Astrophysics Research at UE

Radiation Signatures of Relativistic Outflows.   

From a simulation of electron confinement within a tangled magnetic field (O'Brien, 2005). Spheres represent the electron's trajectory and the color gradient corresponds to the temporal evolution.

Relativistic outflows (often called “jets”) are some of the most interesting environments in the astrophysical inventory. They contain streams of particles that travel at speeds near the speed of light, and produce tremendous amounts of radiation over a wide range of frequencies. Though they are associated with all sorts of intriguing astrophysical objects, we are particularly interested in studying the jets that emanate from a class of objects called “microquasars.” Microquasars are binary systems in which a compact object (a stellar mass black hole or a neutron star) accretes matter from its companion. This process forms an accretion disk around the compact object, and jets flowing (roughly) along the object's spin axis. The study of these jets progresses by modeling the radiation signatures of a plausible range of physical processes thought to exist in the jet environment. These models are then compared and matched to observational data by an iterative process of adjusting physical parameters and then recalculating radiation signatures.

Mosaic image of the microquasar GRS 1915+105 from INTEGRAL satellite observations (Walker, 2005).

Work on this project progresses along two fronts: theoretical modeling and analysis of data from various satellite detectors. The theoretical models currently under development revolve around a simplified picture of a single emission zone, or blob of relativistic leptons. These particles may radiate via their interaction with the jet's magnetic field (synchrotron radiation), or by scattering the ambient photon field to higher energies (inverse Compton radiation). The data analysis investigations have utilized archival data from both the Chandra X-ray observatory, and the Rossi X-ray Timing Explorer (RXTE), and have centered around understanding the operational details of these experiments, and learning how to produce statistically meaningful fits of these data to general theoretical models.

Correlated High Energy Emission in Active Galaxies.   In their 1988 paper, Edelson and Krolik detail the Discrete Correlation Function (DCF), a tool for analyzing temporal correlations in unevenly sampled astrophysical data sets. The DCF is a natural choice for seeking out correlations in active galactic nuclei (AGN) between the gamma-ray emission, and that of other energy bands (such as the X-ray band). In fact, the study of these correlations can provide constraints on models of AGN emission. Thus the DCF is used frequently in studies of gamma-ray fluctuations. However, when applied to gamma-ray data, the results of the DCF algorithm can become difficult to interpret. Our current work has centered on a detailed interpretation of the DCF algorithm for sparsely sampled data (common in gamma-ray observations of AGN). These studies have been conducted on simulated data sets, by postulating (very) simple gamma-ray emission models, constructing appropriate transfer functions, simulating light curves drawn from various energy bands, and then performing the correlation analysis.

C. A. O'Brien, L. M. Boone, & J. D. Hohertz (2006), “Simulating Particle Dynamics in Tangled Magnetic Fields,” 207th Meeting of the American Astronomical Society.

L. M. Boone & T. G. Spears (2004), “Applying the Discrete Correlation Function,” 8th Meeting of the AAS High Energy Astrophysics Division.