10,116 research outputs found
The mass of the black hole in LMC X-3
New high resolution, optical spectroscopy of the high mass X-ray binary LMC X-3, shows the spectral type of the donor star changes with phase due to irradition by the X-ray source. We find the spectral type is likely to be B5V, and only appears as B3V when viewing the heated side of the donor. Combining our measurements with those previously published, and taking into account the effects of X-ray irradiation, results in a value for the donor star radial velocity semi-amplitude of ~km~s. We find the mass of the black hole lies in the range
Principles of Discrete Time Mechanics: IV. The Dirac Equation, Particles and Oscillons
We apply the principles of discrete time mechanics discussed in earlier
papers to the first and second quantised Dirac equation. We use the Schwinger
action principle to find the anticommutation relations of the Dirac field and
of the particle creation operators in the theory. We find new solutions to the
discrete time Dirac equation, referred to as oscillons on account of their
extraordinary behaviour. Their principal characteristic is that they oscillate
with a period twice that of the fundamental time interval T of our theory.
Although these solutions can be associated with definite charge, linear
momentum and spin, such objects should not be observable as particles in the
continuous time limit. We find that for non-zero T they correspond to states
with negative squared norm in Hilbert space. However they are an integral part
of the discrete time Dirac field and should play a role in particle
interactions analogous to the role of longitudinal photons in conventional
quantum electrodynamics.Comment: 27 pages LateX; published versio
Periodicities In The X-Ray Intensity Variations of TV Columbae: An Intermediate Polar
We present results from a temporal analysis of the longest and the most
sensitive X-ray observations of TV Columbae--an intermediate polar. The
observations were carried out with the RXTE PCA, ROSAT PSPC, and ASCA. Data
were analyzed using a 1-dimensional CLEAN and Bayesian algorithms. The presence
of a nearly sinusoidal modulation due to the spin of the white dwarf is seen
clearly in all the data, confirming the previous reports based on the EXOSAT
data. An improved period of 1909.7+/-2.5s is derived for the spin from the RXTE
data.The binary period of 5.5hr is detected unambiguously in X-rays for the
first time. Several side-bands due to the interaction of these periods are
observed in the power spectra, thereby suggesting contributions from both the
disk-fed and the stream-fed accretion for TV Col. The accretion disk could
perhaps be precessing as side-bands due to the influence of 4 day period on the
orbital period are seen. The presence of a significant power at certain
side-bands of the spin frequency indicates that the emission poles are
asymmetrically located. The strong power at the orbital side-bands seen in both
the RXTE and ROSAT data gives an indication for an absorption site fixed in the
orbital frame. Both the spin and the binary modulation are found to be
energy-dependent. Increased hardness ratio during a broad dip in the intensity
at binary phase of 0.75--1.0 confirms the presence of a strong attenuation due
to additional absorbers probably from an impact site of the accretion stream
with the disk or magnetosphere. Hardness ratio variations and the energy
dependent modulation depth during the spin modulation can be explained by
partially covered absorbers in the path of X-ray emission region in the
accretion stream.Comment: 34 pages, including 12 figures, Accepted for publication in
Astronomical Journal, scheduled for January 2004 issue (vol. 127
RXTE confirmation of the intermediate polar status of IGR J15094-6649
Aims. To establish the X-ray properties of the intermediate polar candidate
IGR J15094-6649 and therefore confirm its inclusion into the class.
Methods. 42 856 s of X-ray data from RXTE was analysed. Frequency analysis
was used to constrain temporal variations and spectral analysis used to
characterise the emission and absorption properties.
Results. A spin period of 809.7+-0.6 s is present, revealed as a complex
pulse profile whose modulation depth decreases with increasing X-ray energy.
The spectrum is well fitted by either a 19+-4 keV Bremsstrahlung or
Gamma=1.8+-0.1 power law, with an iron emission line feature and significant
absorption in each case.
Conclusions. IGR J15094-6649 is confirmed to be an intermediate polar.Comment: 3 pages, 5 figures. Submitted to A&
Hyperpolarized ^1H NMR employing low γ nucleus for spin polarization storage
The PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment)(1, 2) and DNP (Dynamic Nuclear Polarization)(3) methods efficiently hyperpolarize biologically relevant nuclei such as 1^H, (31)^P, (13)^C, (15)^N achieving signal enhancement by a factor of ~ 100000 on currently utilized MRI scanners. Recently, many groups have demonstrated the utility of hyperpolarized MR in biological systems using hyperpolarized (13)^C biomarkers with a relatively long spin lattice relaxation time T_1 on the order of tens of seconds.(4-7) Moreover, hyperpolarized (15)^N for biomedical MR has been proposed due to even longer spin lattice relaxations times.(8) An additional increase of up to tens of minutes in the lifetime of hyperpolarized agent in vivo could be achieved by using the singlet states of low gamma (γ) nuclei.(9) However, as NMR receptivity scales as γ^3 for spin 1/2 nuclei, direct NMR detection of low γ nuclei results in a lower signal-to-noise ratio compared to proton detection. While protons are better nuclei for detection, short spin lattice relaxation times prevent direct 1^H hyperpolarized MR in biomedical applications
Evidence for polar jets as precursors of polar plume formation
Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are
utilized to study polar coronal jets and plumes. The study focuses on the
temporal evolution of both structures and their relationship. The data sample,
spanning April 7-8 2007, shows that over 90% of the 28 observed jet events are
associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising
from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV
jets, with the plume haze appearing minutes to hours after the jet was
observed. The remaining jets occurred in areas where plume material previously
existed causing a brightness enhancement of the latter after the jet event.
Short-lived, jet-like events and small transient bright points are seen (one at
a time) at different locations within the base of pre-existing long-lived
plumes. X-ray images also show instances (at least two events) of
collimated-thin jets rapidly evolving into significantly wider plume-like
structures that are followed by the delayed appearance of plume haze in the
EUV. These observations provide evidence that X-ray jets are precursors of
polar plumes, and in some cases cause brightenings of plumes. Possible
mechanisms to explain the observed jet and plume relationship are discussed.Comment: 10 pages, 4 figures, accepted as APJ Lette
Characteristics of Magnetohydrodynamic Oscillations Observed with Michelson Doppler Imager
We report on the spatial distribution of magnetogram oscillatory power and
phase angles between velocity and magnetogram signals as observed with the
Michelson Doppler Imager. The dataset is 151.25 arcsec times 151.25 arcsec
containing sunspot from Dec 2, 1997 with a temporal sampling interval of 60
seconds and spatial sampling of 0.605 arcsec. Simultaneously observed continuum
intensity and surface velocity accompany the magnetic information. We focus on
three frequency regimes: 0.5-1.0, 3.0-3.5 and 5.5-6.0 mHz corresponding roughly
to timescales of magnetic evolution, p-modes and the 3 minute resonant sunspot
oscillation. Significant low frequency magnetogram power is found in lower flux
pixels, 100-300 Gauss, in a striking ring with filamentary structure
surrounding sunspot. Five minute magnetogram power peaks in extended regions of
flux 600-800 Gauss. The 3 minute oscillation is observed in sunspot umbra in
pixels whose flux measures 1300-1500 Gauss. Phase angles of approximately -90
degrees between velocity and magnetic flux in the 3.0-3.5 and 5.5-6.0 mHz
regimes are found in regions of significant cross amplitude.Comment: 4 Pages, 4 Figures -- For better Figure files see:
http://www.astro.ucla.edu/~norton/pub_list.htm
Dysregulation of visual motion inhibition in major depression
Individuals with depression show depleted concentrations of the inhibitory neurotransmitter GABA in
occipital (visual) cortex, predicting weakened inhibition within their visual systems. Yet, visual inhibition
in depression remains largely unexplored. To fill this gap, we examined the inhibitory process of centersurround suppression (CSS) of visual motion in depressed individuals. Perceptual performance in discriminating the direction of motion was measured as a function of stimulus presentation time and
contrast in depressed individuals (n¼27) and controls (n¼22). CSS was operationalized as the accuracy
difference between conditions using large (7.5°) and small (1.5°) grating stimuli. Both depressed and
control participants displayed the expected advantage in accuracy for small stimuli at high contrast. A
significant interaction emerged between subject group, contrast level and presentation time, indicating
that alterations of CSS in depression were modulated by stimulus conditions. At high contrast, depressed
individuals showed significantly greater CSS than controls at the 66 ms presentation time (where the
effect peaked in both groups). The results' specificity and dependence on stimulus features such as
contrast, size and presentation time suggest that they arise from changes in early visual processing, and
are not the results of a generalized deficit or cognitive bias.Accepted versio
- …