A short, but detailed, summary of my past research is given below. To see a broader overview of my research philosophy and what I plan to do in the near future, see my research statement & interests page.

There are many ways to study accretion disks -- I choose to concentrate on the rapid variability seen in compact object systems. Rapid variability tends to go hand-in-hand with higher energy processes deeper inside the gravitational potential well. I also prefer to work in the UV/optical, since this is such a rich region for analysis, especially the velocity information in the emission lines. My tools are thus UV/optical spectroscopy, high-speed photometry, and time series analysis techniques.

Highlights of some of my research are summarized below. The numbers in brackets [#] refer to specific published papers.

Cataclysmic Variables:

AE Aqr: AE Aqr is a remarkable CV, with emission spanning from the radio to TeV gamma-rays [c21]. My work (i) elucidated the origin of the 33s pulsations [4,6]; (ii) scrutinized the mass-donor star with emphasis on using its rotation and orbital velocities to measure the mass of the white dwarf (this technique can also be used to measure the masses of the compact objects in XRBs) [9]; (iii) provided observational motivation and tests for the new ``propeller'' (mass ejector) paradigm of this binary [17].

WZ Sge: The discovery of Ly-alpha absorption in the pulsation spectrum favors the white dwarf pulsation model over the magnetic rotator (intermediate polar) hypothesis [16]. As the prototype of a class of CVs, WZ Sge plays an important role in testing CV evolution and accretion disk instability models. [21, 13, c13]

DQ Her: The determination of a low-mass white dwarf (0.6 M) in this slow nova provided an important test of thermonuclear runaway models for nova eruptions [5].

Flickering in CVs: Rapid, aperiodic fluctuations in luminosity are present in all accretion-powered systems, but its origin is unknown. An investigation using a cellular automaton gave promising results [14]. A spatially resolved image of the flickering was produced through the ``eclipse mapping'' MEM image reconstruction technique [c8]. The spectrum of flickering was extracted from high-speed HST spectrophometry [c5,c17] and shows that the flickering consists of optically thin gas and therefore cannot be described or produced by a standard disk model [c10]. Furthermore, the spectrum of the flickering is timescale-dependent -- the faster flickering has a different spectrum than the slower flickering [c11].

X-Ray Binaries:

XTE J0421+560 (CI Cam): This optically bright and unusually fast X-ray transient went into outburst in March 1998, producing strong radio emission and relativistic jets [c16]. Very high resolution observations of CI Cam reveal an enormously rich emission-line spectrum, with at least 3 thermally and kinematically distinct regions [26,,27,c19]. The extremely strong Fe II emission will be useful as a template for modeling the Fe II emission in AGN. We should soon be able to measure the mass function, helping to determine whether the compact object is black hole or a neutron star. We have been awarded HST time to further study this peculiar beast.

Ellipsoidal Variations: Photometry of Aql X-1 (a neutron star binary [25,c22]), GS2000+25 and J0422+32 (black hole binaries) is being analyzed to measure the photometric variations from the tidally distorted companion star. These variations allow us to refine the orbital periods and inclinations, and with the mass function we better constrain the mass of the central objects.

We are also engaged in long-term synoptic observations of soft X-ray transients (SXTs) during their outbursts using the new 9-m Hobby-Eberly Telescope. In particular, we will watch the accretion disk evolve throughout the outburst and use these observations to constrain ADAF models for the accretion flow.

Active Galactic Nuclei:

Echo Mapping: The broad-line region (BLR) 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 is used to map out the geometry of the BLR gas. Through echo (or ``reverberation'') mapping we were able to rule out a spherical isotropically-emitting BLR in NGC 5548 (a Seyfert 1) [1]; the emission is probably highly anisotropic (radiating back toward the source) because the BLR clouds are optically thick [3].

Echo images of the BLR in AGN: One-dimensional echo map solutions are highly degenerate, e.g., a spherical inflow appears identical to a spherical outflow. Our pedagogical paper [2] showed that the echo mapping technique can be made far more powerful by including line profile information. Illustrative examples of 2-d echo images were presented, showing how one can to solve for both the geometry and flow in the BLR. From the BLR kinematics we can measure the mass of the central object. I am the P.I. of ``HEMP'', a long-term, large-scale project to echo map AGN using the HET.

AGN variability: Very accurate HST spectrophotometric observations of NGC 7469 showed a 4% increase in flux over the span of 10 hours. However, no evidence for ultra-rapid variability was seen: no microvariability flares, no characteristic timescales, and no periodicities were present [18, c15]. A study of the cross-correlation function, the standard tool used to measure time delays, revealed that AGN time lag measurements are both biased and contain a very large variance, throwing into question whether the observed year-to-year changes in lag seen in some AGN are real [23].

And finally, I am a member of the ``Kronos'' project, a proposed MIDEX-class satellite to probe black holes and accretion processes in AGN, X-ray binaries and cataclysmic variables (B.M. Peterson, Ohio State Univ., is the P.I.).

Some of my near-term research goals include precisely measuring the masses of compact objects (from white dwarfs to supermassive black holes), understanding why all accretion disks exhibit rapid ``flickering'' and how this relates to accretion disk viscosity, testing advection-dominated accretion flow models (the current paradigm for accretion onto stellar-mass black holes), and ``echo mapping'' of the centers of AGN. For more information, see my research statement page.

Back to my research page.
This page was last updated on 2000 Jan 10