4,556 research outputs found
The linear rms-flux relation in an Ultraluminous X-ray Source
We report the first detection of a linear correlation between rms variability
amplitude and flux in the Ultraluminous X-ray source NGC 5408 X-1. The rms-flux
relation has previously been observed in several Galactic black hole X-ray
binaries (BHBs), several Active Galactic Nuclei (AGN) and at least one neutron
star X-ray binary. This result supports the hypothesis that a linear rms-flux
relation is common to all luminous black hole accretion and perhaps even a
fundamental property of accretion flows about compact objects. We also show for
the first time the cross-spectral properties of the variability of this ULX,
comparing variations below and above 1 keV. The coherence and time delays are
poorly constrained but consistent with high coherence between the two bands,
over most of the observable frequency range, and a significant time delay (with
hard leading soft variations). The magnitude and frequency dependence of the
lags are broadly consistent with those commonly observed in BHBs, but the
direction of the lag is reversed. These results indicate that ULX variability
studies, using long X-ray observations, hold great promise for constraining the
processes driving ULXs behaviour, and the position of ULXs in the scheme of
black hole accretion from BHBs to AGN.Comment: 4 Pages, 3 figures, accepted for publication by MNRAS
Inclination-dependent spectral and timing properties in transient black hole X-ray binaries
We use a simple one-dimensional parameterisation of timing properties to show
that hard and hard-intermediate state transient black hole X-ray binaries with
the same power-spectral shape have systematically harder X-ray power-law
emission in higher-inclination systems. We also show that the power-spectral
shape and amplitude of the broadband noise (with low-frequency quasi-periodic
oscillations, QPOs, removed) is independent of inclination, confirming that it
is well-correlated with the intrinsic structure of the emitting regions and
that the "type C" QPO, which is inclination-dependent, has a different origin
to the noise, probably geometric. Our findings suggest that the power-law
emission originates in a corona which is flattened in the plane of the disc,
and not in a jet-like structure which would lead to softer spectra at higher
inclinations. However, there is tentative evidence that the
inclination-dependence of spectral shape breaks down deeper into the hard
state. This suggests either a change in the coronal geometry and possible
evidence for contribution from jet emission, or alternatively an even more
optically thin flow in these states.Comment: 6 Pages, 4 Figures, accepted as a Letter by MNRA
Power-Colours: Simple X-ray Binary Variability Comparison
We demonstrate a new method of variability classification using observations
of black hole X-ray binaries. Using `power colours' -- ratios of integrated
power in different Fourier frequency bands -- we can clearly differentiate
different canonical black hole states as the objects evolve during outburst. We
analyse (~ 2400) Rossi X-ray Timing Explorer observations of 12 transient low
mass black hole X-ray binaries and find that the path taken around the power
colour-colour diagram as the sources evolve is highly consistent from object to
object. We discuss how the consistency observed in the power colour-colour
diagram between different objects allows for easy state classification based on
only a few observations, and show how the power-spectral shapes can be simply
classified using a single parameter, the power-spectral `hue'. To illustrate
the benefits of our simple model-independent approach, we show that the
persistent high mass X-ray binary Cyg X-1 shows very similar power-spectral
evolution to the transient black hole sources, with the main difference being
caused by a combination of a lack of quasi-periodic oscillations and an excess
of low-frequency power-law noise in the Cyg X-1 power spectra during the
transitional state. We also compare the transient objects to the neutron star
atoll source Aquila X-1, demonstrating that it traces a different path in the
power colour-colour plot. Thus, power-colours could be an effective method to
classify newly discovered X-ray binaries.Comment: 13 pages, 9 figures, accepted by MNRA
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
Extrafloral nectar production of the ant-associated plant, Macaranga tanarius, is an induced, indirect, defensive response elicited by jasmonic acid
Neutron Capture Cross Sections for the Weak s Process
In past decades a lot of progress has been made towards understanding the
main s-process component that takes place in thermally pulsing Asymptotic Giant
Branch (AGB) stars. During this process about half of the heavy elements,
mainly between 90<=A<=209 are synthesized. Improvements were made in stellar
modeling as well as in measuring relevant nuclear data for a better description
of the main s process. The weak s process, which contributes to the production
of lighter nuclei in the mass range 56<=A<=90 operates in massive stars
(M>=8Msolar) and is much less understood. A better characterization of the weak
s component would help disentangle the various contributions to element
production in this region. For this purpose, a series of measurements of
neutron-capture cross sections have been performed on medium-mass nuclei at the
3.7-MV Van de Graaff accelerator at FZK using the activation method. Also,
neutron captures on abundant light elements with A<56 play an important role
for s-process nucleosynthesis, since they act as neutron poisons and affect the
stellar neutron balance. New results are presented for the (n,g) cross sections
of 41K and 45Sc, and revisions are reported for a number of cross sections
based on improved spectroscopic information
Stellar neutron capture cross sections of ⁴¹K and ⁴⁵Sc
The neutron capture cross sections of light nuclei (
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