4,998 research outputs found
Study made of pneumatic high pressure piping materials /10,000 psi/
Evaluations of five types of steel for use in high pressure pneumatic piping systems include tests for impact strength, tensile and yield strengths, elongation and reduction in area, field weldability, and cost. One type, AISI 4615, was selected as most advantageous for extensive use in future flight vehicles
Smooth critical points of planar harmonic mappings
In a work in 1992, Lyzzaik studies local properties of light harmonic
mappings. More precisely, he classifies their critical points and accordingly
studies their topological and geometrical behaviours. We will focus our study
on smooth critical points of light harmonic maps. We will establish several
relationships between miscellaneous local invariants, and show how to connect
them to Lyzzaik's models. With a crucial use of Milnor fibration theory, we get
a fundamental and yet quite unexpected relation between three of the numerical
invariants, namely the complex multiplicity, the local order of the map and the
Puiseux pair of the critical value curve. We also derive similar results for a
real and complex analytic planar germ at a regular point of its Jacobian
level-0 curve. Inspired by Whitney's work on cusps and folds, we develop an
iterative algorithm computing the invariants. Examples are presented in order
to compare the harmonic situation to the real analytic one.Comment: 36 pages, 5 figure
Stability of the Forward/Reverse Shock System Formed by the Impact of a Relativistic Fireball on an Ambient Medium
We analyze the stability of a relativistic double (forward/reverse) shock
system which forms when the fireball of a Gamma-Ray Burst (GRB) impacts on the
surrounding medium. We find this shock system to be stable to linear global
perturbations for either a uniform or a wind (r^{-2}) density profile of the
ambient medium. For the wind case, we calculate analytically the frequencies of
the normal modes which could modulate the early short-term variability of GRB
afterglows. We find that perturbations in the double shock system could induce
oscillatory fluctuations in the observed flux on short (down to seconds) time
scales during the early phase of an afterglow.Comment: ApJ, submitted, 26 pages, 5 figure
Cosmic Variance In the Transparency of the Intergalactic Medium After Reionization
Following the completion of cosmic reionization, the mean-free-path of
ionizing photons was set by a population of Ly-limit absorbers. As the
mean-free-path steadily grew, the intensity of the ionizing background also
grew, thus lowering the residual neutral fraction of hydrogen in ionization
equilibrium throughout the diffuse intergalactic medium (IGM). Ly-alpha photons
provide a sensitive probe for tracing the distribution of this residual
hydrogen at the end of reionization. Here we calculate the cosmic variance
among different lines-of-sight in the distribution of the mean Ly-alpha optical
depths. We find fractional variations in the effective post-reionization
optical depth that are of order unity on a scale of ~100 co-moving Mpc, in
agreement with observations towards high-redshift quasars. Significant
contributions to these variations are provided by the cosmic variance in the
density contrast on the scale of the mean-free-path for ionizing photons, and
by fluctuations in the ionizing background induced by delayed or enhanced
structure formation. Cosmic variance results in a highly asymmetric
distribution of transmission through the IGM, with fractional fluctuations in
Ly-alpha transmission that ar larger than in Ly-beta transmission.Comment: 7 pages 3 figures. Replaced with version accepted for publication in
Ap
Variability of GRB Afterglows Due to Interstellar Turbulence
Gamma-Ray Burst (GRB) afterglows are commonly interpreted as synchrotron
emission from a relativistic blast wave produced by a point explosion in an
ambient medium, plausibly the interstellar medium of galaxies. We calculate the
amplitude of flux fluctuations in the lightcurve of afterglows due to
inhomogeneities in the surrounding medium. Such inhomogeneities are an
inevitable consequence of interstellar turbulence, but could also be generated
by variability and anisotropy in a precursor wind from the GRB progenitor.
Detection of their properties could provide important clues about the
environments of GRB sources. We apply our calculations to GRB990510, where an
rms scatter of 2% was observed for the optical flux fluctuations on the 0.1--2
hour timescale during the first day of the afterglow, consistent with it being
entirely due to photometric noise (Stanek et al. 1999). The resulting upper
limits on the density fluctuations on scales of 20-200 AU around the source of
GRB990510, are lower than the inferred fluctuation amplitude on similar scales
in the Galactic interstellar medium. Hourly monitoring of future optical
afterglows might therefore reveal fractional flux fluctuations at the level of
a few percent.Comment: 18 pages, submitted to Ap
Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra
The discovery of Gamma-Ray Burst (GRB) afterglows revealed a new class of
variable sources at optical and radio wavelengths. At present, the environment
and precise redshift of the detected afterglows are still unknown. We show that
if a GRB source resides in a compact (<100pc) gas-rich environment, the
afterglow spectrum will show time-dependent absorption features due to the
gradual ionization of the surrounding medium by the afterglow radiation.
Detection of this time-dependence can be used to constrain the size and density
of the surrounding gaseous system. For example, the MgII absorption line
detected in GRB970508 should have weakened considerably during the first month
if the absorption occurred in a gas cloud of size <100pc around the source. The
time-dependent HI or metal absorption features provide a precise determination
of the GRB redshift.Comment: 13 pages, 4 figures, submitted to ApJ
Self-Regulated Growth of Supermassive Black Holes in Galaxies as the Origin of the Optical and X-ray Luminosity Functions of Quasars
We postulate that supermassive black-holes grow in the centers of galaxies
until they unbind the galactic gas that feeds them. We show that the
corresponding self-regulation condition yields a correlation between black-hole
mass (Mbh) and galaxy velocity dispersion (sigma) as inferred in the local
universe, and recovers the observed optical and X-ray luminosity functions of
quasars at redshifts up to z~6 based on the hierarchical evolution of galaxy
halos in a Lambda-CDM cosmology. With only one free parameter and a simple
algorithm, our model yields the observed evolution in the number density of
optically bright or X-ray faint quasars between 2<z<6 across 3 orders of
magnitude in bolometric luminosity and 3 orders of magnitude in comoving
density per logarithm of luminosity. The self-regulation condition identifies
the dynamical time of galactic disks during the epoch of peak quasar activity
(z~2.5) as the origin of the inferred characteristic quasar lifetime of ~10
million years. Since the lifetime becomes comparable to the Salpeter e-folding
time at this epoch, the model also implies that the Mbh-sigma relation is a
product of feedback regulated accretion during the peak of quasar activity. The
mass-density in black-holes accreted by that time is consistent with the local
black-hole mass density of ~(0.8-6.3) times 10^5 solar masses per cubic Mpc,
which we have computed by combining the Mbh-sigma relation with the measured
velocity dispersion function of SDSS galaxies (Sheth et al.~2003). Applying a
similar self-regulation principle to supernova-driven winds from starbursts, we
find that the ratio between the black hole mass and the stellar mass of
galactic spheroids increases with redshift as (1+z)^1.5 although the Mbh-sigma
relation is redshift-independent.Comment: 10 pages, 5 figures, submitted to Ap
Effect of Gravitational Lensing on Measurements of the Sunyaev-Zel'dovich Effect
The Sunyaev-Zel'dovich (SZ) effect of a cluster of galaxies is usually
measured after background radio sources are removed from the cluster field.
Gravitational lensing by the cluster potential leads to a systematic deficit in
the residual intensity of unresolved sources behind the cluster core relative
to a control field far from the cluster center. As a result, the measured
decrement in the Rayleigh-Jeans temperature of the cosmic microwave background
is overestimated. We calculate the associated systematic bias which is
inevitably introduced into measurements of the Hubble constant using the SZ
effect. For the cluster A2218, we find that observations at 15 GHz with a beam
radius of 0'.4 and a source removal threshold of 100 microJy underestimate the
Hubble constant by 6-10%. If the profile of the gas pressure declines more
steeply with radius than that of the dark matter density, then the ratio of
lensing to SZ decrements increases towards the outer part of the cluster.Comment: 11 pages, 3 figures, submitted to ApJ
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
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