Research

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. In addition to nuclear reactors and man-made accelerators, neutrinos have also been detected from Sun and, during a brief 10 seconds, from a supernova (the explosion of a dying, massive star) near our own galaxy in 1987. Unlike protons and electrons, neutrinos have no electric charge, and neutrinos hardly ever interact with matter. Indeed, a neutrino produced by the Sun could travel more than one light year in water before stopping. 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. It is generally believed that a kilometer-scale detector is needed to search the skies for neutrino sources beyond our Sun.

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.

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 (>100 GeV) gamma-rays. As of spring 2009, approximately 80 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 has demonstrated that a detector with a wide field of view (2sr) and nearly 100% duty cycle can discover new sources of TeV gamma rays, and map the diffuse emission from the plane of our Galaxy. The HAWC (High Altitude Water Cherenkov) observatory builds on the experience and technology of Milagro to make a second-generation high-sensitivity detector. This unique detector will be capable of continuously surveying the TeV sky for steady and transient sources from 100 GeV to 100 TeV.

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 is a field of gravitational physics that uses numerical techniques to solve the Einstein field equations for unknown spacetimes. Because of the complexity of the Einstein equations, very few exact solutions are known, and numerical techniques are often the only avenue for exploring general relativity. One of the cornerstone problems in numerical relativity is the binary black hole problem (BBH). BBHs that are coalescing due to the emission of gravitational radiation are of interest as a fundamental solution to the Einstein equations and as an important phenomenon in the Universe. In one sense, the BBH problem is an elegantly simple two-body problem in general relativity with no matter terms involved. However, the black-hole singularities have made this problem formidable, taking many years to solve until just recently breakthroughs in the formulation, gauges and computational algorithms have succeeded (Pretorius, Campanelli et al and Centrella et al). Now that many of the technical difficulties have been resolved, the field is moving forward rapidly, tackling a variety of key problems. Recently, a lot of excitment has been generated in some of the results from the collision of two black holes. If black holes collide under assymetric conditions, the final black hole gets a kick, or gravitaitonal recoil. The magnitide and direction of the black-hole kick has plays an important role in understanding on how large-scale structures in the Universe evolved. BBH systems also play an important role in gravitational wave astrophysics as discussed below.

Gravitational Wave Astrophysics (Laguna, Shoemaker)

Gravitational physics is about to become observationally driven with facilities such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) having reached their initial design sensitivity and conducting science runs and the future ESA/NASA mission LISA. Fundamental to the success of the search for gravitational waves is the theoretical modeling of the sources that produce them. One of those sources is the system of binary black holes mentioned above. Systems of BBH of interest to LIGO and LISA span a large parameter space consisting of orbital configurations, masses and spins among others. While analytic techniques exist for the early and late stages of the BBH coalescence, the merger portion requires numerical solutions that can be computationally very demanding. There is a wide erray of active research in using the information we are obtaining in numerical relativity to aid in gravitational wave detection including 1) testing template banks currently used to search for signals in the data stream 2) creating phenomenological or hybrid templates based on numerical relativity waveforms and 3) using the waveforms directly in searches.