X-ray Point Source Populations
The X-ray point source populations of nearby galaxies consist almost
entirely of accreting X-ray binary stars and supernova remnants
(SNRs). Thanks to the superb spatial resolution and sensitivity of
the Chandra X-ray
Observatory, we can now actually study these objects
in detail in galaxies beyond our own Galactic neighborhood (the Milky
Way and its neighbors, called the Local Group). X-ray binaries are
classified by the mass of the star that is accreting matter onto the
compact object as either high-mass or low-mass (HMXBs and LMXBs).
These are further classified by the type of compact object as either
black holes or neutron stars.
Since the mass-donors in HMXB systems are short-lived high-mass stars,
they are accurate tracers of active star formation within a galaxy.
Conversely, LMXBs are associated with longer lived
progenitor stars, and are thus an accurate tracer of the total stellar
mass in a galaxy. The X-ray bright SNRs are an independent tracer of
However, the X-ray data alone is not enough to classify the majority
of sources that we see. We must therefore rely on other wavelengths:
optical to find stellar counterparts, radio to find SNRs, IR as a
tracer of dust and of star-formation, UV as a tracer of mass, etc.
Combining all these data provides the complete picture of X-ray sources in
galaxies, and what they can tell us about the star-formation histories
of those galaxies.
Other Research Interests
My primary research interest beyond X-ray populations is in deep X-ray
surveys and, in particular, the normal galaxy contribution to the
X-ray background. As we probe to ever fainter fluxes with X-ray
telescopes, we are rapidly approaching the point where most of the
objects detected will be normal (non-AGN) galaxies. It is important
to understand these galaxies not only nearby, but at higher redshift,
in order to completely understand the evolution of galaxies and their
I am also interested in statistical challenges in X-ray astronomy.
The high quality of modern X-ray telescopes pushes the limit of our
understanding of small number statistics. Cooperative work between
statisticians and astronomers is beginning to introduce new techniques
in source detection, spectral fitting, and other unique methods of
For students who may be interested in getting involved in research, here are some projects I'm currently working on:
- Ultraluminous X-ray Sources: or ULXs, are X-ray sources whose luminosities exceed the Eddington limit for mass accretion onto a typical stellar-mass black hole. That is to say, they get too bright to be explained by comparison with typical sources in the Milky Way. It is becoming clear that many of these sources are probably extremely massive stellar-mass black holes: the most massive black holes that can be created from a single parent star. However, some of these sources are too luminous even for that explanation. Those sources may be intermediate-mass black holes, with masses between 100 and 10,000 times that of the Sun. They may represent the "missing link" between stellar-mass and supermassive black holes.
- Diffuse X-ray emission in galaxies: Though much of the X-ray emission from nearby spiral galaxies comes from the discrete point sources, there is also diffuse X-ray emission from hot gas. This gas is largely the result of supernovae. Studying the elemental abundances in this gas can teach us about the progenitor stars that produced the supernovae, and studying the distribution and intensity of the diffuse X-ray emission provides an independent measure of star-formation distribution in the galaxies.
- Multiwavelength observations of dwarf galaxies: I am working on multiwavelengh analyses of two very different Milky Way satellite galaxies. The Fornax Dwarf Spheroidal galaxy is the miniature version of an elliptical galaxy, with little on-going star formation. IC 10 is a dwarf starburst, with very active star formation and a large population of Wolf-Rayet stars. Contrasting these two opposite ends of the star-formation spectrum can provide many insights into the process of star formation.
- X-ray source stacking: In X-ray observations, just because a source isn't detected, that doesn't mean it isn't really there. By stacking the signal at the positions of lots of sources detected in another wavelength (for example, starburst galaxies detected in the infrared), it is possible to detect some flux from these sources and even determine some properties of the sources.
- X-ray binary variability: Most Galactic HMXBs do not have well-established periods and little is known about the long-term optical variability of these systems. Using the 24-inch Perkin telescope here at Wesleyan, it is possible to monitor bright Galactic HMXBs to sample this important parameter space.
My Papers on ADS