2,562 research outputs found
Graphical programming and the use of simulation for space-based manipulators
Robotic manipulators are difficult to program even without the special requirements of a zero-gravity environment. While attention should be paid to investigating the usefulness of industrial application programming methods to space manipulators, new methods with potential application to both environments need to be invented. These methods should allow various levels of autonomy and human-in-the-loop interaction and simple, rapid switching among them. For all methods simulation must be integrated to provide reliability and safety. Graphical programming of manipulators is a candidate for an effective robot programming method despite current limitations in input devices and displays. A research project in task-level robot programming has built an innovative interface to a state-of-the-art commercial simulation and robot programming platform. The prototype demonstrates simple augmented methods for graphical programming and simulation which may be of particular interest to those concerned with Space Station applications; its development has also raised important issues for the development of more sophisticated robot programming tools. Both aspects of the project are discussed
Magnetohydrodynamic simulations of black hole accretion
We discuss the results of three-dimensional magnetohydrodynamic simulations,
using a pseudo-Newtonian potential, of thin disk (h/r ~ 0.1) accretion onto
black holes. We find (i) that magnetic stresses persist within the marginally
stable orbit, and (ii) that the importance of those stresses for the dynamics
of the flow depends upon the strength of magnetic fields in the disk outside
the last stable orbit. Strong disk magnetic fields (alpha > 0.1) lead to a
gross violation of the zero-torque boundary condition at the last stable orbit,
while weaker fields (alpha ~ 0.01) produce results more akin to traditional
models for thin disk accretion onto black holes. Fluctuations in the magnetic
field strength in the disk could lead to changes in the radiative efficiency of
the flow on short timescales.Comment: 6 pages, to appear in proceedings, 20th Texas Symposium on
Relativistic Astrophysics, eds J.C. Wheeler and H. Marte
Implications of the X-ray Variability for the Mass of MCG-6-30-15
The bright Seyfert 1 galaxy \mcg shows large variability on a variety of time
scales. We study the \aproxlt 3 day time scale variability using a set of
simultaneous archival observations that were obtained from \rxte and the {\it
Advanced Satellite for Cosmology and Astrophysics} (\asca). The \rxte\
observations span nearly sec and indicate that the X-ray Fourier Power
Spectral Density has an rms variability of 16%, is flat from approximately
10^{-6} - 10^{-5} Hz, and then steepens into a power law
with \alpha\aproxgt 1. A further steepening to occurs
between 10^{-4}-10^{-3} Hz. The shape and rms amplitude are comparable to what
has been observed in \ngc and \cyg, albeit with break frequencies that differ
by a factor of 10^{-2} and 10^{4}, respectively. If the break frequencies are
indicative of the central black hole mass, then this mass may be as low as
. An upper limit of ks for the relative lag
between the 0.5-2 keV \asca band compared to the 8-15 keV \rxte band was also
found. Again by analogy with \ngc and \cyg, this limit is consistent with a
relatively low central black hole mass.Comment: 5 pages, 3 figures, LaTeX, uses emulateapj.sty and apjfonts.sty,
revised version, accepted for publication in ApJ Letter
General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes
Coalescing supermassive black hole binaries are produced by the mergers of
galaxies and are the most powerful sources of gravitational waves accessible to
space-based gravitational observatories. Some such mergers may occur in the
presence of matter and magnetic fields and hence generate an electromagnetic
counterpart. In this Letter, we present the first general relativistic
simulations of magnetized plasma around merging supermassive black holes using
the general relativistic magnetohydrodynamic code Whisky. By considering
different magnetic field strengths, going from non-magnetically dominated to
magnetically dominated regimes, we explore how magnetic fields affect the
dynamics of the plasma and the possible emission of electromagnetic signals. In
particular we observe a total amplification of the magnetic field of ~2 orders
of magnitude which is driven by the accretion onto the binary and that leads to
much stronger electromagnetic signals, more than a factor of 10^4 larger than
comparable calculations done in the force-free regime where such amplifications
are not possible.Comment: 7 pages, 5 figures. Minor changes to match version accepted for
publication on The Astrophysical Journal Letter
Eggshell appearance does not signal maternal corticosterone exposure in Japanese quail:an experimental study with brown-spotted eggs
Reproduction is a critical period for birds as they have to cope with many stressful events. One consequence of an acute exposure to stress is the release of corticosterone, the avian stress hormone. Prolonged stress can have negative impacts on the immune system, resulting in, for example, increased oxidative stress. Through maternal effects, females are known to modulate their investment in eggs content according to their own physiological condition. Less is known about maternal investment in eggshells, especially in pigments. The two main eggshell pigments may possess opposite antioxidant properties: protoporphyrin (brown) is a pro-oxidant, whereas biliverdin (blue-green) is an antioxidant. In Japanese quail, we know that the deposition of both pigments is related to female body condition. Thus, a chronic stress response may be reflected in eggshell coloration. Using female Japanese quails that lay brown-spotted eggs, we explored whether physiological exposure to corticosterone induces a change in female basal stress and antioxidant factors, and eggshell pigment concentration, spectrophotometric reflectance, and maculation coverage. We supplemented adult females over a 2 week period with either peanut oil (control) or corticosterone (treatment). We collected pre- and post-supplementation eggs and analysed the effect of corticosterone treatment on female physiology and eggshell appearance parameters. Except for corticosterone-fed birds which laid eggs with brighter spots, supplementation had no significant effect on female physiology or eggshell pigment concentration, reflectance and maculation. The change in eggshell spot brightness was not detected by a photoreceptor noise-limited color opponent model of avian visual perception. Our data confirms that eggshell reflectance in spotted eggs varies over the laying sequence, and spot reflectance may be a key factor that is affected by females CORT exposure, even if the changes are not detected by an avian visual model
Disposal Alternatives for Intermittent Sand Filter Scrapings Utilization and Sand Recovery
A three phase study is used to develop disposal alternatives and cost analysis for algae laden sand scrapings removed from intermittent sand filters used to polish wastewater lagoon effluent. Phase I, Irrigation Technique, explores the feasibility of supplying sufficient water to sand scrapings to leach out entrapped material. Details of removal with amount of water applied are presented. Phase II, Soil Application, tests with lysimeters soil response to application of the algae laden sand material. Physical and chemical parameters are not markedly altered where algae laden sand is applied to the soil surface. Phase III, Plant Bioassays, grew tall fescue on lysimeters which had algae laden sand scrapings applied and compared this growth response with lysimeters having no additives to a clay soil and lysimeters having ammonium nitrate added as fertilizer. Results indicate that all three disposal alternatives are viable recourses for sewage sand filter sand deposition and utilization. Cost analyses indicate that an irrigation technique may be less expensive
XMM-EPIC observation of MCG-6-30-15: Direct evidence for the extraction of energy from aspinning black hole?
We present XMM-Newton European Photon Imaging Camera (EPIC) observations of
the bright Seyfert 1 galaxy MCG-6-30-15, focusing on the broad Fe K
line at ~6keV and the associated reflection continuum, which is believed to
originate from the inner accretion disk. We find these reflection features to
be extremely broad and red-shifted, indicating its origin from the very most
central regions of the accretion disk. It seems likely that we have caught this
source in the ``deep minimum'' state first observed by Iwasawa et al. (1996).
The implied central concentration of X-ray illumination is difficult to
understand in any pure accretion disk model. We suggest that we are witnessing
the extraction and dissipation of rotational energy from a spinning black hole
by magnetic fields connecting the black hole or plunging region to the disk.Comment: 6 pages and one postscript figure. Accepted for publication in MNRAS
letter
Current and Future constraints on Very-Light Axion-Like Particles from X-ray observations of cluster-hosted Active Galaxies
We discuss our recent constraints on the coupling of Very-Light Axion-Like
Particles (of masses ) to electromagnetism from
observations of the cluster-hosted Active Galactic Nuclei (AGN)
H1821+643 and NGC1275. In both cases, the inferred high-quality AGN spectra
excluded all photon-ALP couplings at the level, respectively, based on
the non-detection of spectral distortions attributed to photon-ALP
inter-conversion along the cluster line-of-sight. Finally, we present the
prospects of tightening current bounds on such ALPs by up to a factor of 10
with next-generation X-ray observatories such as , and
given their improved spectral and spatial resolution and collecting area
compared to current missions.Comment: 4 pages, 1 figure. Accepted for publication to Memorie della SAIt for
the Proceedings of the European Astronomical Society 2022 (EAS 2022) Annual
Meeting Symposium S3 "The Dark matter multi-messenger challenge
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