58 research outputs found
Analysis of structure and movement of neural growth cone
Growth cone is a sensory and motile part at the tip of a neuron and its main function is path finding. In previous works many chemical cascades and simple structure of growth cone were revealed. However, to understand the whole function of growth cone, deeper research on structure, especially on filopodia, is desired. Here, in this thesis, we developed a new method to segment growth cone and also count and categorize filopodia. Furthermore, evaluation of previous method and this new method were also established. By using this new software, deeper and more detailed research of structure of growth cone is now possible. The method has been tested on real data and evaluated against manually annotated ground truth with promising results
ASCA Observations of OAO 1657-415 and its Dust-Scattered X-Ray Halo
We report on two ASCA observations of the high-mass X-ray binary pulsar OAO
1657-415. A short observation near mid-eclipse caught the source in a
low-intensity state, with a weak continuum and iron emission dominated by the
6.4-keV fluorescent line. A later, longer observation found the source in a
high-intensity state and covered the uneclipsed through mid-eclipse phases. In
the high-intensity state, the non-eclipse spectrum has an absorbed continuum
component due to scattering by material near the pulsar and 80 per cent of the
fluorescent iron emission comes from less than 19 lt-sec away from the pulsar.
We find a dust-scattered X-ray halo whose intensity decays through the eclipse.
We use this halo to estimate the distance to the source as 7.1 +/- 1.3 kpc.Comment: Accepted for publication in MNRA
A Search for X-Ray Evidence of a Compact Companion to the Unusual Wolf-Rayet Star HD 50896 (EZ CMa)
We analyze results of a approx.25 ksec ASCA X-ray observation of the unusual Wolf-Rayet star HD 50896 (= EZ CMa). This WN5 star shows optical and ultraviolet variability at a 3.766 day period, which has been interpreted as a possible signature of a compact companion. Our objective was to search for evidence of hard X-rays (greater than or equal to 5 keV) which could be present if the WN5 wind is accreting onto a compact object. The ASCA spectra are dominated by emission below 5 keV and show no significant emission in the harder 5-10 keV range. Weak emission lines are present, and the X-rays arise in an optically thin plasma which spans a range of temperatures from less than or equal to 0.4 keV up to at least approx. 2 keV. Excess X-ray absorption above the interstellar value is present, but the column density is no larger than N(sub H) approx. 10(exp 22)/sq cm. The absorption-corrected X-ray luminosity L(sub x)(0.5 - 10 keV) = 10(exp 32.85) erg/s gives L(sub x)/ L(sub bol) approx. 10(exp -6), a value that is typical of WN stars. No X-ray variability was detected. Our main conclusion is that the X-ray properties of HD 50896 are inconsistent with the behavior expected for wind accretion onto a neutron star or black hole companion. Alternative models based on wind shocks can explain most aspects of the X-ray behavior, and we argue that the hotter plasma near approx. 2 keV could be due to the WR wind shocking onto a normal (nondegenerate) companion
Study of the Long Term Stability of two Anomalous X-ray Pulsars 4U 0142+61 and 1E 1048.1-5937 with ASCA
We present new observations of two anomalous X-ray pulsars (AXP) 4U 0142+61
and 1E 1048.1-5937 made in 1998 with the ASCA. The energy spectra of these two
AXPs are found to consist of two components, a power-law and a blackbody
emission from the neutron star surface. These observations, when compared to
earlier ASCA observations in 1994 show remarkable stability in the intensity,
spectral shape and pulse profile. However, we find that the spin-down rate in
1E 1048.1-5937 is not constant. In this source, we have clearly identified
three epochs with spin-down rates different from each other and the average
value. This has very strong implications for the magnetar hypothesis of AXPs.
We also note that the spin-down rate and its variations in 1E 1048.1-5937 are
much larger than what can normally be produced by an accretion disk with very
low mass accretion rate corresponding to its low X-ray luminosity.Comment: 12 pages, 7 postscript figures included, To appear in the
Astrophysical Journa
High resolution X-ray spectrum of the accreting binary X-ray pulsar GX 1+4
We present here high resolution X-ray spectrum of the accreting binary X-ray
pulsar GX 1+4 obtained with the High Energy Transmission Grating (HETG)
instrument of the Chandra X-ray Observatory. This was supplemented by a
simultaneous observation with the Proportional Counter Array (PCA) of the Rossi
X-ray Timing Explorer (RXTE). During this observation, the source was in a
somewhat low intensity state and the pulse profile with both Chandra and RXTE
shows a narrow dip, characteristic of GX 1+4 in medium and low intensity
states. The continuum X-ray spectrum obtained with the HETG and PCA can be
fitted well with a high energy cutoff power-law model with line of sight
absorption. Interestingly, we find that this low state is accompanied by a
relatively small absorption column density. A 6.4 keV narrow emission line with
an equivalent width of 70 eV is clearly detected in the HETG spectrum. The
fluorescence iron line, or at least part of it is produced in the neutral or
lowly ionized iron in the circumstellar material that also causes most of the
line of sight absorption. In the HETG spectrum, we have found evidence for a
weak (equivalent width ~30 eV) emission line at 6.95 keV. This line is
identified as Ly_alpha emission line from hydrogen-like iron and the spectrum
does not show emission lines from helium-like iron. We discuss various emission
regions for the hydrogen-like iron emission line, like gas diffused into the
Alfven sphere or an accretion curtain flowing from the inner accretion disk to
the magnetic poles.Comment: 15 pages, 4 postscript figures, accepted for publication in The
Astrophysical Journa
Quinazolinobenzodiazepine Derivatives, Novobenzomalvins A–C: Fibronectin Expression Regulators from Aspergillus novofumigatus
Three new quinazolinobenzodiazepine derivatives, novobenzomalvins A (1), B (2), and C (3), have been isolated as fibronectin expression regulators from Aspergillus novofumigatus CBS117520. The structures of 1 to 3 were established by spectroscopic and physicochemical analysis, and chemical investigation including the total synthesis of 1. Treatment with novo-benzomalvins A (1), B (2), C (3), and N-methylnovobenzomalvin A (5) increased the expression of fibronectin in normal human neonatal dermal fibroblast cells
X-ray Spectral Study of the Photoionized Stellar Wind in Vela~X-1
We present results from quantitative modeling and spectral analysis of the
high mass X-ray binary Vela X-1 obtained with the Chandra HETGS. The spectra
exhibit emission lines from H-like and He-like ions driven by photoionization,
as well as fluorescent emission lines from several elements in lower charge
states. In order to interpret and make full use of the high-quality data, we
have developed a simulator, which calculates the ionization and thermal
structure of a stellar wind photoionized by an X-ray source, and performs Monte
Carlo simulations of X-ray photons propagating through the wind. The emergent
spectra are then computed as a function of the viewing angle accurately
accounting for photon transport in three dimensions including dynamics. From
comparisons of the observed spectra with the simulation results, we are able to
find the ionization structure and the geometrical distribution of material in
Vela X-1 that can reproduce the observed spectral line intensities and
continuum shapes at different orbital phases remarkably well. It is found that
a large fraction of X-ray emission lines from highly ionized ions are formed in
the region between the neutron star and the companion star. We also find that
the fluorescent X-ray lines must be produced in at least three distinct regions
--(1)the extended stellar wind, (2)reflection off the stellar photosphere, and
(3)in a distribution of dense material partially covering and possibly trailing
the neutron star, which may be associated with an accretion wake. Finally, from
detailed analysis of the emission lines, we demonstrate that the stellar wind
is affected by X-ray photoionization.Comment: 22 pages, 7 tables, 24 figures, accepted for publication in The
Astrophysical Journa
Ionization States and Plasma Structures of Mixed-morphology SNRs Observed with ASCA
We present the results of a systematic study using ASCA of the ionization
state for six ``mixed-morphology'' supernova emnants (MMSNRs): IC 443, W49B,
W28, W44, 3C391, and Kes 27. MMSNRs show centrally filled thermal X-ray
emission, which contrasts to shell-like radio morphology, a set of
haracteristics at odds with the standard model of SNR evolution (e.g., the
Sedov model). We have therefore studied the evolution of the MMSNRs from the
ionization conditions inferred from the X-ray spectra, independent of X-ray
morphology. We find highly ionized plasmas approaching ionization equilibrium
in all the mmsnrs. The degree of ionization is systematically higher than the
plasma usually seen in shell-like SNRs. Radial temperature gradients are also
observed in five remnants, with cooler plasma toward the limb. In IC 443 and
W49B, we find a plasma structure consistent with shell-like SNRs, suggesting
that at least some MMSNRs have experienced similar evolution to shell-like
SNRs. In addition to the results above, we have discovered an ``overionized''
ionization state in W49B, in addition to that previously found in IC 443.
Thermal conduction can cause the hot interior plasma to become overionized by
reducing the temperature and density gradients, leading to an interior density
increase and temperature decrease. Therefore, we suggest that the
``center-filled'' X-ray morphology develops as the result of thermal
conduction, and should arise in all SNRs. This is consistent with the results
that MMSNRs are near collisional ionization equilibrium since the conduction
timescale is roughly similar to the ionization timescale. Hence, we conclude
that MMSNRs are those that have evolved over yr. We call this phase
as the ``conduction phase.''Comment: 34 pages, 20 figures, 9 tables, accepted for publication in The
Astrophysical Journa
Nature of the Soft Spectral Component in the X-ray Pulsars SMC X-1 and LMC X-4
We present here the results of an investigation of the pulse averaged and
pulse phase resolved energy spectra of two high luminosity accretion powered
X-ray pulsars SMC X-1 and LMC X-4 made with ASCA. The phase averaged energy
spectra definitely show the presence of a soft excess in both the sources. If
the soft excess is modeled as a separate black-body or thermal bremsstrahlung
type component, pulse phase resolved spectroscopy of SMC X-1 shows that the
soft component also has a pulsating nature. Same may be true for LMC X-4,
though a very small pulse fraction limits the statistical significance. The
pulsating soft component is found to have a nearly sinusoidal profile,
dissimilar to the complex profile seen at higher energies, which can be an
effect of smearing. Due to very high luminosity of these sources, the size of
the emission zone required for the soft component is large (radius ~300-400
km). We show that the pulsating nature of the soft component is difficult to
explain if a thermal origin is assumed for it. We further investigated with
alternate models, like inversely broken power-law or two different power-law
components and found that these models can also be used to explain the excess
at low energy. A soft power-law component may be a common feature of the
accreting X-ray pulsars, which is difficult to detect because most of the HMXB
pulsars are in the Galactic plane and experience large interstellar absorption.
In LMC X-4, we have also measured two additonal mid-eclipse times, which
confirm the known orbital decay.Comment: 28 pages (AAS preprint), 10 Postscript figures, ApJ accepte
- …