A method of mass measurement in black hole binaries using timing and high-resolution X-ray spectroscopy

In X-ray binaries, several percent of the compact object luminosity is intercepted by the surface of the normal companion and reradiated through Compton reflection and the K fluorescence. This reflected emission follows the variability of the compact object with a delay approximately equal to the orbital radius divided by the speed of light. This provides the possibility of measuring the orbital radius and thus substantially refining the compact object mass determination compared to using optical data alone. We demonstrate that it may be feasible to measure the time delay between the direct and reflected emission using cross-correlation of the light curves observed near the K alpha line and above the K edge of neutral iron. In the case of Cygnus X-1, the time delay measurement is feasible with a 3 x 10(5)-10(6) s observation by a telescope with a 1000 cm(2) effective area near 6.4 keV and with a similar to 5 eV energy resolution. With longer exposures, it may be possible to obtain mass constraints even if an X-ray source in the binary system lacks an optical counterpart.