17,859 research outputs found
Interpreting the High Frequency QPO Power Spectra of Accreting Black Holes
In the context of a relativistic hot spot model, we investigate different
physical mechanisms to explain the behavior of quasi-periodic oscillations
(QPOs) from accreting black holes. The locations and amplitudes of the QPO
peaks are determined by the ray-tracing calculations presented in Schnittman &
Bertschinger (2004a): the black hole mass and angular momentum give the
geodesic coordinate frequencies, while the disk inclination and the hot spot
size, shape, and overbrightness give the amplitudes of the different peaks. In
this paper additional features are added to the existing model to explain the
broadening of the QPO peaks as well as the damping of higher frequency
harmonics in the power spectrum. We present a number of analytic results that
closely agree with more detailed numerical calculations. Four primary pieces
are developed: the addition of multiple hot spots with random phases, a finite
width in the distribution of geodesic orbits, Poisson sampling of the detected
photons, and the scattering of photons from the hot spot through a corona of
hot electrons around the black hole. Finally, the complete model is used to fit
the observed power spectra of both type A and type B QPOs seen in XTE
J1550-564, giving confidence limits on each of the model parameters.Comment: 30 pages, 5 figures, submitted to Ap
A Hot Spot Model for Black Hole QPOs
In at least two black hole binary systems, the Rossi X-Ray Timing Explorer
has detected high frequency quasi-periodic oscillations (HFQPOs) with a 2:3
frequency commensurability. We propose a simple hot spot model to explain the
positions, amplitudes, and widths of the HFQPO peaks. Using the exact geodesic
equations for the Kerr metric, we calculate the trajectories of massive test
particles, which are treated as isotropic, monochromatic emitters in their rest
frames. By varying the hot spot parameters, we are able to explain the
different features observed in ``Type A'' and ``Type B'' QPOs from XTE
J1550-564. In the context of this model, the observed power spectra allow us to
infer values for the black hole mass and angular momentum, and also constrain
the parameters of the model.Comment: 4 pages, 2 figures, to be published in X-Ray Timing 2003: Rossi and
Beyond, ed. P. Kaaret, F. K. Lamb, & J. H. Swank (Melville, NY: American
Institute of Physics
Ray casting implicit fractal surfaces with reduced affine arithmetic
A method is presented for ray casting implicit surfaces defined by fractal combinations of procedural noise functions. The method is robust and uses affine arithmetic to bound the variation of the implicit function along a ray. The method is also efficient due to a modification in the affine arithmetic representation that introduces a condensation step at the end of every non-affine operation. We show that our method is able to retain the tight estimation capabilities of affine arithmetic for ray casting implicit surfaces made from procedural noise functions while being faster to compute and more efficient to store
Shape and Trajectory Tracking of Moving Obstacles
This work presents new methods and algorithms for tracking the shape and
trajectory of moving reflecting obstacles with broken rays, or rays reflecting
at an obstacle. While in tomography the focus of the reconstruction method is
to recover the velocity structure of the domain, the shape and trajectory
reconstruction procedure directly finds the shape and trajectory of the
obstacle. The physical signal carrier for this innovative method are ultrasonic
beams. When the speed of sound is constant, the rays are straight line segments
and the shape and trajectory of moving objects will be reconstructed with
methods based on the travel time equation and ellipsoid geometry. For variable
speed of sound, we start with the eikonal equation and a system of differential
equations that has its origins in acoustics and seismology. In this case, the
rays are curves that are not necessarily straight line segments and we develop
algorithms for shape and trajectory tracking based on the numerical solution of
these equations. We present methods and algorithms for shape and trajectory
tracking of moving obstacles with reflected rays when the location of the
receiver of the reflected ray is not known in advance. The shape and trajectory
tracking method is very efficient because it is not necessary for the reflected
signal to traverse the whole domain or the same path back to the transmitter.
It could be received close to the point of reflection or far away from the
transmitter. This optimizes the energy spent by transmitters for tracking the
object, reduces signal attenuation and improves image resolution. It is a safe
and secure method. We also present algorithms for tracking the shape and
trajectory of absorbing obstacles. The new methods and algorithms for shape and
trajectory tracking enable new applications and an application to one-hop
Internet routing is presented.Comment: 22 pages, 2 figures, 2 table
Designing Volumetric Truss Structures
We present the first algorithm for designing volumetric Michell Trusses. Our
method uses a parametrization approach to generate trusses made of structural
elements aligned with the primary direction of an object's stress field. Such
trusses exhibit high strength-to-weight ratios. We demonstrate the structural
robustness of our designs via a posteriori physical simulation. We believe our
algorithm serves as an important complement to existing structural optimization
tools and as a novel standalone design tool itself
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