Accretion power is derived from the release of gravitational potential energy. For compact objects the efficiency of accretion can exceed nuclear fusion by a factor of 60! It is for this reason that accretion-powered sources are so highly energetic and important. Furthermore, the only way to study black holes is through their interactions with the accretion flow around them. Accretion disks are believed to be essential components of a host of astrophysical sources, from protostellar objects to AGN. Although ubiquitous and energetically dominant in the universe, accretion processes are still poorly understood.

My research is focused on accretion onto compact objects. These are the white dwarfs in cataclysmic variables (CVs), the neutron stars and black holes in X-ray binaries, and the supermassive black holes in AGN. I have worked most extensively on the white dwarf accretion disk systems, but have recently shifted emphasis towards the X-ray binaries, and have always been interested in AGN. Although these are very different systems, each provides its own clues to the accretion process. The cross-fertilization between fields has proven to be extremely fruitful to the understanding of accretion astrophysics.

I am proud of the breadth, originality, and technical prowess of my research. I am not afraid to tackle difficult problems -- I often find the most challenging problems are the most rewarding; thus my research often pushes the envelope of what is observationally possible. Examples include my series of papers on the bizarre cataclysmic variable AE Aquarii, or my investigation into ``2-d echo mapping'' in AGN. Rather than go into details (which I will be happy to discuss at length), I simply state that my research has had two dominant themes: (1) To elucidate the rapid variability. Rapid variability goes hand-in-hand with high energy processes, usually deep inside the gravitational potential well. This variability can be used as a powerful tool to probe the accretion flows, whether it be in an AGN or a white dwarf binary. Furthermore, all accretion-powered sources show aperiodic ``flickering'', but the cause remains a mystery. Is it related to accretion-disk viscosity? To me, the flickering is a fundamental issue in accretion astrophysics; (2) To determine the system parameters, especially the mass of the compact accreting objects. This is the most important parameter of an accretion-powered system, whether it be a white dwarf or supermassive black hole.

I very much enjoy being able to work both as an AGN astronomer and a stellar astronomer -- applying knowledge and skills gained in one field to the other, and keeping the ``big picture'' in mind. I am as equally adept at discussing how to measure rotation in late-type stars as how to measure winds from accretion disks in AGN. I have a diverse range of interests, and I refuse to rigidly confine my scope to any one topic -- there are too many fascinating things to investigate! I expect that as astronomy progresses, so too will my interests grow, dynamically evolving to keep pace in this ``golden age'' of astronomy.

During the 2-1/2 years I've been at UT, I have played a very active role in the department: I have given 2 seminars every semester (stellar and extra-galactic), organized and led the very successful weekly "High-Gravity Journal Club", been heavily involved in graduate and undergraduate research projects, have sponsored 5 visitors to the Department (including a 6-month sabbatical visit), and have been P.I. or co-P.I. on projects that have brought in over half a million dollars in grants.

My efforts have been recognized by the Department: I have been quickly promoted from post-doc to research associate, I have been asked to serve on the telescope Time Allocation Committee, I was appointed a member of the McDonald Observatory Users Committee (long-term planning and steering), and I am in-line for promotion to the position of research scientist this spring.

Some final thoughts on who I am:
I believe rigorous data analysis is essential; you will see error bars on every observationally--derived value I publish. (As my mentor KDH taught me, ``An observation without an error bar is theory.'') Quite often I write my own analysis software since my research is generally original or beyond the normal capabilities of standard analysis packages (e.g. I rely on ``Numerical Recipes'' far more than I do on IRAF).

While I am primarily an UV/optical astronomer, I expect to extend into the X-ray and infrared regimes in the near future. Just as I believe in the wonderful effects of synergy between fields, the same is true for different wavelengths.

Synergy is indeed an essential component of my ethos. I strongly believe in the exchange of information between fields, wavelengths, and most importantly, people. I greatly enjoy collaborating with my colleagues, and I find the interactions both stimulating and fruitful.

I am energetic, creative, and extremely enthusiastic about my research. I have an abundance of novel research ideas, with topics ranging from planet searches to the diffuse X-ray emission in galaxy clusters. Only the finite length of the day prevents me from pursuing all these ideas.

Although I enjoy being a research associate/scientist, I look forward to being a faculty member, where I can more actively contribute to the enrichment and mission of the Department.

Future Research Plans:
My research will continue to focus on observational analyses of accretion-driven systems, striving to better understand the central compact objects and the accretion flow around them. I will concentrate on studying their variability using spectroscopic time series analysis because time-dependent phenomena offer a wealth of information that steady processes do not, and emission lines offer crucial velocity information.

Over the years I have enjoyed many successful collaborations and have benefited from the expertise, motivation and friendships that these collaborations have fostered. I hope to continue to work with my colleagues, particularly those at the University of Texas, where I am involved in several large projects involving the 9-m Hobby-Eberly Telescope (HET). I am also involved with ``Kronos'', a proposed Explorer-class satellite with simultaneous X-ray, UV and optical spectroscopic instrumentation; Kronos is dedicated to long timescale observations of variability in AGN, soft X-ray transients and CVs.

Below is a representative sample of my near-future research goals. This gives a flavor of some of my research interests, but this synopsis is not all-inclusive - I have several long-term interests that are not described, e.g., astrobiology.

AGN Variability and Echo Mapping:
The broad-line region (BLR) in AGN consists of gas surrounding the supermassive black hole at a distance of 10²-10⁴ gravitational radii (1-200 light days). The BLR is photoionized by the hard continuum produced by the central engine. The emission lines from the BLR respond to variations in the continuum emission, but with a light-travel time delay. This delay can be used to map out the geometry of the BLR gas. I am the P.I. of the HET Echo Mapping Project, a long-term program whose goal is to measure the geometry and motion of gas in the BLR at microarcsecond scales. We will be able to determine a dynamical mass for the black hole, the mass-accretion rate, and the kinematics of the flow in both low- and high-luminosity AGN.

X-Ray Binaries:
X-ray binary star systems are the best way to study black holes. In particular, the Soft X-ray Transients (or X-ray nova) are known to contain accreting black holes. Through spectroscopy one can measure the mass function (via emission and absorption line radial velocities), and through photometry one can measure the inclination of the binary (via ``ellipsoidal variations'' - photometric variations from the tidally distorted companion star as a function of orbital phase). Combining the two gives a precise mass for the black hole. Spectroscopy also allows one to study the accretion flow via the Doppler-broadened emission lines. In addition to this line of research, E.L. Robinson (U. Texas) and I have started a project to use the queue-scheduled HET for long-term synoptic observations of outbursts of SXTs, following the system from the initial rise to late decline (months later). This unique set of observations will strongly test the currently fashionable advection flow models for accretion onto stellar-mass black holes.

Cataclysmic Variables:
Flickering: Rapid, aperiodic fluctuations in luminosity are present in all accretion-powered systems, thus it is a key element of the physics of accretion. However, its origin is unknown. I will continue to work on several projects that have begun to show interesting results, namely: (i) I have been able to obtain a spatially resolved image of the flickering source through the ``eclipse mapping'' technique; (ii) I have extracted the spectrum of flickering, showing it consists of optically thin gas and therefore cannot be described or produced by a standard thick disk model; (iii) I have shown that in some cases the spectrum of the flickering is timescale-dependent; the faster flickering has a different spectrum than the slower flickering. Also, I want to pursue a new and very exciting idea: Using a large telescope one can study rapid emission-line profile changes. If small substructures can be tracked across the emission line (on a timescale of seconds to minutes), then a direct dynamical measure of the mass of the central object can be made. This exciting and powerful technique will also work on X-ray binaries, but it needs to be tested on the brighter and longer-timescale CVs.
DNOs: The dwarf nova oscillations (DNOs) are enigmatic, rapid (2-35 s) sinusoidal variations seen in the light curves of CVs undergoing dwarf nova outbursts. The periods and amplitudes of the DNOs are luminosity-dependent, varying as the disk brightens and fades through eruption. DNOs are high energy events, and may be the low-mass analogues of the kHz QPOs seen in the X-ray binaries. I have excellent ground-based high-speed spectroscopy in hand and am also in a collaboration with simultaneous XTE+EUVE+HST observations of DNOs.
White dwarf masses: White dwarf mass determination continue to play a critical role in testing thermonuclear runaway models in novae and in the progenitors of Type Ia supernovae. In particular, I will attempt to get accurate mass determinations for the so-called ``neon novae''; theory requires that they are very massive. No neon nova white dwarf mass has been measured because this is a difficult task. The techniques used to measure the white dwarf masses in the CV binaries will immediately be applicable to the fainter and more complicated black hole and neutron star binaries.

Teaching Philosophy:

I strongly believe that I can be an effective and motivating instructor. This belief is based on my personal enjoyment of teaching and the understanding that being a good researcher does not make one a good teacher - one must craft each lecture and put an effort into learning the skills required for effective teaching. I have not had the opportunity to teach a class, but I do take pride in the quality of the instruction that I have given: I tutored math as an undergraduate; I gave lectures, created and graded exams as a graduate student T.A.; I have worked with several graduate students and closely supervised one graduate student's dissertation research; and I am currently supervising an undergraduate research project. I find the experience enjoyable and interesting - sometimes frustrating, sometimes highly rewarding. I look forward to formal teaching, especially something like an introductory astronomy class for non-majors, because I enjoy the challenge of motivating students to do their best.

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This page was last updated on 1999 Nov 3