My main research interest in recent years has been is how the central engines of active galaxies work. I observe them in X-rays to probe the hottest parts of the accretion flow onto the supermassive black hole, and in the optical to study the line emitting gas, which may be the fuel for the central engine. I mostly observe radio-loud AGNs with one more goal in mind: getting clues to the process of formation of their relativistic jets. All the details are described in the AGN research page.
As in AGNs, I am interested in the accretion flows of cataclysmic variables. I have studied their X-ray emission to investigate how matter is deposited onto the surface of the white dwarf and their UV and optical spectra to probe the dynamics of the accretion flow. More details of these projects can be found in the dedicated cataclysmic variable research page.
My student Mike Sipior is modeling the population of interacting binaries in starburst galaxies. These are mostly HMXBs,with neutron star or black hole primaries, whose X-ray emission makes up the integrated X-ray luminosity of the Starburst galaxy. Our goal is to predict the population of X-ray binaries in a starburst galaxy and its X-ray luminosity function, which can then be tested using high-resolution X-ray images taken with Chandra. This project has unexpectedly turned out to have a close relation to my work on the X-ray properties of LINERs, which is described in the AGN research page. My original goal was to identify their central engines by observing them with Chandra, but I have found that their host galaxies are full of X-ray binaries.
I am involved in a systematic effort to identify some of the bright, high-energy gamma-ray source,s originally found by the EGRET instrument on the Compton Gamma-Ray Observatory. About 2/3 of these EGRET sources remain unidentified. We are focusing on high-galactic attitude sources which are likely extragalactic and may constitute a previously unrecognized population of gamma-ray blazars. I am also participating in various efforts to identify the optical counterparts of transient and persistent X-ray sources, such as those detected by the BeppoSAX wide-field camera in the Galactic center, or by repeated observations with ROSAT. My role in the above projects is to carry out optical spectroscopy of candidates with the Hobby-Eberly Telescope or the KPNO 2.1m telescope.
I am leading Penn State's observational effort to obtain spectroscopic redshifts of gamma-ray burst afterglows with the Hobby-Eberly Telescope. This is part of a larger effort in collaboration with the other Hobby-Eberly Telescope partner institutions. I have instituted an alert system by which our team members can be notified immediately about bursts detected by space-based observatories. Once an optical counterpart is identified in images taken with smaller telescopes by other groups, our group steps in to take spectra. So far we have been able to take spectra of two bursts: GRB991216 and GRB000301C.
Richard Wade and I have carried out a survey of UV-excess stars found by Howard Lanning in his Two-Color (U, B) Survey of the Galactic Plane, using plates taken by A. Sandage. The aim of this project was to identify the types of stars that make up the population of UV-excess objects. We took moderate-resolution spectra of a large number of Lanning stars with the Hobby-Eberly Telescope and the KPNO 2.1m telescope