10,802 research outputs found
Titanium alloy stress corrosion cracking in presence of dinitrogen tetroxide
Study resulting in a satisfactory stress corrosion cracking test with extremely consistent results produced six new analytical methods. Methods detect and determine differences in the minor constituent composition of different types of dinitrogen tetroxide
Age-Related Decrease in the Schaffer Collateral-Evoked EPSP in Awake, Freely, Behaving Rats
Synaptic response size in the CA1 region of
the hippocampus in aged rats is reduced for a
given stimulus intensity, compared with that
elicited in young rats. Consistent with the in vitro
findings of reduced Schaffer collateral-evoked
CA1 EPSPs in old rats, the population currents
evoked to iontophoretically applied AMPA are
also smaller relative to the presynaptic fiber
potential amplitude. On the other hand, the size of
the presynaptic fiber potential and amplitude of
unitary intra-cellularly recorded EPSP responses
do not change across age in the CA1 region. These
electrophysiological findings are consistent with
the hypothesis that old rats have fewer functional
synaptic contacts per Schaffer collateral axon
than do young rats. The possibility that this age
change arises as a result of a differential tissue
recovery response to in vitro preparation was
examined in the present study. CA1 presynaptic
fiber potential and EPSP amplitudes evoked by
the stimulation of Schaffer collateral afferents
were studied in intact, freely behaving young
and old rats. We confirmed in vivo the pattern of electrophysiophysiological results previously
reported in vitro and found significant
correlations between the synaptic response
amplitudes and the accuracy of spatial behavior
in the Morris swim task. The data suggest that
changes in functional connectivity of old rats
may be a significant contributor to cognitive
changes during aging
Non-Thermal Emission from AGN Coronae
Accretion disk coronae are believed to account for X-ray emission in Active
Galactic Nuclei (AGNs). In this paper the observed emission is assumed to be
due to a population of relativistic, non-thermal electrons (e.g. produced in a
flare) injected at the top of an accretion disk magnetic loop. While electrons
stream along magnetic field lines their energy distribution evolves in time
essentially because of inverse Compton and synchrotron losses. The
corresponding time dependent emission due, in the X-ray energy range, to the
inverse Compton mechanism, has been computed. Since the typical decay time of a
flare is shorter than the integration time for data acquisition in the X-ray
domain, the resulting spectrum is derived as the temporal mean of the real,
time-dependent, emission, as originated by a series of consecutive and
identical flares. The model outcome is compared to both the broad band BeppoSAX
X-ray data of the bright Seyfert 1 NGC 5548, and to a few general X-ray
spectral properties of Seyfert 1s as a class. The good agreement between model
and observations suggests that the presently proposed non-thermal,
non-stationary model could be a plausible explanation of AGN X-ray emission, as
an alternative to thermal coronae models.Comment: 15 pages, 4 postscript figure
Extended sudden approximation model for high-energy nucleon removal reactions
A model based on the sudden approximation has been developed to describe high
energy single nucleon removal reactions. Within this approach, which takes as
its starting point the formalism of Hansen \cite{Anne2}, the nucleon-removal
cross section and the full 3-dimensional momentum distributions of the core
fragments including absorption, diffraction, Coulomb and nuclear-Coulomb
interference amplitudes, have been calculated. The Coulomb breakup has been
treated to all orders for the dipole interaction. The model has been compared
to experimental data for a range of light, neutron-rich psd-shell nuclei. Good
agreement was found for both the inclusive cross sections and momentum
distributions. In the case of C, comparison is also made with the
results of calculations using the transfer-to-the-continuum model. The
calculated 3-dimensional momentum distributions exhibit longitudinal and
transverse momentum components that are strongly coupled by the reaction for
s-wave states, whilst no such effect is apparent for d-waves. Incomplete
detection of transverse momenta arising fromlimited experimental acceptances
thus leads to a narrowing of the longitudinal distributions for nuclei with
significant s-wave valence neutron configurations, as confirmed by the data.
Asymmetries in the longitudinal momentum distributions attributed to
diffractive dissociation are also explored.Comment: 16 figures, submitted to Phys. Rev.
The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory
BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim
X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times
fainter than its historical average value, and remained steady along the whole
observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense
iron line (EW ~600 eV) are best explained assuming that the active nucleus has
switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in
MNRA
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