We report the discovery with the proportional counter array ( PCA ) onboard the Rossi X-ray Timing Explorer ( RXTE ) of a 450 Hz quasiperiodic oscillation ( QPO ) in the 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 \sim 13 keV .
It is both strong and narrow , with a typical rms amplitude of 4.5 % in the 13 - 27 keV range , and a width of \sim 40 Hz ( FWHM ) .
For two observations in which we detect the 450 Hz QPO a previously known \sim 300 Hz QPO is also observed in the 2 - 13 keV band .
We show that these two QPO sometimes appear simultaneously , thus demonstrating the first detection of a pair of high frequency QPO in a black hole system .
Prior to this , pairs of high frequency QPO have only been detected in neutron star systems .
GRO J1655-40 is one of only a handful of black hole systems with a good dynamical mass constraint .
For a non-rotating black hole with mass between 5.5 - 7.9 M _ { \odot } the innermost stable circular orbit ( ISCO ) ranges from 45 - 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 Kepler 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 range the dimensionless angular momentum lies between 0.15 < j < 0.5 .
Moreover , if the modulation is caused by oscillation modes in the disk or Lense-Thirring precession , then this would also require a rapidly rotating hole .
We briefly discuss the implications of our findings for models of X-ray variability in black holes and neutron stars .