RXTEObservations of QPOs in the Black Hole Candidate GRS 1915+105

Abstract

We report on quasi-periodic oscillations (QPOs) in the black hole candidate GRS 1915+105 seen in 31 observations made by the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. We distinguish three different types: a QPO with constant centroid frequency of 67 Hz, a dynamic low-frequency (10-3-10 Hz) QPO with a large variety of amplitudes and widths, and high-amplitude sputters at frequencies of 10-3-10-1 Hz that are among the most extreme X-ray variations ever seen. We discuss the 67 Hz QPO on the assumption that it arises in the inner accretion disk of a black hole binary. If this QPO represents the rotation frequency of the innermost stable orbit around a nonrotating black hole, then the implied mass is 33 M☉. An alternative interpretation of this QPO as g-mode oscillations in the inner disk (Nowak et al. 1996) implies a black hole mass of 10 M☉. Four selected QPOs at lower frequencies (0.067-1.8 Hz) are tracked continuously in five energy bands. The QPO-folded profiles are mostly sinusoidal. Remarkably, all four investigated QPOs are broadened in frequency by a random walk in oscillation phase. At higher photon energies the QPO profiles show larger amplitude and increasing phase lag. Our QPO profile analysis shows that the phase delays are not caused by scattering effects, and we discuss a more direct relation between these QPOs and the origin of the hard X-ray spectrum. There are at least three general shapes of the broadband power continuum, with a typical persistence time scale of several weeks. The combined characteristics of the power spectra, light curves, and energy spectra during the period between 1996 February 21 and 1996 August 15 are interpreted as a succession of four different emission states. None of these states appears identical to any of the canonical states of black hole binaries.