684 research outputs found
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
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
Advancing environmental risk assessment for transgenic biofeedstock crops
Transgenic modification of plants is a key enabling technology for developing sustainable biofeedstocks for biofuels production. Regulatory decisions and the wider acceptance and development of transgenic biofeedstock crops are considered from the context of science-based risk assessment. The risk assessment paradigm for transgenic biofeedstock crops is fundamentally no different from that of current generation transgenic crops, except that the focus of the assessment must consider the unique attributes of a given biofeedstock crop and its environmental release. For currently envisioned biofeedstock crops, particular emphasis in risk assessment will be given to characterization of altered metabolic profiles and their implications relative to non-target environmental effects and food safety; weediness and invasiveness when plants are modified for abiotic stress tolerance or are domesticated; and aggregate risk when plants are platforms for multi-product production. Robust risk assessments for transgenic biofeedstock crops are case-specific, initiated through problem formulation, and use tiered approaches for risk characterization
A model-independent analysis of the variability of GRS 1915+105
We analyzed 163 observations of the microquasar GRS 1915+105 made with the
Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each
observation, we produced light curves and color-color diagrams. We classified
the observations in 12 separate classes, based on their count rate and color
characteristics. From the analysis of these classes, we reduced the variability
of the source to transitions between three basic states: a hard state
corresponding to the non-observability of the innermost parts of the accretion
disk, and two softer states with a fully observable disk. These two soft states
represent different temperatures of the accretion disk, related to different
local values of the accretion rate. The transitions between these states can be
extremely fast. The source moves between these three states following certain
patterns and avoiding others, giving rise to a relatively large but limited
number of variability classes. These results are the first step towards a
linking of the properties of this exceptional source with standard black-hole
systems and with accretion disk models.Comment: Accepted for publication in Astronomy & Astrophysics, 2000 January
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Identification of black hole power spectral components across all canonical states
From a uniform analysis of a large (8.5 Ms) Rossi X-ray Timing Explorer data
set of Low Mass X-ray Binaries, we present a complete identification of all the
variability components in the power spectra of black holes in their canonical
states. It is based on gradual frequency shifts of the components observed
between states, and uses a previous identification in the black hole low hard
state as a starting point. It is supported by correlations between the
frequencies in agreement with those previously found to hold for black hole and
neutron stars. Similar variability components are observed in neutron stars and
black holes (only the component observed at the highest frequencies is
different) which therefore cannot depend on source-specific characteristics
such as the magnetic field or surface of the neutron star or spin of the black
hole. As the same variability components are also observed across the jet-line
the X-ray variability cannot originate from the outer-jet but is most likely
produced in either the disk or the corona. We use the identification to
directly compare the difference in strength of the black hole and neutron star
variability and find these can be attributed to differences in frequency and
strength of high frequency features, and do not require the absence of any
components. Black holes attain their highest frequencies (in the
hard-intermediate and very-high states) at a level a factor ~6 below the
highest frequencies attained by the corresponding neutron star components,
which can be related to the mass difference between the compact objects in
these systems.Comment: 17 pages, 16 figures, accepted for publication in Ap
Simulated Acid Rain Effects On Cool-Season Forage Grasses
University of Tennessee Agricultural Experiment Station Bulletin 670 contains published papers on fescue and forage grasses and their reactions to exposure of high ph water and acid rain
Antenna Design and Implementation for the Future Space Ultra-Long Wavelength Radio Telescope
In radio astronomy, the Ultra-Long Wavelengths (ULW) regime of longer than 10
m (frequencies below 30 MHz), remains the last virtually unexplored window of
the celestial electromagnetic spectrum. The strength of the science case for
extending radio astronomy into the ULW window is growing. However, the
opaqueness of the Earth's ionosphere makes ULW observations by ground-based
facilities practically impossible. Furthermore, the ULW spectrum is full of
anthropogenic radio frequency interference (RFI). The only radical solution for
both problems is in placing an ULW astronomy facility in space. We present a
concept of a key element of a space-borne ULW array facility, an antenna that
addresses radio astronomical specifications. A tripole-type antenna and
amplifier are analysed as a solution for ULW implementation. A receiver system
with a low power dissipation is discussed as well. The active antenna is
optimized to operate at the noise level defined by the celestial emission in
the frequency band 1 - 30 MHz. Field experiments with a prototype tripole
antenna enabled estimates of the system noise temperature. They indicated that
the proposed concept meets the requirements of a space-borne ULW array
facility.Comment: Submitted to Experimental Astronomy, 23 pages, 17 figure
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