Physics
Scientific paper
Apr 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aps..aprk20079s&link_type=abstract
American Physical Society, April Meeting, April 28 - May 1, 2001 Washington, DC Bulletin of the American Physical Society, Vol.
Physics
Scientific paper
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 ~ 13 keV, and is seen preferentially at times when a ~ 300 Hz QPO is observed predominantly in the soft X-ray range. We show that these two QPO 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. The ~ 7 M_solar mass implies an innermost stable circular orbit (ISCO), assuming a non-rotating hole, of 64 km. The radius of an orbit with a frequency of 450 Hz is significantly less than this ( ~ 49 km), indicating that if the modulation is caused by Kepler motion, the black hole must have appreciable spin. 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.
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