13,652 research outputs found
Origin of the X-ray Quasi-Periodic Oscillations and Identification of a Transient Ultraluminous X-Ray Source in M82
The starburst galaxy M82 contains two ultraluminous X-ray sources (ULXs),
CXOM82 J095550.2+694047 (=X41.4+60) and CXOM82 J095551.1+694045 (=X42.3+59),
which are unresolved by XMM-Newton. We revisited the two XMM-Newton
observations of M82 and analyzed the surface brightness profiles using the
known Chandra source positions. We show that the quasi-periodic oscillations
(QPOs) detected with XMM-Newton originate from X41.4+60, the brightest X-ray
source in M82. Correcting for the contributions of the unresolved sources, the
QPO at a frequency of 55.8+/-1.3 mHz on 2001 May 06 had a fractional rms
amplitude of 32%, and the QPO at 112.9+/-1.3 mHz on 2004 April 21 had an
amplitude of 21%. The QPO frequency may possibly be correlated with the source
flux, similar to the type C QPOs in XTE 1550-564 and GRS 1915+105, but at
luminosities two orders of magnitude higher. X42.3+59, the second brightest
source in M82, displayed a strikingly high flux of 1.4E-11 ergs/cm^2/s in the
2-10 keV band on 2001 May 6. A seven-year light curve of X42.3+59 shows extreme
variability over a factor of 1000; the source is not detected in several
Chandra observations. This transient behavior suggests accretion from an
unstable disk. If the companion star is massive, as might be expected in the
young stellar environment, then the compact object would likely be an IMBH.Comment: 9 pages, 6 figures, submitted to ApJ on May 08, 200
Thermoelectric three-terminal hopping transport through one-dimensional nanosystems
A two-site nanostructure (e.g, a "molecule") bridging two conducting leads
and connected to a phonon bath is considered. The two relevant levels closest
to the Fermi energy are connected each to its lead. The leads have slightly
different temperatures and chemical potentials and the nanos- tructure is also
coupled to a thermal (third) phonon bath. The 3 x 3 linear transport
("Onsager") matrix is evaluated, along with the ensuing new figure of merit,
and found to be very favorable for thermoelectric energy conversion.Comment: Accepted by Phys. Rev.
Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4
Using Co-L_(2,3) and O-K x-ray absorption spectroscopy, we reveal that the
charge ordering in La_(1.5)Sr_(0.5)CoO4 involves high spin (S=3/2) Co^2+ and
low spin (S=0) Co^3+ ions. This provides evidence for the spin blockade
phenomenon as a source for the extremely insulating nature of the
La_(2-x)Sr_(x)CoO4 series. The associated e_g^2 and e_g^0 orbital occupation
accounts for the large contrast in the Co-O bond lengths, and in turn, the high
charge ordering temperature. Yet, the low magnetic ordering temperature is
naturally explained by the presence of the non-magnetic (S=0) Co^3+ ions. From
the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow
band material.Comment: 5 pages, 3 figure
Nucleon axial form factors from two-flavour Lattice QCD
We present preliminary results on the axial form factor and the
induced pseudoscalar form factor of the nucleon. A systematic
analysis of the excited-state contributions to form factors is performed on the
CLS ensemble `N6' with and lattice spacing . The relevant three-point functions were computed with
source-sink separations ranging from to $t_s \sim \
1.4 \ \text{fm}$. We observe that the form factors suffer from non-trivial
excited-state contributions at the source-sink separations available to us. It
is noted that naive plateau fits underestimate the excited-state contributions
and that the method of summed operator insertions correctly accounts for these
effects.Comment: 7 pages, 12 figures; talk presented at Lattice 2014 -- 32nd
International Symposium on Lattice Field Theory, 23-28 June, 2014, Columbia
University New York, N
Physical Structure of Planetary Nebulae. I. The Owl Nebula
The Owl Nebula is a triple-shell planetary nebula with the outermost shell
being a faint bow-shaped halo. We have obtained deep narrow-band images and
high-dispersion echelle spectra in the H-alpha, [O III], and [N II] emission
lines to determine the physical structure of each shell in the nebula. These
spatio-kinematic data allow us to rule out hydrodynamic models that can
reproduce only the nebular morphology. Our analysis shows that the inner shell
of the main nebula is slightly elongated with a bipolar cavity along its major
axis, the outer nebula is a filled envelope co-expanding with the inner shell
at 40 km/s, and the halo has been braked by the interstellar medium as the Owl
Nebula moves through it. To explain the morphology and kinematics of the Owl
Nebula, we suggest the following scenario for its formation and evolution. The
early mass loss at the TP-AGB phase forms the halo, and the superwind at the
end of the AGB phase forms the main nebula. The subsequent fast stellar wind
compressed the superwind to form the inner shell and excavated an elongated
cavity at the center, but has ceased in the past. At the current old age, the
inner shell is backfilling the central cavity.Comment: 10 pages, 6 figures, 1 table, to appear in the Astronomical Journa
Nucleon electromagnetic form factors in two-flavour QCD
We present results for the nucleon electromagnetic form factors, including
the momentum transfer dependence and derived quantities (charge radii and
magnetic moment). The analysis is performed using O(a) improved Wilson fermions
in Nf=2 QCD measured on the CLS ensembles. Particular focus is placed on a
systematic evaluation of the influence of excited states in three-point
correlation functions, which lead to a biased evaluation, if not accounted for
correctly. We argue that the use of summed operator insertions and fit
ans\"atze including excited states allow us to suppress and control this
effect. We employ a novel method to perform joint chiral and continuum
extrapolations, by fitting the form factors directly to the expressions of
covariant baryonic chiral effective field theory. The final results for the
charge radii and magnetic moment from our lattice calculations include, for the
first time, a full error budget. We find that our estimates are compatible with
experimental results within their overall uncertainties.Comment: 22 pages, 10 figures, citations modifie
Local electronic structure and magnetic properties of LaMn0.5Co0.5O3 studied by x-ray absorption and magnetic circular dichroism spectroscopy
We have studied the local electronic structure of LaMn0.5Co0.5O3 using
soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found
a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie
temperature. We discovered that samples with lower Curie temperatures contain
low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism
we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned.
We revealed also that the Co^{2+} ions have a large orbital moment:
m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the
presence of a large magnetocrystalline anisotropy in the material and predicts
a non-trivial temperature dependence for the magnetic susceptibility.Comment: 8 pages, 7 figure
Whisper-to-speech conversion using restricted Boltzmann machine arrays
Whispers are a natural vocal communication mechanism, in which vocal cords do not vibrate normally. Lack of glottal-induced pitch leads to low energy, and an inherent noise-like spectral distribution reduces intelligibility. Much research has been devoted to processing of whispers, including conversion of whispers to speech. Unfortunately, among several approaches, the best reconstructed speech to date still contains obviously artificial muffles and suffers from an unnatural prosody. To address these issues, the novel use of multiple restricted Boltzmann machines (RBMs) is reported as a statistical conversion model between whisper and speech spectral envelopes. Moreover, the accuracy of estimated pitch is improved using machine learning techniques for pitch estimation within only voiced (V) regions. Both objective and subjective evaluations show that this new method improves the quality of whisper-reconstructed speech compared with the state-of-the-art approaches
Electronic structure of AuMg and AgMg ( = Eu, Gd, Yb)
We have investigated the electronic structure of the equiatomic EuAuMg,
GdAuMg, YbAuMg and GdAgMg intermetallics using x-ray photoelectron
spectroscopy. The spectra revealed that the Yb and Eu are divalent while the Gd
is trivalent. The spectral weight in the vicinity of the Fermi level is
dominated by the mix of Mg , Au/Ag and bands, and not by the
. We also found that the Au and Ag bands are extraordinarily
narrow, as if the noble metal atoms were impurities submerged in a low density
metal host. The experimental results were compared with band structure
calculations, and we found good agreement provided that the spin-orbit
interaction in the Au an Ag bands is included and correlation effects in an
open shell are accounted for using the local density approximation +
Hubbard scheme. Nevertheless, limitations of such a mean-field scheme to
explain excitation spectra are also evident.Comment: 4 pages, 3 figures, Brief Repor
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