Discovery of a 450 Hz quasi-periodic oscillation from the microquasar GRO J1655-40 with the Rossi X-Ray Timing Explorer

We report the discovery with the Proportional Counter Array on board the Ro ssi X-Ray Timing Explorer of a 450 Hz quasi-periodic oscillation (QPO) in t he hard X-ray flux from the Galactic microquasar GRO J1655-40. This is the highest frequency QPO modulation seen to date from a black hole. The QPO is detected only in the hard X-ray band above similar to 13 keV. It is both s trong and narrow, with a typical rms amplitude of 4.5% in the 13-27 keV ran ge and a width of similar to 40 Hz (FWHM). For two observations in which we detect the 450 Hz QPO, a previously known similar to 300 Hz QPO is also ob served in the 2-13 keV band. We show that these two QPOs sometimes appear s imultaneously, thus demonstrating the first detection of a pair of high-fre quency QPOs in a black hole system. Prior to this, pairs of high-frequency QPOs have been detected only in neutron star systems. GRO J1655-40 is one o f only a handful of black hole systems with a good dynamical mass constrain t. For a nonrotating black hole with mass between 5.5 and 7.9 M., the inner most stable circular orbit (ISCO) ranges from 45 to 70 km. For any mass in this range the radius at which the orbital frequency reaches 450 Hz is less than the ISCO radius, indicating that, if the modulation is caused by Kepl er motion, the black hole must have appreciable spin. If the QPO frequency is set by the orbital frequency of matter at the ISCO, then for this mass r ange the dimensionless angular momentum lies in the range 0.15 < j < 0.5. M oreover, if the modulation is caused by oscillation modes in the disk or Le nse-Thirring precession, then this would also require a rapidly rotating ho le. We briefly discuss the implications of our findings for models of X-ray variability in black holes and neutron stars.