14,643 research outputs found
RXTE and BeppoSAX Observations of MCG -5-23-16: Reflection From Distant Cold Material
We examine the spectral variability of the Seyfert 1.9 galaxy MCG -5-23-16
using RXTE and BeppoSAX observations spanning 2 years from April 1996 to April
1998. During the first year the X-ray source brightens by a factor of ~25% on
timescales of days to months. During this time, the reprocessed continuum
emission seen with RXTE does not respond measurably to the continuum increase.
However, by the end of the second year during the BeppoSAX epoch the X-ray
source has faded again. This time, the reprocessed emission has also faded,
indicating that the reprocessed flux has responded to the continuum. If these
effects are caused by time delays due to the distance between the X-ray source
and the reprocessing region, we derive a light crossing time of between ~1
light day and ~1.5 light years. This corresponds to a distance of 0.001 pc to
0.55 pc, which implies that the reprocessed emission originates between 3x10^15
cm and 1.6x10^18 cm from the X-ray source. In other words, the reprocessing in
MCG -5-23-16 is not dominated by the inner regions of a standard accretion
disk.Comment: Accepted for publication in ApJ. 17 pages, 8 figure
On the area of the symmetry orbits in symmetric spacetimes
We obtain a global existence result for the Einstein equations. We show that
in the maximal Cauchy development of vacuum symmetric initial data with
nonvanishing twist constant, except for the special case of flat Kasner initial
data, the area of the group orbits takes on all positive values. This
result shows that the areal time coordinate which covers these spacetimes
runs from zero to infinity, with the singularity occurring at R=0.Comment: The appendix which appears in version 1 has a technical problem (the
inequality appearing as the first stage of (52) is not necessarily true), and
since the appendix is unnecessary for the proof of our results, we leave it
out. version 2 -- clarifications added, version 3 -- reference correcte
Dynamic Correlation in Wave Propagation in Random Media
We report time-resolved measurements of the statistics of pulsed transmission
through quasi-one-dimensional dielectric media with static disorder. The
normalized intensity correlation function with displacement and polarization
rotation for an incident pulse of linewidth at delay time t is a
function only of the field correlation function, which is identical to that
found for steady-state excitation, and of , the residual
degree of intensity correlation at points at which the field correlation
function vanishes. The dynamic probability distribution of normalized intensity
depends only upon . Steady-state statistics are recovered
in the limit ->0, in which is the steady-state
degree of correlation.Comment: 4 RevTex pages, 4 figure
Laser Doppler technology applied to atmospheric environmental operating problems
Carbon dioxide laser Doppler ground wind data were very favorably compared with data from standard anemometers. As a result of these measurements, two breadboard systems were developed for taking research data: a continuous wave velocimeter and a pulsed Doppler system. The scanning continuous wave laser Doppler velocimeter developed for detecting, tracking and measuring aircraft wake vortices was successfully tested at an airport where it located vortices to an accuracy of 3 meters at a range of 150 meters. The airborne pulsed laser Doppler system was developed to detect and measure clear air turbulence (CAT). This system was tested aboard an aircraft, but jet stream CAT was not encountered. However, low altitude turbulence in cumulus clouds near a mountain range was detected by the system and encountered by the aircraft at the predicted time
The Singularity in Generic Gravitational Collapse Is Spacelike, Local, and Oscillatory
A longstanding conjecture by Belinskii, Khalatnikov, and Lifshitz that the
singularity in generic gravitational collapse is spacelike, local, and
oscillatory is explored analytically and numerically in spatially inhomogeneous
cosmological spacetimes. With a convenient choice of variables, it can be seen
analytically how nonlinear terms in Einstein's equations control the approach
to the singularity and cause oscillatory behavior. The analytic picture
requires the drastic assumption that each spatial point evolves toward the
singularity as an independent spatially homogeneous universe. In every case,
detailed numerical simulations of the full Einstein evolution equations support
this assumption.Comment: 7 pages includes 4 figures. Uses Revtex and psfig. Received
"honorable mention" in 1998 Gravity Research Foundation essay contest.
Submitted to Mod. Phys. Lett.
High velocity spikes in Gowdy spacetimes
We study the behavior of spiky features in Gowdy spacetimes. Spikes with
velocity initially high are, generally, driven to low velocity. Let n be any
integer greater than or equal to 1. If the initial velocity of an upward
pointing spike is between 4n-3 and 4n-1 the spike persists with final velocity
between 1 and 2, while if the initial velocity is between 4n-1 and 4n+1, the
spiky feature eventually disappears. For downward pointing spikes the analogous
rule is that spikes with initial velocity between 4n-4 and 4n-2 persist with
final velocity between 0 and 1, while spikes with initial velocity between 4n-2
and 4n eventually disappear.Comment: discussion of constraints added. Accepted for publication in Phys.
Rev.
Laser Doppler dust devil measurements
A scanning laser doppler velocimeter (SLDV) system was used to detect, track, and measure the velocity flow field of naturally occurring tornado-like flows (dust devils) in the atmosphere. A general description of the dust devil phenomenon is given along with a description of the test program, measurement system, and data processing techniques used to collect information on the dust devil flow field. The general meteorological conditions occurring during the test program are also described, and the information collected on two selected dust devils are discussed in detail to show the type of information which can be obtained with a SLDV system. The results from these measurements agree well with those of other investigators and illustrate the potential for the SLDV in future endeavors
Effect of molecular models on viscosity and thermal conductivity calculations
The effect of molecular models on viscosity and thermal conductivity calculations is investigated. The Direct Simulation Monte Carlo (DSMC) method for rarefied gas flows is used to simulateCouette and Fourier flows as a means of obtaining the transport coefficients. Experimentalmeasurements for argon (Ar) provide a baseline for comparison over a wide temperature range of 100–1,500 K. The variable hard sphere (VHS), variable soft sphere (VSS), and Lennard-Jones (L-J) molecular models have been implemented into a parallel version of Bird’s one-dimensional DSMC code, DSMC1, and the model parameters have been recalibrated to the current experimental data set. While the VHS and VSS models only consider the short-range, repulsive forces, the L-J model also includes constributions from the long-range, dispersion forces. Theoretical results for viscosity and thermal conductivity indicate the L-J model is more accurate than the VSS model; with maximum errors of 1.4% and 3.0% in the range 300–1,500 K for L-J and VSS models, respectively. The range of validity of the VSS model is extended to 1,650 K through appropriate choices for the model parameters
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