8,179 research outputs found
Compressible Turbulence Measurements in a Supersonic Boundary Layer Including Favorable Pressure Gradient Effects
The effect of a favorable pressure gradient on the turbulent flow structure in a Mach 2.9 boundary layer (Re/m approx. 1.5 x 107) is investigated experimentally. Conventional flow and hot film measurements of turbulent fluctuation properties have been made upstream of and along an expansion ramp. Upstream measurements were taken in a zero pressure gradient boundary layer 44 cm from the nozzle throat in a 6.35 cm square test section. Measurements are obtained in the boundary layer, above the expansion ramp, 71.5 cm from the nozzle throat. Mean flow and turbulent flow characteristics are measured in all three dimensions. Comparisons are made between data obtained using single and multiple-overheat cross-wire anemometry as well as conventional mean flow probes. Conventional flow measurements were taken using a Pitot probe and a 10 degree cone static probe. Flow visualization was conducted via imaging techniques (Schlieren and shadowgraph photographs). Results suggest that compressibility effects, as seen through the density fluctuations in the Reynolds shear stress are roughly 10% relative to the mean velocity and are large relative to the velocity fluctuations. This is also observed in the total Reynolds shear stress; compressibility accounts for 50 - 75% of the total shear. This is particularly true in the favorable pressure gradient region, where though the peak fluctuation intensities are diminished, the streamwise component of the mean flow is larger, hence the contribution of the compressibility term is significant in the Reynolds shear
Suzaku Observations of the Black Hole H1743-322 in Outburst
We observed the Galactic black hole candidate H1743-322 with Suzaku for
approximately 32 ksec, while the source was in a low/hard state during its 2008
outburst. We collected and analyzed the data with the HXD/PIN, HXD/GSO and XIS
cameras spanning the energy range from 0.7-200 keV. Fits to the spectra with
simple models fail to detect narrow Fe XXV and Fe XXVI absorption lines, with
90% confidence upper limits of 3.5 eV and 2.5 eV on the equivalent width,
respectively. These limits are commensurate with those in the very high state,
but are well below the equivalent widths of lines detected in the high/soft
state, suggesting that disk winds are partially state-dependent. We discuss
these results in the context of previous detections of ionized Fe absorption
lines in H1743-322 and connections to winds and jets in accreting systems.
Additionally, we report the possible detection of disk reflection features,
including an Fe K emission line.Comment: 16 pages, 4 figures, 4 tables. Accepted for publication in ApJ
Investigating the nature of absorption lines in the Chandra X-ray spectra of the neutron star binary 4U 1820−30
We use four Chandra gratings spectra of the neutron star low-mass X-ray binary 4U 1820–30 to better understand the nature of certain X-ray absorption lines in X-ray binaries, including the Ne II, Ne III, Ne IX, O VII, and O VIII lines. The equivalent widths of the lines are generally consistent between the observations, as expected if these lines originate in the hot interstellar medium. No evidence was found that the lines were blueshifted, again supporting the interstellar medium origin, although this may be due to poor statistics. There is apparent variability in the O VIII Lyα line equivalent width providing some evidence that at least some of the O VIII absorption arises within the system. However, the significance is marginal (2.4 σ), and the lack of variation in the other lines casts some doubt on the reality of the variability. From calculating the equivalent hydrogen column densities for a range of Doppler parameters, we find they are consistent with the interstellar origin of the lines. In addition, we fit the spectra with photoionization models for locally absorbing material, and find that they can reproduce the spectrum well, but only when there is an extremely low filling factor. We conclude that both the ISM and local absorption remain possible for the origin of the lines, but that more sensitive observations are needed to search for low-level variability
Initial measurements of black hole spin in GX 339-4 from Suzaku spectroscopy
We report on a deep Suzaku observation of the stellar-mass black hole GX 339-4 in outburst. A clear, strong, relativistically shaped iron emission line from the inner accretion disk is observed. The broadband disk reflection spectrum revealed is one of the most sensitive yet obtained from an accreting black hole. We fit the Suzaku spectra with a physically motivated disk reflection model, blurred by a new relativistic line function in which the black hole spin parameter is a variable. This procedure yielded a black hole spin parameter of a p. Joint modeling of these Suzaku spectra and prior XMM-Newton spectra obtained in two different 0.89 +/- 0.04 outburst phases yields a spin parameter of a = 0.93 +/- 0.01. The degree of consistency between these results suggests that disk reflection models allow for spin measurements that are not strongly biased by scattering effects. We suggest that the best value of the black hole spin parameter is a = 0.93 +/- 0.01 (statistical) +/- 0.04 (systematic). Although preliminary, these results represent the first direct measurement of nonzero spin in a stellar-mass black hole using relativistic line modeling
The magnetic nature of disk accretion onto black holes
Although disk accretion onto compact objects - white dwarfs, neutron stars,
and black holes - is central to much of high energy astrophysics, the
mechanisms which enable this process have remained observationally elusive.
Accretion disks must transfer angular momentum for matter to travel radially
inward onto the compact object. Internal viscosity from magnetic processes and
disk winds can in principle both transfer angular momentum, but hitherto we
lacked evidence that either occurs. Here we report that an X-ray-absorbing wind
discovered in an observation of the stellar-mass black hole binary GRO J1655-40
must be powered by a magnetic process that can also drive accretion through the
disk. Detailed spectral analysis and modeling of the wind shows that it can
only be powered by pressure generated by magnetic viscosity internal to the
disk or magnetocentrifugal forces. This result demonstrates that disk accretion
onto black holes is a fundamentally magnetic process.Comment: 15 pages, 2 color figures, accepted for publication in Nature.
Supplemental materials may be obtained by clicking
http://www.astro.lsa.umich.edu/~jonmm/nature1655.p
Microflares in accretion disks
We have investigated the phenomenon of explosive chromospheric evaporation
from an accretion disk as a mechanism for fast variability in accreting sources
such as low mass X-ray binaries and active galactic nuclei. This has been done
in the context of advection dominated accretion flows, allowing both high and
low states to be considered. This mechanism can in principle produce
sub-millisecond timescales in binaries and sub-minute timescales in active
galaxies. However, even considering the possibility that large numbers of these
microflares may be present simultaneously, the power emitted from these
microflares probably amounts to only a small fraction of the total X-ray
luminosity.Comment: 5 pages, 1 figure, uses older A&A class file; accepted for
publication in A&
Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H~1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow
We observed the bright phase of the 2003 outburst of the Galactic black hole
candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four
occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV
and (H-like) Fe XXVI resonance absorption lines. In the first observation, the
Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200
km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe
XXV line is observed to vary significantly on a timescale of a few hundred
seconds in the first observation, which corresponds to the Keplerian orbital
period at approximately 1 E+4 gravitational radii. Our models for the
absorption geometry suggest that a combination of geometric effects and
changing ionizing flux are required to account for the large changes in line
flux observed between observations, and that the absorption likely occurs at a
radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the
absorption geometry include cyclic absorption due to an accretion disk
structure, absorption in a clumpy outflowing disk wind, or possibly a
combination of these two. If the wind in H 1743-322 has unity filling factor,
the highest implied mass outflow rate is 20 percent of the Eddington mass
accretion rate. This wind may be a hot precursor to the Seyfert-like,
outflowing "warm absorber" geometries recently found in the Galactic black
holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of
ionized Fe absorption lines found in the spectra of other Galactic sources, and
connections to warm absorbers, winds, and jets in other accreting systems.Comment: 18 pages, 7 figures, 5 in color, subm. to ApJ. Uses emulateapj.sty
and apjfonts.st
Genome scan of Diabrotica virgifera virgifera for genetic variation associated with crop rotation tolerance
Crop rotation has been a valuable technique for control of Diabrotica virgifera virgifera for almost a century. However, during the last two decades, crop rotation has ceased to be effective in an expanding area of the US corn belt. This failure appears to be due to a change in the insect's oviposition behaviour, which, in all probability, has an underlying genetic basis. A preliminary genome scan using 253 amplified fragment-length polymorphism (AFLP) markers sought to identify genetic variation associated with the circumvention of crop rotation. Samples of D. v. virgifera from east-central Illinois, where crop rotation is ineffective, were compared with samples from Iowa at locations that the behavioural variant has yet to reach. A single AFLP marker showed signs of having been influenced by selection for the circumvention of crop rotation. However, this marker was not diagnostic. The lack of markers strongly associated with the trait may be due to an insufficient density of marker coverage throughout the genome. A weak but significant general heterogeneity was observed between the Illinois and Iowa samples at microsatellite loci and AFLP markers. This has not been detected in previous population genetic studies of D. v. virgifera and may indicate a reduction in gene flow between variant and wild-type beetles
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