Professors Howes, Kaaret, Lang, McEntaffer, Gayley, Mutel
Students: Quentin Roper, Tom Brantseg
High energy astrophysics at Iowa includes the study black holes, neutron stars, supernova remnants, the interstellar medium, colliding winds from massive stars, the nuclear region of the Milky Way, and ultra-high energy neutrinos and spans an energy range from 100 eV to 1022 eV including the study of X-rays, gamma-rays, and neutrinos. Iowa faculty are frequent users of NASA observatories such as Chandra and Swift in X-rays and Hubble in the optical. The time from idea to publication in observing projects is typically one to two years and Iowa students often participate in and sometimes lead such projects. Iowa faculty also build X-ray instrumentation for launch on sounding rockets and satellites, specifically the GEMS X-ray polarimetry mission. Students are involved in these projects (make link to instrumentation page). UI is a member of the VERITAS gamma-ray observatory and a frequent user of the radio Very Large Array including a novel application of the VLA for neutrino detection.
Black holes and neutron stars
The concentrated release of energy near accreting black holes and neutron stars (compact objects) produces X-rays via thermal emission from gas at temperatures of millions to tens of millions of degrees Kelvin and non-thermal emission from highly energetic particles. Key research goals include measuring the fundamental parameters, mass and spin, of compact objects and understanding the dynamics of accretion of matter onto compact objects. Kaaret's current work concerns the highly luminous X-ray sources in nearby galaxies which have been interpreted as being intermediate-mass or "medium-sized" black holes, jet ejection from black holes, and the polarization of X-ray emission from black holes and neutron stars. Professor McEntaffer is studying how soft X-ray emission and absorption lines can help understand the dynamics of accretion flows around neutron stars and the origin of the X-ray emission. Professor Howes employs high-performance numerical computations to determine the heating of the plasma comprising the black hole accretion disk due to the dissipation of turbulence, a key physicalmechanism governing the emitted radiation that is observed at Earth.
Supernova remnants and the interstellar medium
Supernova blast waves heat gas in the surrounding interstellar medium to millions of Kelvin and accelerate particles up to energies of 1015 eV. Understanding supernova remnants is key to understanding the chemical evolution of our Galaxy and the power sources that energize the interstellar medium. McEntaffer studies soft X-ray emission lines that provide detailed information about the interactions of supernova blast waves with moderate density clouds in the spherical cavity shell produced by the precursor stellar wind. Data for these areas of interest are obtained from the Chandra X-ray Observatory and sounding rocket payloads built through collaboration between UI and the University of Colorado.
Viewed from Earth, the soft X-ray sky is dominated by apparently thermal emission that suggests we live in a bubble of million degree gas. However, this putative signature of hot gas could be “faked” by charge exchange of solar wind ions with interstellar neutrals. High resolution soft X-ray spectroscopy done with sounding rokcets offers the best means to find out.
The nuclear region of the Milky Way
Professor Lang has recently completed the first comprehensive high resolution survey of neutral hydrogen in the galactic center region. This is an important study which provides fundamental insights concerning the kinematics of the central region (including the massive black hole at its center), as well as the dynamic of star formation and ionization. She has also studied the puzzling filamentary structures seen in radio maps of the galactic center. Although they are clearly a result of fine-scale magnetic fields, the larger question of how these fields form and what relation they have to the massive black hole at the center, is still unresolved. These studies are of fundamental importance, since the Milky Way galaxy’s central black hole is [by far] the nearest massive black hole available for detailed study.
Ultra-high energy neutrinos
Ultra-high energy (E > 1019 eV) neutrino astronomy is a new window to high-energy astrophysical processes. Potential UHE neutrino sources include Active Galactic Nuclei (AGN) primaries, GZK-induced showers from UHE cosmic rays, Z-bursts from massive primordial remnant particles, and topological defects. For distant ( > 50 Mpc, 1 Mpc = 3 x 1022 m) sources, neutrinos are the only way to probe physical processes at UHE energies. This is because cosmic ray primaries interact strongly with Cosmic Microwave Background photons. The past 20 years have seen numerous experiments aimed at observing these cosmic messengers, however, no attempts have yielded a detection.
Project RESUN (Radio EVLA Search for UHE Neutrinos) utilizes multiple antennas in the Expanded Very Large Array (EVLA) to search the lunar limb for nanosecond-duration Cerenkov radio bursts at 1.4 GHz. These bursts are created when (primarily cosmogenic) UHE neutrinos annihilate in the lunar regolith, producing a particle shower which in turn produces charge currents and consequent Cerenkov radio emission. The EVLA is currently the best ground-based radio array in the world to search for these bursts in this energy range until the completion of the SKA. The RESUN search will either make the first UHE neutrino detections, or will provide a new lower limit to neutrino flux in the important energy range just above the GZK limit (1019.5eV), approximately one order of magnitude lower than previous searches. More details of this search can be found at the RESUN website.
Gayley, G., Mutel, R., and Jaeger, T. 2009, 'Analytic Aperture Calculation and Scaling Laws for Radio Detection of Lunar-Target UHE Neutrinos, ApJ 706, 1156 (doi 10.1088/0004-637X/706/1556).
Gayley, K. 2009, Asymptotic Opening Angles for Colliding-Wind Bow Shocks: The Characteristic-Angle Approximation, Ap. J. 703,89.
Jaeger, T., Mutel, R., and Gayley, 2009, 'UHE neutrino searches using a Lunar target: First Results from the RESUN search, submitted to MNRAS.
Kaaret, P. and Feng, H. 2009, X-ray Monitoring of Ultraluminous X-ray Sources
Kaaret, P., Feng, H. and Gorski, M. 2009, A Major X-Ray Outburst From an Ultraluminous X-Ray Source in M82
Lang, C., Kaaret, P. , Corbel, S. and Mercer, A. 2007, Radio Nebula Surrounding the Ultraluminous X-Ray Source in NGC 5408, Ap. J. 666, 79.