Physics Colloquia Fall 2011

General theory of relativity predicts that binary systems consisting of massive, compact objects (e.g. neutron stars and/or black holes) radiate their energy in the form of gravitational waves. In the last stage of this process, when the compact objects inspiral in close orbits and eventually coalesce, the gravitational-wave signal is the strongest and is detectable by the ground based detectors such as LIGO and Virgo. If detected, it will not only serve as a direct confirmation of the general theory of relativity, but also provide invaluable information on the gravity in the strong field regime and the physics of neutron stars and black holes. In anticipation of the start of the advanced LIGO and Virgo detectors around 2015, I will review the methods and strategies that are used to search for gravitation waves from coalescing binaries illustrating them with some of the results from the recent scientific runs of the initial LIGO and Virgo detectors. When discussing prospects of observing coalescing binaries with advanced detector network I will focus primarily on new and rapidly developing multi-messenger program in which central role is given to the low-latency searches with followup observations of interesting candidates by optical, X-ray and radio telescopes. While presenting new challenges for data analysis, this approach offers exciting possibility of observing binary coalescence in multi-wavelengths thus providing a complete picture of the process.

Brachytherapy is the first conformal radiation therapy. A sealed source is placed in or in contact with the tumor providing dose to the tumor with small volumes of normal tissue irradiated. It became more sophisticated with the usage of remote afterloader. With this, our responsibility as physicists increased and became critical in ensuring an accurate and precise treatment planning and delivery. The ability to deliver a correct treatment is conditioned by the precision of the source placement at the dwell point/points predicted by the treatment planning system (TPS). The introducing of the CT based planning gave the possibility of reconstructing not only the correct shape of a treated volume but the correct reconstruction of the applicators, the catheters, and localizing the first dwell position (the stopping point) of the radioactive source that will deliver the treatment.

This colloquium will provide an introduction into the understanding of High Dose Radiation (HDR) Therapy and into organizing a safe HDR program in a cancer center.

Cataclysmic variable (CV) systems are mass transfer binary star systems with white dwarf primaries, low-mass main sequence secondaries, and an accretion disk that typically provides most of the system luminosity.

The previously little-studied cataclysmic variable system V344 Lyr has been observed by the Kepler instrument at a 1-min cadence since 2009 June 20, and as a result it is now the CV with the best time-series data set in history. The system is rich in its behavior and promises to be a touchstone for CV studies for the foreseeable future. The Kepler data reveal that two physical sources yield positive superhumps: viscous dissipation within the periodically flexing disk, and the signal generated as the accretion stream bright spot sweeps around the rim of the non-axisymmetric disk. The V344 Lyr data also reveal negative superhumps arising from accretion onto a tilted disk precessing in the retrograde direction. The changing negative superhump period results from a changing precession rate, which in turn results from a changing mass distribution within the disk.

At present, the source of accretion disk tilt in cataclysmic variables is unknown, however the Kepler data show that in one of the DN outbursts, negative superhumps are excited. Further study of this system may conclusively reveal the physical source of accretion disk tilt as well as severely constrain the microphysics and macroscopic effects of viscosity in differentially-rotating astrophysical plasmas.

The Milky way did not form in isolation, but is the product of a complex evolution of generations of merges, collapse, star formation, supernova and collisional heating, radiative and collisional cooling, and ejected nucleosynthesis. Moreover, all of this occurs in the context of the cosmic expansion, the formation of cosmic filaments, dark-matter haloes, spiral density waves, and emerging dark energy. In this talk I will summarize recent calculations of the formation and evolution of Local-Group like systems derived from simulations of large scale structure. We begin with a variety of simulations on the scale of (400-800 Mpc)$^3$. We then scan for poor clusters which contain two large spirals presently separated by $\sim 800$ kpc. We compare simulated properties with various observed properties of the Local Group. Among the generic features of these systems is the tendency for galactic halos to form within the dark matter filaments that define a super-galactic plane. Gravitational interaction along this structure leads to a streaming flow toward the two dominant galaxies in the cluster. We analyze this alignment and streaming flow and compare with observed properties of Local-Group galaxies. Our comparison with Local Group properties suggests that some dwarf galaxies in the Local Group are part of a local streaming flow. These simulations also suggest that a significant fraction of the Galactic halo formed as at large distances and then arrived later along these streaming flows.

Armin Fuchs (Florida Atlantic University), 18 November
( Note: special place, 303 Behavioral Science)