1,733 research outputs found
Classical and Quantum Cosmology of Multigravity
Recently, a multigraviton theory on a simple closed circuit graph
corresponding to the discretization of compactification of the
Kaluza-Klein (KK) theory has been considered. In the present paper, we extend
this theory to that on a general graph and study what modes of particles are
included. Furthermore, we generalize it in a possible nonlinear theory based on
the vierbein formalism and study classical and quantum cosmological solutions
in the theory. We found that scale factors in a solution for this theory repeat
acceleration and deceleration.Comment: 17 pages, 15 figures, RevTeX4.1, revised versio
Ocean temperature and salinity components of the Madden-Julian oscillation observed by Argo floats
New diagnostics of the Madden-Julian Oscillation (MJO) cycle in ocean temperature and, for the first time, salinity are presented. The MJO composites are based on 4 years of gridded Argo float data from 2003 to 2006, and extend from the surface to 1,400 m depth in the tropical Indian and Pacific Oceans. The MJO surface salinity anomalies are consistent with precipitation minus evaporation fluxes in the Indian Ocean, and with anomalous zonal advection in the Pacific. The Argo sea surface temperature and thermocline depth anomalies are consistent with previous studies using other data sets. The near-surface density changes due to salinity are comparable to, and partially offset, those due to temperature, emphasising the importance of including salinity as well as temperature changes in mixed-layer modelling of tropical intraseasonal processes. The MJO-forced equatorial Kelvin wave that propagates along the thermocline in the Pacific extends down into the deep ocean, to at least 1,400 m. Coherent, statistically significant, MJO temperature and salinity anomalies are also present in the deep Indian Ocean
Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67
The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic
oscillations (QPOs) and red noise variability as well as coherent pulsations at
the 0.130 Hz neutron star spin frequency. Power density spectra of observations
made with the Rossi X-ray Timing Explorer show significant sidebands separated
from the pulsar spin frequency (and its harmonics) by the QPO frequency. These
show that the instantaneous amplitude of the coherent pulsations is modulated
by the amplitude of the QPOs. This phenomenon is expected in models such as the
magnetospheric beat frequency model where the QPOs originate near the polar
caps of the neutron star. In the 4--8 keV energy range, however, the
lower-frequency sidebands are significantly stronger than their
higher-frequency complements; this is inconsistent with the magnetospheric beat
frequency model. We suggest that the 0.048 Hz QPOs are instead produced by a
structure orbiting the neutron star at the QPO frequency. This structure
crosses the line of sight once per orbit and attenuates the pulsar beam,
producing the symmetric (amplitude modulation) sidebands. It also reprocesses
the pulsar beam at the beat frequencies between the neutron star spin frequency
and the QPOs, producing the excess variability observed in the lower-frequency
sidebands. Quite independently, we find no evidence that the red noise
variability modulates the amplitude of the coherent pulsations. This is also in
contrast to the expectations of the magnetospheric beat frequency model and
differs from the behavior in some high-mass X-ray binary pulsars.Comment: 8 pages, 3 figures, AAS macros v4.0. To appear in ApJ Letter
Sonic-Point Model of Kilohertz Quasi-Periodic Brightness Oscillations in Low-Mass X-ray Binaries
Strong, coherent, quasi-periodic brightness oscillations (QPOs) with
frequencies ranging from about 300 Hz to 1200 Hz have been discovered with the
Rossi X-ray Timing Explorer in the X-ray emission from some fifteen neutron
stars in low-mass binary systems. Two simultaneous kilohertz QPOs differing in
frequency by 250 to 350 Hertz have been detected in twelve of the fifteen
sources. Here we propose a model for these QPOs. In this model the X-ray source
is a neutron star with a surface magnetic field of 10^7 to 10^10 G and a spin
frequency of a few hundred Hertz, accreting gas via a Keplerian disk. The
frequency of the higher-frequency QPO in a kilohertz QPO pair is the Keplerian
frequency at a radius near the sonic point at the inner edge of the Keplerian
flow whereas the frequency of the lower-frequency QPO is approximately the
difference between the Keplerian frequency at a radius near the sonic point and
the stellar spin frequency. This model explains naturally many properties of
the kilohertz QPOs, including their frequencies, amplitudes, and coherence. We
show that if the frequency of the higher-frequency QPO in a pair is an orbital
frequency, as in the sonic-point model, the frequencies of these QPOs place
interesting upper bounds on the masses and radii of the neutron stars in the
kilohertz QPO sources and provide new constraints on the equation of state of
matter at high densities. Further observations of these QPOs may provide
compelling evidence for the existence of a marginally stable orbit, confirming
a key prediction of general relativity in the strong-field regime.Comment: 67 pages, including 15 figures and 5 tables; uses aas2pp4; final
version to appear in the Astrophysical Journal on 1 December 199
The frequency in Japanese of genetic variants of 22 proteins III. Phosphoglucomutase-1, phosphoglucomutase-2, 6-phosphogluconate dehydrogenase, adenylate kinase, and adenosine deaminase
Five enzyme systems, PGM 1 , PGM 2 , ADA, 6-PGD and AK, were examined by electrophoresis in over 4000 samples from Hiroshima and Nagasaki for the frequencies of common and rare variants. In the PGM 1 , system, the PGM 2 1 allele and PGM 7 1 ; allele were found in polymorphic proportions. I n addition, five kinds of slow variants and three types of fast variants of PGM 1 were detected. The PGM 3 NGS 1 1 allele was found in five individuals from Nagasaki, but was not observed in samples from Hiroshima. There were no variants of PGM 2 . Three kinds of fast variants of 6-PGD were detected. NO variation in AK was observed. There were no rare variants of ADA. The 6-PGD c allele had a frequency of 0.084 in Hiroshima, and 0.093 in Nagasaki, and the ADA 2 allele frequencies of 0.025 in Hiroshima and 0.032 in Nagasaki.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65524/1/j.1469-1809.1977.tb01912.x.pd
Spontaneous magnetization of the Ising model on the Sierpinski carpet fractal, a rigorous result
We give a rigorous proof of the existence of spontaneous magnetization at
finite temperature for the Ising spin model defined on the Sierpinski carpet
fractal. The theorem is inspired by the classical Peierls argument for the two
dimensional lattice. Therefore, this exact result proves the existence of
spontaneous magnetization for the Ising model in low dimensional structures,
i.e. structures with dimension smaller than 2.Comment: 14 pages, 8 figure
High-resolution X-ray spectroscopy of the ultracompact LMXB pulsar 4U 1626-67
[abridged] We report results from four recent observations of the
ultracompact LMXB pulsar 4U 1626-67. All the observations obtained
high-resolution X-ray spectra of the system, two from the Chandra X-ray
Observatory using the HETGS, and two from the XMM-Newton Observatory using the
RGS as well as the EPIC PN and MOS. These data allow us to study in detail the
prominent Ne and O emission line complexes which make 4U 1626-67 unique among
LMXBs. The observations were spaced over a period of 3 years for a total
observing time of 238 ks, allowing us to monitor the line regions as well as
the overall source flux, continuum spectrum, and timing properties. The
structure of the emission lines and the ratios of the components of the
helium-like Ne IX and O VII triplets support the hypothesis that they are
formed in the high-density environment of the accretion disk. We do not find
any significant changes in the line widths or ratios over this time period,
though we note that the line equivalent widths decrease. We are able to place
constraints on the strengths of the Ne K, Fe L, and O K photoelectric
absorption edges, and find that the data do not require an overabundance of Ne
or O in the system relative to the expected ISM values. We find that the pulsar
is still spinning down, and note that the pulse profile has changed
significantly from what was found prior to the torque reversal in 1990,
suggesting that this event may be linked to a change in the geometry of the
accretion column. The flux of 4U 1626-67 continues to decrease, in keeping with
the trend of the last approximately 30 years over which it has been observed.
Taking into consideration current theory on disk stability, we expect that 4U
1626-67 will enter a period of quiescence in 2-15 years.Comment: 10 pages, 7 figures, submitted to Ap
Dose ratio proton radiography using the proximal side of the Bragg peak
Purpose: In recent years there has been a movement towards single-detector proton radiography, due to its potential ease of implementation within the clinical environment. One such single-detector technique is the dose ratio method, in which the dose maps from two pristine Bragg peaks are recorded beyond the patient. To date, this has only been investigated on the distal side of the lower energy Bragg peak, due to the sharp fall-off. We investigate the limits and applicability of the dose ratio method on the proximal side of the lower energy Bragg peak, which has the potential to allow a much wider range of water-equivalent thicknesses (WET) to be imaged. Comparisons are made with the use of the distal side of the Bragg peak. Methods: Using the analytical approximation for the Bragg peak we generated theoretical dose ratio curves for a range of energy pairs, and then determined how an uncertainty in the dose ratio would translate to a spread in the WET estimate. By defining this spread as the accuracy one could achieve in the WET estimate, we were able to generate look-up graphs of the range on the proximal side of the Bragg peak that one could reliably use. These were dependent on the energy pair, noise level in the dose ratio image and the required accuracy in the WET. Using these look-up graphs we investigated the applicability of the technique for a range of patient treatment sites. We validated the theoretical approach with experimental measurements using a complementary metal oxide semiconductor active pixel sensor (CMOS APS), by imaging a small sapphire sphere in a high energy proton beam. Results: Provided the noise level in the dose ratio image was 1% or less, a larger spread of WETs could be imaged using the proximal side of the Bragg peak (max 5.31 cm) compared to the distal side (max 2.42 cm). In simulation it was found that, for a pediatric brain, it is possible to use the technique to image a region with a square field equivalent size of 7.6 cm2, for a required accuracy in the WET of 3 mm and a 1% noise level in the dose ratio image. The technique showed limited applicability for other patient sites. The CMOS APS demonstrated a good accuracy, with a root-mean-square-error of 1.6 mm WET. The noise in the measured images was found to be Ï =1.2% (standard deviation) and theoretical predictions with a 1.96Ï noise level showed good agreement with the measured errors. Conclusions: After validating the theoretical approach with measurements, we have shown that the use of the proximal side of the Bragg peak when performing dose ratio imaging is feasible, and allows for a wider dynamic range than when using the distal side. The dynamic range available increases as the demand on the accuracy of the WET decreases. The technique can only be applied to clinical sites with small maximum WETs such as for pediatric brains
Genetic heterogeneity within an electrophoretic phenotype of phosphoglucose isomerase in a Japanese population
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65884/1/j.1469-1809.1979.tb00662.x.pd
On the relationship between the periodic and aperiodic variability of accreting X-ray pulsars
Besides the narrow peaks originating from the periodic signal, the power
spectra of accreting X-ray pulsars display continuum components usually
increasing towards low frequencies; these arise from the source aperiodic
variability. Most studies so far adopted the view that the periodic and
aperiodic variations are independent. However any aperiodic variability in the
emission from the accretion column(s) towards the magnetic neutron star should
be modulated at the X-ray pulsar period, by virtue of the same rotation-induced
geometric effects which give rise to the periodic signal. We develop here a
simple shot noise model to test the presence of a coupling between the periodic
and aperiodic variability of X-ray pulsars. The model power spectrum is fit to
the power spectra of three X-ray pulsars, Vela X-1, 4U 1145-62 and Cen X-3
observed with EXOSAT. In the first two cases we find that a highly significant
coupling is present, as testified by a substantialbroadening in the wings of
the power spectrum peaks due to the periodic modulation. We find also that
these wings can mimick the presence of a knee in the continuum power spectrum
components around the fundamental of the periodic modulation, therefore
questioning the correlation reported by Takeshima (1992) between the X-ray
pulsar frequency and the knee frequency, beyond which the continuum power
spectral component steepens.Comment: 19 pages + 5 figures, Accepted for publication in the Astrophysical
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