1,149 research outputs found
Probing the Intergalactic Medium with Fast Radio Bursts
The recently discovered fast radio bursts (FRBs), presumably of
extra-galactic origin, have the potential to become a powerful probe of the
intergalactic medium (IGM). We point out a few such potential applications. We
provide expressions for the dispersion measure and rotation measure as a
function of redshift, and we discuss the sensitivity of these measures to the
HeII reionization and the IGM magnetic field. Finally we calculate the
microlensing effect from an isolate, extragalctic stellar-mass compact object
on the FRB spectrum. The time delays between the two lensing images will induce
constructive and destructive interference, leaving a specific imprint on the
spectra of FRBs. With a high all-sky rate, a large statistical sample of FRBs
is expected to make these applications feasible.Comment: 4 pages, 1 figure; Typos for the variable x in Eq.6 corrected;
Published in ApJ; Originally the Appendix E of arXiv:1402.4766; Separated
from the main paper upon the referee's reques
Precision measurements of large scale structure with future type Ia supernova surveys
Type Ia supernovae are currently the best known standard candles at
cosmological distances. In addition to providing a powerful probe of dark
energy they are an ideal source of information about the peculiar velocity
field of the local universe. Even with the very small number of supernovae
presently available it has been possible to measure the dipole and quadrupole
of the local velocity field out to z~0.025. With future continuous all-sky
surveys like the LSST project the luminosity distances of tens of thousands of
nearby supernovae will be measured accurately. This will allow for a
determination of the local velocity structure of the universe as a function of
redshift with unprecedented accuracy, provided the redshifts of the host
galaxies are known. Using catalogues of mock surveys we estimate that future
low redshift supernova surveys will be able to probe sigma-8 to a precision of
roughly 5% at 95% C.L. This is comparable to the precision in future galaxy and
weak lensing surveys and with a relatively modest observational effort it will
provide a crucial cross-check on future measurements of the matter power
spectrum.Comment: 18 pages, 9 figures, submitted to JCA
Precision Pointing of IBEX-Lo Observations
Post-launch boresight of the IBEX-Lo instrument onboard the Interstellar
Boundary Explorer (IBEX) is determined based on IBEX-Lo Star Sensor
observations. Accurate information on the boresight of the neutral gas camera
is essential for precise determination of interstellar gas flow parameters.
Utilizing spin-phase information from the spacecraft attitude control system
(ACS), positions of stars observed by the Star Sensor during two years of IBEX
measurements were analyzed and compared with positions obtained from a star
catalog. No statistically significant differences were observed beyond those
expected from the pre-launch uncertainty in the Star Sensor mounting. Based on
the star observations and their positions in the spacecraft reference system,
pointing of the IBEX satellite spin axis was determined and compared with the
pointing obtained from the ACS. Again, no statistically significant deviations
were observed. We conclude that no systematic correction for boresight geometry
is needed in the analysis of IBEX-Lo observations to determine neutral
interstellar gas flow properties. A stack-up of uncertainties in attitude
knowledge shows that the instantaneous IBEX-Lo pointing is determined to within
\sim 0.1\degr in both spin angle and elevation using either the Star Sensor
or the ACS. Further, the Star Sensor can be used to independently determine the
spacecraft spin axis. Thus, Star Sensor data can be used reliably to correct
the spin phase when the Star Tracker (used by the ACS) is disabled by bright
objects in its field-of-view. The Star Sensor can also determine the spin axis
during most orbits and thus provides redundancy for the Star Tracker.Comment: 22 pages, 18 figure
Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots
Several widespread changes in the ecology of old-growth tropical forests have recently been documented for the late twentieth century, in particular an increase in stem turnover (pan-tropical), and an increase in above-ground biomass (neotropical). Whether these changes are synchronous and whether changes in growth are also occurring is not known. We analysed stand-level changes within 50 long-term. monitoring plots from across South America spanning 1971-2002. We show that: (i) basal area (BA: sum of the cross-sectional areas of all trees in a plot) increased significantly over time (by 0.10 +/- 0.04 m(2) ha(-1) yr(-1), mean +/- 95% CI); as did both (ii) stand-level BA growth rates (sum of the increments of BA of surviving trees and BA of new trees that recruited into a plot); and (iii) stand-level BA mortality rates (sum of the cross-sectional areas of all trees that died in a plot). Similar patterns were observed on a per-stem basis: (i) stem density (number of stems per hectare; 1 hectare is 10(4) m(2)) increased significantly over time (0.94 +/- 0.63 stems ha(-1) yr(-1)); as did both (ii) stem recruitment rates; and (iii) stem mortality rates. In relative terms, the pools of BA and stem density increased by 0.38 +/- 0.15% and 0.18 +/- 0.12% yr(-1), respectively. The fluxes into and out of these pools-stand-level BA growth, stand-level BA mortality, stem recruitment and stem mortality rates-increased, in relative terms, by an order of magnitude more. The gain terms (BA growth, stem recruitment) consistently exceeded the loss terms (BA loss, stem mortality) throughout the period, suggesting that whatever process is driving these changes was already acting before the plot network was established. Large long-term increases in stand-level BA growth and simultaneous increases in stand BA and stem density imply a continent-wide increase in resource availability which is increasing net primary productivity and altering forest dynamics. Continent-wide changes in incoming solar radiation, and increases in atmospheric concentrations of CO2 and air temperatures may have increased resource supply over recent decades, thus causing accelerated growth and increased dynamism across the world's largest tract of tropical forest
Identifying runoff sources across scales in Amazon watersheds: an LBA synthesis effort.
Abstract ID: 54
Correlations in Two-Dimensional Vortex Liquids
We report on a high temperature perturbation expansion study of the
superfluid-density spatial correlation function of a Ginzburg-Landau-model
superconducting film in a magnetic field. We have derived a closed form which
expresses the contribution to the correlation function from each graph of the
perturbation theory in terms of the number of Euler paths around appropriate
subgraphs. We have enumerated all graphs appearing out to 10-th order in the
expansion and have evaluated their contributions to the correlation function.
Low temperature correlation functions, obtained using Pad\'{e} approximants,
are in good agreement with Monte Carlo simulation results and show that the
vortex-liquid becomes strongly correlated at temperatures well above the vortex
solidification temperature.Comment: 18 pages (RevTeX 3.0) and 4 figures, available upon request,
IUCM93-01
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Status of the DIII-D 110 GHz ECH system
The DIII-D program is presently commissioning the first NM gyrotron of a planned 3 MW, I 10 GHz electron cyclotron heating (ECH) system for off-axis electron heating and current drive. Advanced tokamak (AT) research in DIII-D and other tokamaks requires the ability to control the current density profile. ECH offers the ability to localize the heating and driven current in a controllable manner and is not dependent upon, the local plasma conditions, so it appears to be an ideal tool for AT research. The planned rf sources for the DIII-D system are I MW state-of-the-art internal mode-converter gyrotrons, with one gyrotron being manufactured by GYCOM, a Russian company, and two gyrotrons being manufactured by CPI (formerly Varian). The GYCOM gyrotron has been tested at the factory to 960 kW, 2 seconds and has been shipped to GA where it is now undergoing initial checkout and testing. The first CPI gyrotron has been assembled and factory tested to 530 kW, 2 seconds and 350 1352 kW, 10 seconds. Both the GYCOM and CPI gyrotrons are limited in pulse length at full power by thermal limits on the output window. The second CPI gyrotron is expected to be ready for testing in April 1996. This paper will report on the initial experiences of using the GYCOM I MW, 110 GHz internal mode- converter gyrotron, at General Atomics, and the observed effects the ECRH power has on the DIII-D plasma
Giant Sparks at Cosmological Distances?
Millisecond-duration bright radio pulses at 1.4 GHz with high dispersion measures (DMs) were reported by Lorimer et al., Keane et al., and Thornton et al. Their all-sky rate is ≈10^4 day^(–1) above ~1 Jy. Related events are "Perytons"—similar pulsed, dispersed sources, but most certainly local. Suggested models of fast radio bursts (FRBs) can originate in Earth's atmosphere, in stellar coronae, in other galaxies, and even at cosmological distances. Using physically motivated assumptions combined with observed properties, we explore these models. In our analysis, we focus on the Lorimer event: a 30 Jy, 5 ms duration burst with DM = 375 cm^(–3) pc, exhibiting a steep frequency-dependent pulse width (the Sparker). To be complete, we drop the assumption that high DMs are produced by plasma propagation and assume that the source produces pulses with frequency-dependent arrival time ("chirped signals"). Within this framework, we explore a scenario in which Perytons, the Sparker, and the FRBs are all atmospheric phenomena occurring at different heights. This model is ad hoc in that we cannot explain why Perytons at higher altitudes show greater DMs or exhibit narrower pulses. Nonetheless, we argue that the Sparker may be a Peryton. We end with two remarks. First, the detection of a single FRB by an interferometer with a kilometer (or longer) baseline will prove that FRBs are of extraterrestrial origin. Second, we urge astronomers to pursue observations and understanding of Perytons since they form (at least) a formidable foreground for the FRBs
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