8,385 research outputs found
Pulse ignition characterization of mercury ion thruster hollow cathode using an improved pulse ignitor
An investigation of the high voltage pulse ignition characteristics of the 8 cm mercury ion thruster neutralizer cathode identified a low rate of voltage rise and long pulse duration as desirable factors for reliable cathode starting. Cathode starting breakdown voltages were measured over a range of mercury flow rates and tip heater powers for pulses with five different rates of voltage rise. Breakdown voltage requirements for the fastest rising pulse (2.5 to 3.0 kV/micro sec) were substantially higher (2 kV or more) than for the slowest rising pulse (0.3 to 0.5 kV/micro sec) for the same starting conditions. Also described is an improved, low impedance pulse ignitor circuit which reduces power losses and eliminates problems with control and packaging associated with earlier designs
Disappearing Pulses in Vela X-1
We present results from a 20 h RXTE observation of Vela X-1, ncluding a
peculiar low state of a few hours duration, during which the pulsation of the
X-ray emission ceased, while significant non-pulsed emission remained. This
``quiescent state'' was preceded by a ``normal state'' without any unusual
signs and followed by a ``high state'' of several hours of increased activity
with strong, flaring pulsations. while there is clear spectral evolution from
the normal state to the low state, the spectra of the following high state are
surprisingly similar to those of the low state.Comment: 5 pages, 5 figures, Proceedings of the 5th Compton Symposium, AIP, in
pres
Discovery of a New 89 Second X-ray Pulsar XTE J1906+09
We report on the discovery of a new pulsating X-ray source during Rossi X-ray
Timing Explorer observations of a low galactic latitude field centered at RA
(J2000) = 19 hr 05 m 43 s and Dec (J2000) = +08 deg 58 arcmin 48 arcsec.
Significant pulsations were detected by both the PCA and HEXTE instruments
aboard RXTE at a fundamental period of 89.17 +/- 0.02 seconds, with higher
harmonics also visible in the 2-10 keV power spectrum. The folded lightcurve
from the source is multiply peaked at lower energies, and changes to single
peaked morphology above ~20 keV. The phase averaged spectrum from the source is
well fit by strongly absorbed power law or thermal bremsstrahlung spectral
models of photon index 1.9 +/- 0.1 or temperature 19.5 +/- 4.6 keV,
respectively. The mean neutral hydrogen column density is approximately 10^23
cm^-2, suggesting a distance of >10 kpc to the source and a minimum 2-10 keV
X-ray luminosity of 2*10^{35} ergs s^{-1}. By comparison with other pulsars
with similar periods and luminosities, we suggest that XTE J1906+09 has a
supergiant companion with an underfilled Roche lobe. We speculate further that
one of the M stars in a peculiar M star binary system may be the companion.Comment: 14 pages, 2 figures. Accepted by ApJ Letter
Multiple Cyclotron Lines in the Spectrum of 4U 0115+63
We report phase resolved spectroscopy of the transient accreting pulsar,
4U0115+63. For the first time, more than two cylotron resonance scattering
features are detected in the spectrum of an X-ray pulsar. The shape of the
fundamental line appears to be complex, and this is in agreement with
predictions of Monte-Carlo models. As in other pulsars, the line energies and
optical depths are strong functions of pulse phase. One possible model for this
is an offset of the dipole of the neutron star magnetic field.Comment: 5 pages. To appear in "Proceedings of the 5th Compton Symposium
Macroscopic equations for the adiabatic piston
A simplified version of a classical problem in thermodynamics -- the
adiabatic piston -- is discussed in the framework of kinetic theory. We
consider the limit of gases whose relaxation time is extremely fast so that the
gases contained on the left and right chambers of the piston are always in
equilibrium (that is the molecules are uniformly distributed and their
velocities obey the Maxwell-Boltzmann distribution) after any collision with
the piston. Then by using kinetic theory we derive the collision statistics
from which we obtain a set of ordinary differential equations for the evolution
of the macroscopic observables (namely the piston average velocity and
position, the velocity variance and the temperatures of the two compartments).
The dynamics of these equations is compared with simulations of an ideal gas
and a microscopic model of gas settled to verify the assumptions used in the
derivation. We show that the equations predict an evolution for the macroscopic
variables which catches the basic features of the problem. The results here
presented recover those derived, using a different approach, by Gruber, Pache
and Lesne in J. Stat. Phys. 108, 669 (2002) and 112, 1177 (2003).Comment: 13 pages, 7 figures (revTeX4) The paper has been completely rewritten
with new derivation and results, supplementary information can be found at
http://denali.phys.uniroma1.it/~cencini/Papers/cppv07_supplements.pd
Discovery of a Cyclotron Resonance Scattering Feature in the X-ray Spectrum of XTE J1946+274
Observations of the transient accreting pulsar XTE J1946+274 made with the
Rossi X-ray Timing Explorer during the course of the 1998 September-November
outburst, reveal a cyclotron resonance scattering feature (or "cyclotron line")
in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2
+0.5/-0.7 keV, which implies a magnetic field strength of 3.1(1+z)x10^12 G,
where z is the gravitational redshift of the scattering region. The optical
depth, Tau = 0.33 +0.07/-0.06, and width, sigma = 3.37 +0.92/-0.75 keV, are
typical of known cyclotron lines in other pulsars. This discovery makes XTE
J1946+274 one of thirteen pulsars with securely detected cyclotron lines
resulting in direct magnetic field measurements.Comment: Five pages including four postscript figures and two tables. Uses
emulateapj5. Published in ApJ Letters:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001ApJ...563L..35
Spectrum of the gamma-ray diffuse component observed from HEAO-1
The spectrum of the diffuse X and gamma ray background was measured between 15 keV and 4 MeV with the scintillation detectors aboard the HEAO 1 satellite. The apertures of the detectors were modulated on time scales of hours and the difference in counting rates measured the diffuse component flux. The observed spectrum is presented and compared with other measurements. At least two components are indicated, one below -100 keV and the other above. Possible origins are discussed
2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate
Based on recent work on spectral decomposition of the emission of
star-forming galaxies, we assess whether the integrated 2-10 keV emission from
high-mass X-ray binaries (HMXBs), L_{2-10}^{HMXB}, can be used as a reliable
estimator of ongoing star formation rate (SFR). Using a sample of 46 local (z <
0.1) star forming galaxies, and spectral modeling of ASCA, BeppoSAX, and
XMM-Newton data, we demonstrate the existence of a linear SFR-L_{2-10}^{HMXB}
relation which holds over ~5 decades in X-ray luminosity and SFR. The total
2-10 keV luminosity is not a precise SFR indicator because at low SFR (i.e., in
normal and moderately-starbursting galaxies) it is substantially affected by
the emission of low-mass X-ray binaries, which do not trace the current SFR due
to their long evolution lifetimes, while at very high SFR (i.e., for very
luminous FIR-selected galaxies) it is frequently affected by the presence of
strongly obscured AGNs. The availability of purely SB-powered galaxies - whose
2-10 keV emission is mainly due to HMXBs - allows us to properly calibrate the
SFR-L_{2-10}^{HMXB} relation. The SFR-L_{2-10}^{HMXB} relation holds also for
distant (z ~ 1) galaxies in the Hubble Deep Field North sample, for which we
lack spectral information, but whose SFR can be estimated from deep radio data.
If confirmed by more detailed observations, it may be possible to use the
deduced relation to identify distant galaxies that are X-ray overluminous for
their (independently estimated) SFR, and are therefore likely to hide strongly
absorbed AGNs.Comment: Astronomy & Astrophysics, in press (15 pages, 7 figures, 4 tables
Modeling biological face recognition with deep convolutional neural networks
Deep convolutional neural networks (DCNNs) have become the state-of-the-art
computational models of biological object recognition. Their remarkable success
has helped vision science break new ground and recent efforts have started to
transfer this achievement to research on biological face recognition. In this
regard, face detection can be investigated by comparing face-selective
biological neurons and brain areas to artificial neurons and model layers.
Similarly, face identification can be examined by comparing in vivo and in
silico multidimensional "face spaces". In this review, we summarize the first
studies that use DCNNs to model biological face recognition. On the basis of a
broad spectrum of behavioral and computational evidence, we conclude that DCNNs
are useful models that closely resemble the general hierarchical organization
of face recognition in the ventral visual pathway and the core face network. In
two exemplary spotlights, we emphasize the unique scientific contributions of
these models. First, studies on face detection in DCNNs indicate that
elementary face selectivity emerges automatically through feedforward
processing even in the absence of visual experience. Second, studies on face
identification in DCNNs suggest that identity-specific experience and
generative mechanisms facilitate this particular challenge. Taken together, as
this novel modeling approach enables close control of predisposition (i.e.,
architecture) and experience (i.e., training data), it may be suited to inform
long-standing debates on the substrates of biological face recognition.Comment: 41 pages, 2 figures, 1 tabl
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