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ResearchHigh-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)
A neutrino sky map will reveal the still unknown sources of the highest energy cosmic rays. For over forty years we have known that somewhere in the Universe particles, typically protons and atomic nuclei, are being accelerated to energies in excess of 1 Joule, or 10 million times more energy than particles accelerated in machines built by human-kind. Well established knowledge in particle physics predicts that sources of high energy cosmic rays are also sources of high energy neutrinos. We detect neutrinos by looking for a tiny amount of light that is produced when a neutrino collides with matter. IceCube is being built at the South Pole because we need a large volume of highly transparent material, and the ice down there is so pure that we can see light from several hundred meters away. IceCube's detection principle is simple: drill holes into the glacial ice and sink in optical sensors (Photo-multiplier tubes) in a three dimensional array that can see the light due to a neutrino interaction. Theoretical physicists have a long list of objects that can produce neutrinos. Active Galactic Nuclei, which are driven by supermassive black holes at the center of galaxies; micro-quasars in which a solar mass black holes devour matter from a companion star; the remanents of supernova explosions; and, the subject of my research, Gamma-ray bursts. A detection of neutrinos in the direction of and in coincidence with a Gamma-ray burst would be a great advance for science. It would provide with a smoking gun for the source of the highest energy cosmic rays, it would give us a fantastic tool to study these mysterious objects and would increase our knowledge into the properties and behavior of neutrinos themselves. With IceCube already operating at half its final size, and with the end of construction only three years away, the promise of extragalactic neutrino astronomy is within our grasp. High Energy Gamma-ray Astrophysics with HAWC (Taboada)
HAWC will be built by a collaboration of scientists from the US and Mexico with joint support. The HAWC site is Sierra Negra, Mexico, which is a very high altitude (4100m) site near existing infrastructure and collaborating universities. The HAWC observatory will utilize water Cherenkov technology (as proven by Milagro) and many of the Milagro components. The first phase of HAWC can be operational quickly, surpassing Milagro's sensitivity within two years of the onset of funding. Because of the increased altitude, the increased physical area, and optimized design, HAWC will have an improved angular resolution, larger effective area, lower energy threshold and better background rejection. These improvements will result in a sensitivity of 10-15x (depending on source spectrum) better than that of Milagro and can be accomplished without any new technology, but only a modest upgrade to the existing electronics. We have used the existing Milagro data and simulations to verify these calculations. HAWC will enable very high energy gamma-ray studies that are unattainable with the current suite of instruments. HAWC will map the Galactic diffuse gamma-ray emission above 1 TeV and thereby measure the cosmic-ray flux and spectrum throughout the Galaxy. This map will allow us to look for regions of strong emission above that expected from correlations with matter: a signature of cosmic-ray acceleration. HAWC with its improved angular and energy resolution plus enhanced background rejection will discover the highest energy gamma-ray sources in the Galaxy. Milagro has already observed gamma rays from one source, MGROJ1908+06, above 100 TeV. Measurement of high-energy spectra will allow us to determine whether these sources are also sources of the galactic cosmic rays. HAWC will perform an unbiased sky survey with a detection threshold of ~30 mCrab in two years, enabling the monitoring of known sources and the discovery of new classes of diffuse and point-like TeV gamma ray sources. HAWC, in one year, will be more sensitive at energies above ~6 TeV in its entire field of view than imaging Air Cherenkov Telescopes with 50 hours of observation on a point source. With the sensitivity to detect a flux of 5 times that of the Crab in just 10 minutes over the entire overhead sky, HAWC will observe AGN flares that are unobservable by other instruments, including TeV orphan flares. Multi-wavelength observations of AGN flares from radio to TeV probe the environment up to within a few hundred AU of the super-massive black hole constraining models of gamma ray production and acceleration of charged particles. Low energy sensitivity and continuous operation are unique and essential to measure the prompt emission from gamma-ray bursts. HAWC can detect GRBs out to z~1 if, as predicted, their TeV fluence is comparable to their keV fluence, while for closer GRBs much lower fluences can be detected. If Fermi sees a single GRB photon above 100 GeV, HAWC will see hundreds, revealing the high energy behavior of GRBs and allowing us to probe the bulk Lorentz factor and size of the emitting region. Numerical Relativity (Laguna, Shoemaker)
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