5,336 research outputs found
Calibration of the LIGO displacement actuators via laser frequency modulation
We present a frequency modulation technique for calibration of the
displacement actuators of the LIGO 4-km-long interferometric gravitational-wave
detectors. With the interferometer locked in a single-arm configuration, we
modulate the frequency of the laser light, creating an effective length
variation that we calibrate by measuring the amplitude of the frequency
modulation. By simultaneously driving the voice coil actuators that control the
length of the arm cavity, we calibrate the voice coil actuation coefficient
with an estimated 1-sigma uncertainty of less than one percent. This technique
enables a force-free, single-step actuator calibration using a displacement
fiducial that is fundamentally different from those employed in other
calibration methods.Comment: 10 pages, 5 figures, submitted to Classical and Quantum Gravit
âLandsatTS': an R package to facilitate retrieval, cleaning, crossâcalibration, and phenological modeling of Landsat time series data
The Landsat satellites provide decades of nearâglobal surface reflectance measurements that are increasingly used to assess interannual changes in terrestrial ecosystem function. These assessments often rely on spectral indices related to vegetation greenness and productivity (e.g. Normalized Difference Vegetation Index, NDVI). Nevertheless, multiple factors impede multiâdecadal assessments of spectral indices using Landsat satellite data, including ease of data access and cleaning, as well as lingering issues with crossâsensor calibration and challenges with irregular timing of cloudâfree acquisitions. To help address these problems, we developed the âLandsatTS' package for R. This software package facilitates sampleâbased time series analysis of surface reflectance and spectral indices derived from Landsat sensors. The package includes functions that enable the extraction of the full Landsat 5, 7, and 8 records from Collection 2 for point sample locations or small study regions using Google Earth Engine accessed directly from R. Moreover, the package includes functions for 1) rigorous data cleaning, 2) crossâsensor calibration, 3) phenological modeling, and 4) time series analysis. For an example application, we show how âLandsatTS' can be used to assess changes in annual maximum vegetation greenness from 2000 to 2022 across the Noatak National Preserve in northern Alaska, USA. Overall, this software provides a suite of functions to enable broader use of Landsat satellite data for assessing and monitoring terrestrial ecosystem function during recent decades across local to global geographic extents
Lidar Remote Sensing Variables Predict Breeding Habitat of a Neotropical Migrant Bird
A topic of recurring interest in ecological research is the degree to which vegetation structure influences the distribution and abundance of species. Here we test the applicability of remote sensing, particularly novel use of waveform lidar measurements, for quantifying the habitat heterogeneity of a contiguous northern hardwoods forest in the northeastern United States. We apply these results to predict the breeding habitat quality, an indicator of reproductive output of a well-studied Neotropical migrant songbird, the Black-throated Blue Warbler (Dendroica caerulescens). We found that using canopy vertical structure metrics provided unique information for models of habitat quality and spatial patterns of prevalence. An ensemble decision tree modeling approach (random forests) consistently identified lidar metrics describing the vertical distribution and complexity of canopy elements as important predictors of habitat use over multiple years. Although other aspects of habitat were important, including the seasonality of vegetation cover, the canopy structure variables provided unique and complementary information that systematically improved model predictions. We conclude that canopy structure metrics derived from waveform lidar, which will be available on future satellite missions, can advance multiple aspects of biodiversity research, and additional studies should be extended to other organisms and regions
The Role of Bilayer Tilt Difference in Equilibrium Membrane Shapes
Lipid bilayer membranes below their main transition have two tilt order
parameters, corresponding to the two monolayers. These two tilts may be
strongly coupled to membrane shape but only weakly coupled to each other. We
discuss some implications of this observation for rippled and saddle phases,
bilayer tubules, and bicontinuous phases. Tilt difference introduces a length
scale into the elastic theory of tilted fluid membranes. It can drive an
instability of the flat phase; it also provides a simple mechanism for the
spontaneous breaking of inversion symmetry seen in some recent experiments.Comment: Latex file; .ps available at
http://dept.physics.upenn.edu/~nelson/saddle.p
Very Extended X-ray and H-alpha Emission in M82: Implications for the Superwind Phenomenon
We discuss the properties and implications of a 3.7x0.9 kpc region of
spatially-coincident X-ray and H-alpha emission about 11.6 kpc to the north of
the galaxy M82 previously discussed by Devine and Bally (1999). The PSPC X-ray
spectrum is fit by thermal plasma (kT=0.80+-0.17 keV) absorbed by only the
Galactic foreground column density. We evaluate the relationship of the
X-ray/H-alpha ridge to the M82 superwind. The main properties of the X-ray
emission can all be explained as being due to shock-heating driven as the
superwind encounters a massive ionized cloud in the halo of M82. This encounter
drives a slow shock into the cloud, which contributes to the excitation of the
observed H-alpha emission. At the same time, a fast bow-shock develops in the
superwind just upstream of the cloud, and this produces the observed X-ray
emission. This interpretation would imply that the superwind has an outflow
speed of roughly 800 km/s, consistent with indirect estimates based on its
general X-ray properties and the kinematics of the inner kpc-scale region of
H-alpha filaments. The gas in the M82 ridge is roughly two orders-of-magnitude
hotter than the minimum "escape temperature" at this radius, so this gas will
not be retained by M82.
(abridged)Comment: 24 pages (latex), 3 figures (2 gif files and one postscript),
accepted for publication in Part 1 of The Astrophysical Journa
Phase ordering and shape deformation of two-phase membranes
Within a coupled-field Ginzburg-Landau model we study analytically phase
separation and accompanying shape deformation on a two-phase elastic membrane
in simple geometries such as cylinders, spheres and tori. Using an exact
periodic domain wall solution we solve for the shape and phase ordering field,
and estimate the degree of deformation of the membrane. The results are
pertinent to a preferential phase separation in regions of differing curvature
on a variety of vesicles.Comment: 4 pages, submitted to PR
Gravitational waves from Sco X-1: A comparison of search methods and prospects for detection with advanced detectors
The low-mass X-ray binary Scorpius X-1 (Sco X-1) is potentially the most
luminous source of continuous gravitational-wave radiation for interferometers
such as LIGO and Virgo. For low-mass X-ray binaries this radiation would be
sustained by active accretion of matter from its binary companion. With the
Advanced Detector Era fast approaching, work is underway to develop an array of
robust tools for maximizing the science and detection potential of Sco X-1. We
describe the plans and progress of a project designed to compare the numerous
independent search algorithms currently available. We employ a mock-data
challenge in which the search pipelines are tested for their relative
proficiencies in parameter estimation, computational efficiency, robust- ness,
and most importantly, search sensitivity. The mock-data challenge data contains
an ensemble of 50 Scorpius X-1 (Sco X-1) type signals, simulated within a
frequency band of 50-1500 Hz. Simulated detector noise was generated assuming
the expected best strain sensitivity of Advanced LIGO and Advanced VIRGO ( Hz). A distribution of signal amplitudes was then
chosen so as to allow a useful comparison of search methodologies. A factor of
2 in strain separates the quietest detected signal, at
strain, from the torque-balance limit at a spin frequency of 300 Hz, although
this limit could range from (25 Hz) to (750 Hz) depending on the unknown frequency of Sco X-1. With future
improvements to the search algorithms and using advanced detector data, our
expectations for probing below the theoretical torque-balance strain limit are
optimistic.Comment: 33 pages, 11 figure
- âŠ