Seminar given at: the Kavli Institute for Theoretical Physics program on accretion disks on January 10 at 2pm, at the Harvard/Smithsonian Center for Astrophysics on January 12 at 11am, and at MIT on January 13 at 2pm.
X-ray spectral-timing is a new field that seeks to investigate how matter behaves in strong gravitational fields. Observations suggest that different types of quasi-periodic oscillations (QPOs) are associated with different emitting-region geometries (e.g. disk-like or jet-like) in the innermost part of the X-ray binary, close to the neutron star or black hole. We developed a technique for phase-resolved spectroscopy of QPOs, and are applying it to Type B and Type C low-frequency QPOs from the black hole X-ray binary GX 339-4. On the QPO time-scale, we find that the energy spectrum changes not only in normalization, but also in spectral shape. We can quantify how the spectral shape changes as a function of QPO phase, and the two different QPOs show markedly different spectral changes. In our previous work, we inferred that the Type B QPO could be caused by a large-scale-height (i.e., jet-like) precessing region illuminating and heating overlapping azimuthal regions of the inner accretion disk. Preliminary results of the Type C QPO indicate that a small-scale-height (disk-like) precessing region may be responsible for the observed spectral changes.