1,647 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
The broad-band X-ray spectrum of the dipping Low Mass X-ray Binary EXO0748--676
We present results of a 0.1-100 keV BeppoSAX observation of the dipping LMXRB
EXO 0748-676 performed in 2000 November. During the observation the source
exhibited X-ray eclipses, type I X-ray bursts and dipping activity over a wide
range of orbital phases. The 0.1-100keV "dip-free"(ie. dipping and eclipsing
intervals excluded) spectrum is complex,especially at low-energies where a soft
excess is present. Two very different spectral models give satisfactory fits.
The first is the progressive covering model, consisting of separately absorbed
black body and cut-off power-law components.The second model is an absorbed
cut-off power-law together with a moderately ionized absorber with a sub-solar
abundance of Fe and a 2.13 keV absorption feature (tentatively identified with
Si xiii). This ionized absorber may be the same feature as seen by Chandra
during dips from EXO 0748-676.Comment: 7 pages, 5 figures, paper accepted for publication in Astronomy and
Astrophysic
High-frequency Quasi-Periodic Oscillations from GRS 1915+105 in its C state
We report the results of a systematic timing analysis of RXTE observations of
GRS 1915+105 when the source was in its variability class theta, characterized
by alternating soft and hard states on a time scale of a few hundred seconds.
The aim was to examine the high-frequency part of the power spectrum in order
to confirm the hecto-Hertz Quasi-Periodic Oscillations (QPO) previously
reported from observations from mixed variability behaviours. During the hard
intervals (corresponding to state C in the classification of Belloni et al.,
2000, A&A, 35, 271), we find a significant QPO at a frequency of ~170 Hz,
although much broader (Q~2) than previously reported. No other significant peak
is observed at frequencies >30 Hz. A time-resolved spectral analysis of
selected observations shows that the hard intervals from class theta show a
stronger and steeper (Gamma=2.8-3.0) power-law component than hard intervals
from other classes. We discuss these results in the framework of hecto-Hertz
QPOs reported from GRS 1915+105 and other black-hole binaries.Comment: 7 pages, 8 figures, accepted for publication on MNRA
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
Fast determination of coarse grained cell anisotropy and size in epithelial tissue images using Fourier transform
Mechanical strain and stress play a major role in biological processes such
as wound healing or morphogenesis. To assess this role quantitatively, fixed or
live images of tissues are acquired at a cellular precision in large fields of
views. To exploit these data, large numbers of cells have to be analyzed to
extract cell shape anisotropy and cell size. Most frequently, this is performed
through detailed individual cell contour determination, using so-called
segmentation computer programs, complemented if necessary by manual detection
and error corrections. However, a coarse grained and faster technique can be
recommended in at least three situations. First, when detailed information on
individual cell contours is not required, for instance in studies which require
only coarse-grained average information on cell anisotropy. Second, as an
exploratory step to determine whether full segmentation can be potentially
useful. Third, when segmentation is too difficult, for instance due to poor
image quality or too large a cell number. We developed a user-friendly, Fourier
transform-based image analysis pipeline. It is fast (typically cells per
minute with a current laptop computer) and suitable for time, space or ensemble
averages. We validate it on one set of artificial images and on two sets of
fully segmented images, one from a Drosophila pupa and the other from a chicken
embryo; the pipeline results are robust. Perspectives include \textit{in vitro}
tissues, non-biological cellular patterns such as foams, and stacks.Comment: 13 pages; 9 figure
MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets
We discuss the jet kinematics of a complete flux-density-limited sample of
135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to
investigate the structure and evolution of parsec-scale jet phenomena. Our
analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained
between 2002 and 2007, but includes our previously published observations made
at the same wavelength, and is supplemented by VLBA archive data. In all, we
have used 2424 images spanning the years 1994-2007 to study and determine the
motions of 526 separate jet features in 127 jets. The data quality and temporal
coverage (a median of 15 epochs per source) of this complete AGN jet sample
represents a significant advance over previous kinematics surveys. In all but
five AGNs, the jets appear one-sided, most likely the result of differential
Doppler boosting. In general the observed motions are directed along the jet
ridge line, outward from the optically thick core feature. We directly observe
changes in speed and/or direction in one third of the well-sampled jet
components in our survey. While there is some spread in the apparent speeds of
separate features within an individual jet, the dispersion is about three times
smaller than the overall dispersion of speeds among all jets. This supports the
idea that there is a characteristic flow that describes each jet, which we have
characterized by the fastest observed component speed. The observed maximum
speed distribution is peaked at ~10c, with a tail that extends out to ~50c.
This requires a distribution of intrinsic Lorentz factors in the parent
population that range up to ~50. We also note the presence of some rare
low-pattern speeds or even stationary features in otherwise rapidly flowing
jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal;
online only material is available from
http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi
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