2,890 research outputs found
On the kHz QPO frequency correlations in bright neutron star X-ray binaries
We re-examine the correlation between the frequencies of upper and lower kHz
quasi-periodic oscillations (QPO) in bright neutron-star low-mass X-ray
binaries. By including the kHz QPO frequencies of the X-ray binary Cir X-1 and
two accreting millisecond pulsars in our sample, we show that the full sample
does not support the class of theoretical models based on a single resonance,
while models based on relativistic precession or Alfven waves describe the data
better. Moreover, we show that the fact that all sources follow roughly the
same correlation over a finite frequency range creates a correlation between
the linear parameters of the fits to any sub-sample.Comment: Accepted for publication in MNRAS; 7 pages, 4 figure
State transitions in the 2001/2002 outburst of XTE J1650-500
We present a study of the X-ray transient and black hole candidate XTE
J1650-500 during its 2001/2002 outburst. The source made two state transitions
between the hard and soft states, at luminosity levels that differed by a
factor of ~5-10. The first transition, between hard and soft, lasted for ~30
days and showed two parts; one part in which the spectral properties evolve
smoothly away from the hard state and another that we identify as the 'steep
power law state'. The two parts showed different behavior of the Fe K emission
line and QPO frequencies. The second transition, from soft to hard, lasted only
\~15 days and showed no evidence of the presence of the 'steep power law
state'. Comparing observations from the early rise and the decay of the
outburst, we conclude that the source can be in the hard state in a range of
more 10^4 in luminosity. We briefly discuss the state transitions in the
framework of a two-flow model.Comment: 4 pages, 5 figures. To appear in Proc. of the II BeppoSAX Meeting:
"The Restless High-Energy Universe" (Amsterdam, May 5-8, 2003), E.P.J. van
den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Ed
Applications of microlocal analysis to some hyperbolic inverse problems
This thesis compiles my work on three inverse problems: ultrasound recovery in thermoacoustic tomography, cancellation of singularities in synthetic aperture radar, and the injectivity and stability of some generalized Radon transforms. Each problem is approached using microlocal methods. In the context of thermoacoustic tomography under the damped wave equation, I show uniqueness and stability of the problem with complete data, provide a reconstruction algorithm for small attenuation with complete data, and obtain stability estimates for visible singularities with partial data. The chapter on synthetic aperture radar constructs microlocally several infinite-dimensional families of ground reflectivity functions which appear microlocally regular when imaged using synthetic aperture radar. Finally, based on a joint work with Hanming Zhou, we show the analytic microlocal regularity of a class of analytic generalized Radon transforms, using this to show injectivity and stability for a generic class of generalized Radon transforms defined on analytic Riemannian manifolds
Circular polarization measurement in millimeter-wavelength spectral-line VLBI observations
This paper considers the problem of accurate measurement of circular
polarization in imaging spectral-line VLBI observations in the lambda=7 mm and
lambda=3 mm wavelength bands. This capability is especially valuable for the
full observational study of compact, polarized SiO maser components in the
near-circumstellar environment of late-type, evolved stars. Circular VLBI
polarimetry provides important constraints on SiO maser astrophysics, including
the theory of polarized maser emission transport, and on the strength and
distribution of the stellar magnetic field and its dynamical role in this
critical circumstellar region. We perform an analysis here of the data model
containing the instrumental factors that limit the accuracy of circular
polarization measurements in such observations, and present a corresponding
data reduction algorithm for their correction. The algorithm is an enhancement
of existing spectral line VLBI polarimetry methods using autocorrelation data
for calibration, but with innovations in bandpass determination,
autocorrelation polarization self-calibration, and general optimizations for
the case of low SNR, as applicable at these wavelengths. We present an example
data reduction at mm and derive an estimate of the predicted
accuracy of the method of m_c < 0.5% or better at lambda=7 mm and m_c < 0.5-1%
or better at lambda=3 mm. Both the strengths and weaknesses of the proposed
algorithm are discussed, along with suggestions for future work.Comment: 23 pages, 13 figure
Stress Coping Strategies for Commercial Flight Crewmembers
This paper provides a selective examination of the current stress management literature and its practices. The author introduces strategies that can be utilized by commercial pilots to handle stressful events in the often-challenging flight environment and establishes a foundation for further empirical research in the area of human stress control in aviation settings. Stress management techniques adopted from sport psychology and management training programs are reviewed. First, stress and distress are defined and the physical, physiological, and emotional responses to stress are discussed. This is followed by an overview of the biology of stress. Next, traditional forms of stress avoidance are examined and the concept of stress hardiness is identified. A holistic management model, adapted to the aviation environment, is proposed to position stress resistance training within the context of the overall stress experience. Finally, a support program, the Air Line Pilots Association\u27s Critical Incident Response Program, which assists commercial pilots who face emotional work-related traumas, is discussed
Swift X-ray Telescope study of the Black Hole Binary MAXI J1659-152: Variability from a two component accretion flow
We present an energy dependent X-ray variability study of the 2010 outburst
of the black hole X-ray binary MAXI J1659-152 with the Swift X-ray Telescope
(XRT). The broad-band noise components and the quasi periodic oscillations
(QPO) observed in the power spectra show a strong and varied energy dependence.
Combining Swift XRT data with data from the Rossi X-ray Timing Explorer, we
report, for the first time, an rms spectrum (fractional rms amplitude as a
function of energy) of these components in the 0.5-30 keV energy range. We find
that the strength of the low-frequency component (< 0.1 Hz) decreases with
energy, contrary to the higher frequency components (> 0.1 Hz) whose strengths
increase with energy. In the context of the propagating fluctuations model for
X-ray variability, we suggest that the low-frequency component originates in
the accretion disk (which dominates emission below ~ 2 keV) and the higher
frequency components are formed in the hot flow (which dominates emission above
~ 2 keV). As the properties of the QPO suggest that it may have a different
driving mechanism, we investigate the Lense-Thirring precession of the hot flow
as a candidate model. We also report on the QPO coherence evolution for the
first time in the energy band below 2 keV. While there are strong indications
that the QPO is less coherent at energies below 2 keV than above 2 keV, the
coherence increases with intensity similar to what is observed at energies
above 2 keV in other black-hole X-ray binaries.Comment: 12 pages, 6 Figures, 2 Tables, Accepted for publication in
Astrophysical Journa
High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)
We present multi-frequency, high resolution VLBA circular polarization images
of the radio source 3C 84 in the center of NGC 1275. Our images reveal a
complex distribution of circular polarization in the inner parsec of the radio
jet, with local levels exceeding 3% polarization, the highest yet detected with
VLBI techniques. The circular polarization changes sign along the jet, making
3C 84 also the first radio jet to show both signs of circular polarization
simultaneously. The spectrum and changing sign of the circular polarization
indicate that it is unlikely to be purely intrinsic to the emitted synchrotron
radiation. The Faraday conversion process makes a significant and perhaps
dominant contribution to the circular polarization, and the observed spectrum
suggests the conversion process is near saturation. The sign change in the
circular polarization along the jet may result from this saturation or may be
due to a change in magnetic field order after an apparent bend in the jet. From
the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels
of circular polarization inferred for the intra-day variable source PKS
1519-273, we suggest a connection between small spatial scales and efficient
production of circular polarization.Comment: 4 pages, accepted in ApJ Letter
Radio Band Observations of Blazar Variability
The properties of blazar variability in the radio band are studied using the
unique combination of temporal resolution from single dish monitoring and
spatial resolution from VLBA imaging; such measurements, now available in all
four Stokes parameters, together with theoretical simulations, identify the
origin of radio band variability and probe the characteristics of the radio jet
where the broadband blazar emission originates. Outbursts in total flux density
and linear polarization in the optical-to-radio bands are attributed to shocks
propagating within the jet spine, in part based on limited modeling invoking
transverse shocks; new radiative transfer simulations allowing for shocks at
arbitrary angle to the flow direction confirm this picture by reproducing the
observed centimeter-band variations observed more generally, and are of current
interest since these shocks may play a role in the gamma-ray flaring detected
by Fermi. Recent UMRAO multifrequency Stokes V studies of bright blazars
identify the spectral variability properties of circular polarization for the
first time and demonstrate that polarity flips are relatively common.
All-Stokes data are consistent with the production of circular polarization by
linear-to-circular mode conversion in a region that is at least partially
self-absorbed. Detailed analysis of single-epoch, multifrequency, all-Stokes
VLBA observations of 3C 279 support this physical picture and are best
explained by emission from an electron-proton plasma.Comment: 6 pages, 5 figures, uses, jaa.sty. Invited talk presented at the
conference Multifrequency Variability of Blazars, Guangzhou, China, September
22-24, 2010. To appear in J. Astrophys. Ast
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