Research

• High-Energy Astrophysics
• Black Hole and Galaxy Evolution
• High Energy Neutrino Astrophysics with IceCube
• High Energy Gamma-ray Astrophysics with HAWC
• Numerical Relativity
• Gravitational Wave Astrophysics

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.

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.

Neutrinos are subatomic particles associated with radioactive decays. Neutrinos have no electric charge, and neutrinos hardly ever interact with matter. Neutrinos travel through walls, planets or across the Universe without difficulty. This ghost-like behavior of neutrinos is both their advantage and disadvantage. Because they travel unimpeded from their source, neutrinos can allow us to peer inside astronomical objects where no light could escape. But a very large detector is needed in order to detect just a handful of them. IceCube, a gigaton scale detector operating at the geographical South Pole is searching for sources of high energy (>10¹²eV) neutrinos. In this search IceCube may find the sources of cosmic rays, that have been unknown for 100 years; it may observe dark matter; it will provide information about neutrino oscillations and it will provide detailed information about neutrino emission from the next supernova in our own galaxy.

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.

Gamma-rays are high energy electromagnetic radiation invisible to the human eye. The past 15 years have seen an explosion in the number of known sources of high energy (>10¹¹ eV) gamma-rays. As of late 2011, approximately 120 confirmed sources are known. High energy gamma rays probe the most extreme astrophysical environments including those that produce the highest energy cosmic-ray particles. The Milagro observatory discovered several of these sources and demonstrated that the air shower array method is viable. As compared to the other detection technique (air Cherenkov telescopes or ACT, such as Veritas), air shower arrays have the advantage of very wide field of view, (2 sr vs. a few degrees for ACTs) and nearly 100% duty cycle (vs. 10% for ACTs). Air shower arrays excel at unbiased surveys of the sky and in searching for transient sources such as Gamma Ray Bursts (GRBs) and Active Galactic Nuclei (AGNs).

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

In a paper recently published by Physical Review Letters (Editor's Suggestion), Georgia Tech researches James Healy and Deirdre Shoemaker teamed up with Janna Levin from Barnard College and Columbia University to investigate zoom-whirl behavior in the coalescence of two black holes. Zoom-whirl behavior is most often seen in binaries where one of the components is significantly larger than the other. The smaller component will whirl around its big partner before zooming away and repeating the process. They found that despite the dissipative nature of more equal-mass mergers, the zoom-whirl behavior stilled played out. In fact, the paper showed that the black holes merged during whirly phases before the system could circularize. In the figure shown here, the gravitational radiation is plotted as a function of the simulation time. The bursts of radiation are due to the whirling phases of the binary before they finally merge.

Can LIGO “hear” different black-hole spin orientations in bursts of gravitational waves?

When two black holes collide in the universe, the resulting gravitational radiation carries an imprint of their component masses, spin magnitudes and spin direction (each black hole can spin around its axis like a top). At the very end of the collision, the black holes produce a strong burst of gravitational radiation which ground-based gravitational wave observatories like the NSF funded LIGO may detect. It is almost like LIGO hears the gravitational waves from these cataclysmic events. Laura Cadonati, Satyanarayan Mohapatra (U. of Massachusetts-Amherst), Sebastian Fischetti (UCSB), James Healy, Lionel London and Deirdre Shoemaker (GT) just completed an investigation into how well one particular burst algorithm (Omega) for LIGO can differentiate the orientation of one of the black-hole spins with the last burst of gravitational radiation from the merger. The figure shows one of the results from the paper submitted to Physics Review D, plotting the distance reach of the detector versus the angle the spin vector makes with the orbital angular momentum (blue). The result is that LIGO can see spins aligned with the orbital angular momentum further away from the Earth than those unaligned for total mass of the binary greater than 80 times that of the sun. The figure also plots the total radiated angular momentum (red) of the black holes at each orientation. It illustrates that the radiated angular momentum can act as a guide to the response of the prediction reach of the detector.