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Research
High-Energy Astrophysics (Ballantyne)
High-energy astrophysics is the study of fundamental physics within the most violent environments imaginable. X-ray and gamma-ray sources are used as laboratories to explore physical processes at temperatures, densities and energies so extreme that Earth-based experiments would be impossible. For example, the gravitational field of a neutron star or black hole is strong enough that they can accrete gas from a companion star by stripping material off of the star's surface. The infalling gas is significantly heated as it spirals down the potential well and radiates strongly in X-rays. By studying this emission, one can test theories of strong gravity (e.g., general relativity) as well as the interaction of matter with intense radiation and magnetic fields. This type of research can therefore provide direct tests of many of the basic ideas of modern physics. Accretion physics is one of the most complex and important problems in modern astrophysics. The energy released in disks of gas flowing onto collapsed relativistic objects power most of the X-ray sources in the Universe. Star and planet formation all occur via accretion through a disk of gas and dust. Accretion disk theory requires combining magnetohydrodynamics, radiative transfer and photoionization physics. It is important to test these theories by determining the physical properties of accretion disks, and discovering how changes in their structure relate to different observational characteristics of accretion powered sources, such as the various manifestations of the active galactic nuclei (AGN) phenomenon. Black Hole and Galaxy Evolution (Ballantyne)
Over the last decade, multiple observational campaigns have produced compelling evidence that super-massive black holes (SMBHs) exist at the centers of almost all massive galaxies. The data also revealed a very significant correlation between the masses of the central SMBH and the host galaxy, implying a close connection between the growth of the SMBH and the surrounding galaxy. Understanding this connection is a goal for many of the next-generation telescopes and instruments (e.g., SCUBA-2, ALMA, LSST, TMT, JWST, Herschel and JDEM). When black holes grow by accreting gas and dust from their surroundings they release a tremendous amount of energy that can be detected to great distances and across many decades in photon energy. These accreting black holes are known as AGN, and thus studying the evolution of AGN over cosmic time provides an unique view of the formation of all massive galaxies. A strategy to answer these questions is to study the AGN that produce the cosmic X-ray background. Deep observations by Chandra and XMM-Newton have resolved 80--90% of the X-ray background into individual sources, identified as mostly obscured AGN. Optical follow-up observations of these obscured AGN have yielded two interesting surprises. First, the redshift distribution of the sources peaks at roughly the same point in cosmic history where the cosmic star-formation rate also reaches its maximum. Second, the luminosities of these AGN are relatively low, implying a small accretion rate onto the SMBHs in these galaxies. Both of these properties are in stark contrast to the high-luminosity AGN population observed at larger redshifts, and suggests that a completely different mode of galaxy evolution is ongoing in the X-ray background sources. High Energy Neutrino Astrophysics with IceCube (Taboada)
Activities of the Georgia Tech group are focused on Gamma Ray Bursts (GRBs) and related phenomena. GRBs are very dramatic explosions that, for a few seconds, can outshine the rest of the visible Universe. GRBs have long been speculated as a potential source of the highest energy neutrinos. Choked-GRBs are hypothezised objects that create a link between GRBs and supernovae. In searching for neutrinos with IceCube, the Georgia Tech group may find evidence for these objects. High Energy Gamma-ray Astrophysics with HAWC (Taboada)
The Georgia Tech group is involved in the design and construction of HAWC, a second generation air shower array. The efforst of the Georgia Tech group are focused on the scaler electronics system. This system can be used for monitoring the health of the detector, studying particle emission by the Sun and for studying GRBs. GRBs are dramatic explosions, that for a few seconds can outshine the rest of the visible Universe. The Georgia Tech group has shown, that, for the first time, a ground-based detector has a realistic opportunity to detect GRBs at energies higher than 3x1010 eV. Zooms, Whirls and Bursts
Gravitational Wave Astrophysics (Laguna, Shoemaker)Can LIGO “hear” different black-hole spin orientations in bursts of gravitational waves?
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