The Soft X-Ray Transients are short-period, X-ray binary stars in which a compact star - either a neutron star or a black hole - is stripping matter from a relatively-normal companion star. The stripped matter spirals down to the compact star, releasing gravitational potential energy and forming a hot accretion disk. It is this superheated accretion disk that is the source of the X-ray emission. Such bursts of X-ray emission may last a few months and can reach extraordinary luminosities, 100,000 times the sun's luminosity. The bursts are caused by an increase in the flow of mass through the disk. The X-ray luminosity between outbursts is low, so the quiescent binary system is undistorted by strong X-ray irradiation, allowing reliable measurements of system properties. The high proportion of black holes among the short-period X-ray binaries, their variable accretion flows, and their low X-ray luminosity between outbursts make them superb laboratories for studying black holes and black hole accretion. Although they are generally discovered at X-ray wavelengths, disentangling their properties requires observations at optical as well as X-ray wavelengths. The known binaries are, however, all faint at optical wavelengths during quiescence and, thus, have been slow to yield their secrets.
This project will measure the dynamical masses of the x-ray binary black holes, measure the structure and evolution of the accretion disk and other accretion flows by Doppler Tomography of the emission lines, and will take advantage of the queue-scheduled operating mode of the 8-meter Hobby-Eberly Telescope of McDonald Observatory to obtain synoptic measurements of the optical spectral energy distribution of any northern x-ray transient that goes into outburst, mapping their full outburst behavior for the first time.
