77,390 research outputs found
Is SGR 1900+14 a Magnetar?
We present RXTE observations of the soft gamma--ray repeater SGR 1900+14
taken September 4-18, 1996, nearly 2 years before the 1998 active period of the
source. The pulsar period (P) of 5.1558199 +/- 0.0000029 s and period
derivative (Pdot) of (6.0 +/- 1.0) X 10^-11 s/s measured during the 2-week
observation are consistent with the mean Pdot of (6.126 +/- 0.006) X 10^-11 s/s
over the time up to the commencement of the active period. This Pdot is less
than half that of (12.77 +/- 0.01) X 10^-11 s/s observed during and after the
active period. If magnetic dipole radiation were the primary cause of the
pulsar spindown, the implied pulsar magnetic field would exceed the critical
field of 4.4 X 10^13 G by more than an order of magnitude, and such field
estimates for this and other SGRs have been offered as evidence that the SGRs
are magnetars, in which the neutron star magnetic energy exceeds the rotational
energy. The observed doubling of Pdot, however, would suggest that the pulsar
magnetic field energy increased by more than 100% as the source entered an
active phase, which seems very hard to reconcile with models in which the SGR
bursts are powered by the release of magnetic energy. Because of this, we
suggest that the spindown of SGR pulsars is not driven by magnetic dipole
radiation, but by some other process, most likely a relativistic wind. The
Pdot, therefore, does not provide a measure of the pulsar magnetic field
strength, nor evidence for a magnetar.Comment: 14 pages, aasms4 latex, figures 1 & 2 changed, accepted by ApJ
letter
An investigation of the optimization of parameters affecting the implementation of fourier transform spectroscopy at 20-500 micron from the C-141 airborne infrared observatory
A program for 20-500 micron spectroscopy from the NASA flying C141 infrared observatory is being carried out with a Michelson interferometer. The parameters affecting the performance of the instrument are studied and an optimal configuration for high performance on the C-141 aircraft is recommended. As each parameter is discussed the relative merits of the two modes of mirror motion (rapid scan or step and integrate) are presented
Research on graphite reinforced glass matrix composites
A broad group of fibers and matrices were combined to create a wide range of composite properties. Primary material fabrication procedures were developed which readily permit the fabrication of flat plate and shaped composites. Composite mechanical properties were measured under a wide range of test conditions. Tensile, flexure mechanical fatigue, thermal fatigue, fracture toughness, and fatigue crack growth resistance were evaluated. Selected fiber-matrix combinations were shown to maintain their strength at up to 1300 K when tested in an inert atmosphere. Composite high temperature mechanical properties were shown to be limited primarily by the oxidation resistance of the graphite fibers. Composite thermal dimensional stability was measured and found to be excellent
Laser cooling in the Penning trap: an analytical model for cooling rates in the presence of an axializing field
Ions stored in Penning traps may have useful applications in the field of
quantum information processing. There are, however, difficulties associated
with the laser cooling of one of the radial motions of ions in these traps,
namely the magnetron motion. The application of a small radio-frequency
quadrupolar electric potential resonant with the sum of the two radial motional
frequencies has been shown to couple these motions and to lead to more
efficient laser cooling. We present an analytical model that enables us to
determine laser cooling rates in the presence of such an 'axializing' field. It
is found that this field leads to an averaging of the laser cooling rates for
the two motions and hence improves the overall laser cooling efficiency. The
model also predicts shifts in the motional frequencies due to the axializing
field that are in qualitative agreement with those measured in recent
experiments. It is possible to determine laser cooling rates experimentally by
studying the phase response of the cooled ions to a near resonant excitation
field. Using the model developed in this paper, we study the expected phase
response when an axializing field is present.Comment: 22 pages, 7 figure
Method of purifying metallurgical grade silicon employing reduced pressure atmospheric control
A method in which a quartz tube is charged with chunks of metallurgical grade silicon and/or a mixture of such chunks and high purity quartz sand, and impurities from a class including aluminum, boron, as well as certain transition metals including nickel, iron, and manganese is described. The tube is then evacuated and heated to a temperature within a range of 800 C to 1400 C. A stream of gas comprising a reactant, such as silicon tetrafluoride, is continuously delivered at low pressures through the charge for causing a metathetical reaction of impurities of the silicon and the reactant to occur for forming a volatile halide and leaving a residue of silicon of an improved purity. The reactant which included carbon monoxide gas and impurities such as iron and nickel react to form volatile carbonyls
Support for Instructional Leadership: Supervision, Mentoring, and Professional Development for U.S. School Leaders - Findings from the American School Leader Panel
With school leadership second only to teaching among school-related influences on student learning, principals can play an important role in school success. But how do their districts promote their effectiveness, especially in improving teaching? Based on a survey of the American School Leader Panel, a representative sample of principals from across the United States, this report explores the prevalence and quality of three important on-the-job supports for school leaders: supervision, mentorship and professional development (as defined by at least a day focused on principals). The good news is that two-thirds of principals report receiving some support. The bad news is that more than two thirds (68 percent) report that they don't receive all three sources of help. Mentoring, for example, is typically available only to first- or second-year principals or those encountering difficulties on the job, and only a minority of principals report that their districts require mentoring, even for first-year (49 percent of respondents) or struggling principals (21 percent). Also, the prevalence of support a principal receives may depend on the size of his or her school district. Both mentoring and professional development are more readily available in larger and medium-sized districts than smaller ones. The value principals place on the support they receive is linked to whether the support emphasizes the key aspect of principals' job—improving teachers' instruction. For example, all of the principals (100 percent) who reported that their mentors focused on instruction to a great extent also said that they prized the mentoring. That compares with a minority (40 percent) of principals who said their mentors devoted little to no time to instruction
Aerobee 350, flight 17.03 GE - Instrumentation information and calibration data
Airborne instrumentation system on Aerobee 350 sounding rocke
Effects of temperature fluctuations of IUE data quality
Analysis of IUE calibration lamp images shows that variation in the temperature of the scientific instrument causes shifts in the location of the spectral format with respect to the reseau grid on the detector and in the location of the reseaux themselves. In high dispersion, a camera head amplifier temperature difference of 6C corresponds to a shift of 4 pixels in the spectral format for LWR and 2 pixels for SWP along the dispersion direction. Shifts perpendicular to the disperson (for the same temperature difference) are less than one pixel for both cameras. In low dispersion spectra, the shifts are similar but orthogonal to those described above with the larger motion lying in the direction perpendicular to the dispersion. In both dispersion modes, the observed shifts are apparently independent of wavelength. In high dispersion, the constant pixel shift mimics a constant velocity error
The Reionization of Carbon
Observations suggest that CII was more abundant than CIV in the intergalactic
medium towards the end of the hydrogen reionization epoch. This transition
provides a unique opportunity to study the enrichment history of intergalactic
gas and the growth of the ionizing background (UVB) at early times. We study
how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic
simulation that includes a self-consistent multifrequency UVB as well as a
well-constrained model for galactic outflows to disperse metals. Our predicted
UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement
given the uncertainties. Nonetheless, we use a calibration in post-processing
to account for Lyman-alpha forest measurements while preserving the predicted
spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way
that it always exceeds the volume average in regions where metals are found.
This implies both that a spatially-uniform UVB is a poor approximation and that
metal absorption is not sensitive to the epoch when HII regions overlap
globally even at column densites of 10^{12} cm^{-2}. We find, consistent with
observations, that the CII mass fraction drops to low redshift while CIV rises
owing the combined effects of a growing UVB and continued addition of carbon in
low-density regions. This is mimicked in absorption statistics, which broadly
agree with observations at z=6-3 while predicting that the absorber column
density distributions rise steeply to the lowest observable columns. Our model
reproduces the large observed scatter in the number of low-ionization absorbers
per sightline, implying that the scatter does not indicate a partially-neutral
Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA
Relativistic Winds from Compact Gamma-Ray Sources: II. Pair Loading and Radiative Acceleration in Gamma-ray Bursts
We consider the effects of rapid pair creation by an intense pulse of
gamma-rays propagating ahead of a relativistic shock. Side-scattered photons
colliding with the main gamma-ray beam amplify the density of scattering
charges. The acceleration rate of the pair-loaded medium is calculated, and its
limiting bulk Lorentz factor related to the spectrum and compactness of the
photon source. One obtains, as a result, a definite prediction for the relative
inertia in baryons and pairs. The deceleration of a relativistic shock in the
moving medium, and the resulting synchrotron emissivity, are compared with
existing calculations for a static medium. The radiative efficiency is
increased dramatically by pair loading. When the initial ambient density
exceeds a critical value, the scattering depth traversed by the main gamma-ray
pulse rises above unity, and the pulse is broadened. These considerations place
significant constraints on burst progenitors: a pre-burst mass loss rate
exceeding 10^{-5} M_\odot per year is difficult to reconcile with individual
pulses narrower than 10 s, unless the radiative efficiency is low. An
anisotropic gamma-ray flux (on an angular scale \Gamma^{-1} or larger) drives a
large velocity shear that greatly increases the energy in the seed magnetic
field forward of the propagating shock.Comment: 19 pp., LaTeX (aaspp4.sty), revised 12/23/99, Ap. J. in press;
summary section added and several minor improvements in presentatio
- …