Discovery of a second high-frequency quasi-periodic oscillation from the microquasar GRS 1915+105

We report the discovery in archival Rossi X-Ray Timing Explorer data of a s imilar to 40 Hz quasi-periodic oscillation (QPO) in the hard X-ray flux fro m the Galactic microquasar GRS 1915+105. The QPO is detected only in the ha rd X-ray band above similar to 13 keV and was discovered in observations in which the previously known 67 Hz QPO is present. The 40 Hz QPO has a typic al rms amplitude of similar to2% in the 13-27 keV band and a width of simil ar to5.5 Hz (FWHM). We show that the 67 and 40 Hz QPOs are detected in the same observations in 1997 July and November. However, the QPO is not detect ed in observations from 1996 April, May, and June in which the 67 Hz QPO wa s first discovered. The frequency of the 67 Hz QPO is significantly higher in the 1997 observations by about 5% compared with the 1996 data. The ident ification of the 40 Hz QPO makes GRS 1915+105 the second black hole binary to show a pair of simultaneous high-frequency QPOs (the other being GRO J16 55-40). The similarities between the properties of the 67 Hz QPO in GRS 191 5+105 and the recently discovered 450 Hz QPO in GRO J1655-40 suggest that t he pairs of frequencies in these systems may be produced by the same physic al mechanism, with the frequency differences between the two being likely d ue to different black hole masses in the two systems. We discuss the implic ations of our result for the mass and spin of GRS 1915+105 as well as for m odels of X-ray variability in black holes and neutron stars.