6,611 research outputs found
A Silver Anniversary Observation of the X-ray Emitting SN1978K in NGC 1313
We describe the results of a 2003 Chandra ACIS-I observation of SN1978K. The
spectrum shows little flux below 0.6 keV, in contrast to the 2002 ACIS-S
observation that showed flux to 0.4 keV. Fitting the ACIS-I spectrum alone
leads to two solutions depending upon the value of the column density. A joint
fit using a dual thermal plasma model applied to the ACIS-I and a
contemporaneous XMM spectrum, which if fit alone also leads to a two-column
solution, yields a single column density fit. The fitted temperature of the
joint fit for the soft component remains constant with the errors from previous
Chandra, XMM, and ASCA data. The hard temperature recovers from its 2000-2002
decline and corresponds to an increase in the column density during that time.
The hard (2-10 keV) light curve is confirmed to be declining. The derived
number density represents a lower limit of 1e5 depending upon the adopted
filling factor of the emitting volume, leading to an estimated mass cooling
rate of 0.1-0.15 solar masses per year.Comment: accepted A
Development of a 1000V, 200A, low-loss, fast-switching, gate-assisted turn-off thyristor
Feasibility was demonstrated for a thyristor that blocks 1000V forward and reverse, conducts 200A, and turns on in little more than 2 microsec with only 2A of gate drive. Its features include a turn-off time of 3 microsec achieved with 2A of gate assist current of a few microseconds duration and an energy dissipation of only 12 mJ per pulse for a 20 microsec half sine wave, 200A pulse. Extensive theoretical and experimental study of the electrical behavior of thyristors having a fast turn-off time have significantly improved the understanding of the physics of turning thyristor off. Thyristors of two new designs were fabricated and evaluated. The high speed and low power were achieved by a combination of gate amplification, cathode shunting, and gate-assisted turn-off. Two techniques for making this combination practical are described
Soft X-ray Emission from the Spiral Galaxy NGC 1313
The nearby barred spiral galaxy NGC 1313 has been observed with the PSPC
instr- ument on board the ROSAT X-ray satellite. Ten individual sources are
found. Three sources (X-1, X-2 and X-3 [SN~1978K]) are very bright (~10^40
erg/s) and are unusual in that analogous objects do not exist in our Galaxy. We
present an X-ray image of NGC~1313 and \xray spectra for the three bright
sources. The emission from the nuclear region (R ~< 2 kpc) is dominated by
source X-1, which is located ~1 kpc north of the photometric (and dynamical)
center of NGC~1313. Optical, far-infrared and radio images do not indicate the
presence of an active galactic nucleus at that position; however, the compact
nature of the \xray source (X-1) suggests that it is an accretion-powered
object with central mass M >~ 10^3 Msun. Additional emission (L_X ~ 10^39
erg/s) in the nuclear region extends out to ~2.6 kpc and roughly follows the
spiral arms. This emission is from 4 sources with luminosity of several x 10^38
erg/s, two of which are consistent with emission from population I sources
(e.g., supernova remnants, and hot interstellar gas which has been heated by
supernova remnants). The other two sources could be emission from population II
sources (e.g., low-mass \xray binaries). The bright sources X-2 and SN~1978K
are positioned in the southern disk of NGC~1313. X-2 is variable and has no
optical counterpart brighter than 20.8 mag (V-band). It is likely that it is an
accretion-powered object in NGC~1313. The type-II supernova SN~1978K (Ryder
\etal 1993) has become extra- ordinarily luminous in X-rays 13 years
after optical maximum.Comment: to appear in 10 Jun 1995 ApJ, 30 pgs uuencoded compressed postscript,
25 pgs of figures available upon request from colbert, whole preprint
available upon request from Sandy Shrader ([email protected]),
hopefully fixed unknown problem with postscript fil
New spectral classification technique for X-ray sources: quantile analysis
We present a new technique called "quantile analysis" to classify spectral
properties of X-ray sources with limited statistics. The quantile analysis is
superior to the conventional approaches such as X-ray hardness ratio or X-ray
color analysis to study relatively faint sources or to investigate a certain
phase or state of a source in detail, where poor statistics does not allow
spectral fitting using a model. Instead of working with predetermined energy
bands, we determine the energy values that divide the detected photons into
predetermined fractions of the total counts such as median (50%), tercile (33%
& 67%), and quartile (25% & 75%). We use these quantiles as an indicator of the
X-ray hardness or color of the source. We show that the median is an improved
substitute for the conventional X-ray hardness ratio. The median and other
quantiles form a phase space, similar to the conventional X-ray color-color
diagrams. The quantile-based phase space is more evenly sensitive over various
spectral shapes than the conventional color-color diagrams, and it is naturally
arranged to properly represent the statistical similarity of various spectral
shapes. We demonstrate the new technique in the 0.3-8 keV energy range using
Chandra ACIS-S detector response function and a typical aperture photometry
involving background subtraction. The technique can be applied in any energy
band, provided the energy distribution of photons can be obtained.Comment: 11 pages, 9 figures, accepted for publication in Ap
Effect of electron heating on self-induced transparency in relativistic intensity laser-plasma interaction
The effective increase of the critical density associated with the
interaction of relativistically intense laser pulses with overcritical plasmas,
known as self-induced transparency, is revisited for the case of circular
polarization. A comparison of particle-in-cell simulations to the predictions
of a relativistic cold-fluid model for the transparency threshold demonstrates
that kinetic effects, such as electron heating, can lead to a substantial
increase of the effective critical density compared to cold-fluid theory. These
results are interpreted by a study of separatrices in the single-electron phase
space corresponding to dynamics in the stationary fields predicted by the
cold-fluid model. It is shown that perturbations due to electron heating
exceeding a certain finite threshold can force electrons to escape into the
vacuum, leading to laser pulse propagation. The modification of the
transparency threshold is linked to the temporal pulse profile, through its
effect on electron heating.Comment: 13 pages, 12 figures; fixed some typos and improved discussion of
review materia
Evaluation of Static Work Capabilities in a Hot Environment
The effects of heat stress on an individual's static work capabilities were investigated. Various indicators of strength and endurance were measured under differing heat stress levels. One measure which exhibited a significant change at higher heat stress levels was a combined strength-endurance measurement involving a series of maximal voluntary contractions throughout a one hour exposure. At 35° C WBGT the decrease in MVC from the beginning of the exposure to the end was 43% compared to only 6% at 18° C WBGT. Core temperature was also monitored throughout each exposure.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Development of a 1000V, 200A, low-loss, fast-switching, gate-assisted turn-off thyristor
The results of a program to develop a fast high power thyristor that can operate in switching circuits at frequencies of 10 to 20 kHz with very low power loss are given. Feasibility was demonstrated for a thyristor that blocks 1000V forward and reverse, conducts 200A, turns on in little more than 2 more microseconds with only 2A of gate drive, turns off in 3 microseconds with 2A of gate assist current and has an energy dissipation of only 12 mJ per pulse for a 20 microsecond half sine wave 200A pulse. Data were generated that clearly showed the tradeoffs that can be made between the turn off time and forward drop. The understanding of this relationship is necessary in the selection of deliverable thyristors with turn off times up to 7 microseconds to give improved efficiency in a series resonant dc to dc inverter application
Food Plants Utilized by Migratory Waterfowl at Cheyenne Bottoms Waterfowl Refuge
The purpose of this problem was to gather information on food habits of migratory water fowl using Cheyenne Bottoms Waterfowl Refuge, and to determine the most important duck-food species. Ducks were examined as hunters checked through the headquarters building. Food staples were removed from the crops of these birds and numbered. The species of duck, age, sex, plumage, and date of collection were recorded. Contents of the samples were examined and identified. Plant species represented in the crop samples were tabulated by the percent frequency method, and the most important plant species determined. Food preferred by each species of duck was determined by this method. Grain sorghum (Sorghum vulgare), spikerush (Eleocharis palustris), sprangletop (Leptochloa fascicularis), barnyard grass (Echinochloa crusgalli), and purple ammannia (Ammannia coccinea) were the most important plant species
The Effect of Interstellar Absorption on Measurements of the Baryon Acoustic Peak in the Lyman-{\alpha} Forest
In recent years, the autocorrelation of the hydrogen Lyman-{\alpha} forest
has been used to observe the baryon acoustic peak at redshift 2 < z < 3.5 using
tens of thousands of QSO spectra from the BOSS survey. However, the
interstellar medium of the Milky-Way introduces absorption lines into the
spectrum of any extragalactic source. These lines, while weak and undetectable
in a single BOSS spectrum, could potentially bias the cosmological signal. In
order to examine this, we generate absorption line maps by stacking over a
million spectra of galaxies and QSOs. We find that the systematics introduced
are too small to affect the current accuracy of the baryon acoustic peak, but
might be relevant to future surveys such as the Dark Energy Spectroscopic
Instrument (DESI). We outline a method to account for this with future
datasets.Comment: MNRAS accepted. Minor change
- …