact parameter , the
coefficient , the relative magnitudes and the angular position
of the relativistic images are larger (smaller) than the
results obtained in the Kerr black hole, but the coefficient , the
deflection angle and the angular separation are smaller
(larger) than that in the Kerr black hole. These features may offer a way to
probe special properties for some rotating compact objects by the astronomical
instruments in the future.Comment: 17 pages, 4 figures. Accepted for publication in JHE
RNA polymerase II stalling promotes nucleosome occlusion and pTEFb recruitment to drive immortalization by Epstein-Barr virus
Epstein-Barr virus (EBV) immortalizes resting B-cells and is a key etiologic agent in the development of numerous cancers. The essential EBV-encoded protein EBNA 2 activates the viral C promoter (Cp) producing a message of ~120 kb that is differentially spliced to encode all EBNAs required for immortalization. We have previously shown that EBNA 2-activated transcription is dependent on the activity of the RNA polymerase II (pol II) C-terminal domain (CTD) kinase pTEFb (CDK9/cyclin T1). We now demonstrate that Cp, in contrast to two shorter EBNA 2-activated viral genes (LMP 1 and 2A), displays high levels of promoter-proximally stalled pol II despite being constitutively active. Consistent with pol II stalling, we detect considerable pausing complex (NELF/DSIF) association with Cp. Significantly, we observe substantial Cp-specific pTEFb recruitment that stimulates high-level pol II CTD serine 2 phosphorylation at distal regions (up to +75 kb), promoting elongation. We reveal that Cp-specific pol II accumulation is directed by DNA sequences unfavourable for nucleosome assembly that increase TBP access and pol II recruitment. Stalled pol II then maintains Cp nucleosome depletion. Our data indicate that pTEFb is recruited to Cp by the bromodomain protein Brd4, with polymerase stalling facilitating stable association of pTEFb. The Brd4 inhibitor JQ1 and the pTEFb inhibitors DRB and Flavopiridol significantly reduce Cp, but not LMP1 transcript production indicating that Brd4 and pTEFb are required for Cp transcription. Taken together our data indicate that pol II stalling at Cp promotes transcription of essential immortalizing genes during EBV infection by (i) preventing promoter-proximal nucleosome assembly and ii) necessitating the recruitment of pTEFb thereby maintaining serine 2 CTD phosphorylation at distal regions
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