GRO J1744-28 and Scorpius X-1: First evidence for photon bubble oscillations and turbulence

We discuss our recent Rossi X-Ray Timing Explorer (RXTE) observations of GRO J1744-28, which discovered quasi-periodic oscillations (QPOs) of the intensities in the energy band 3-12 keV during observations starting on 1996 January 18.77 UT. We have found that the power spectrum in the frequency band 5-1000 Hz consists of two red-noise components that can be characterized by two power laws, each with an index of -5/3 and a QPO peak centered at 40 Hz. We suggest that the power peak is due to a newly discovered form of turbulence in the accretion column of super-Eddington accretion-powered pulsars driven by photon bubble instabilities. These instabilities give rise to strong power peaks at frequencies characteristic of photon diffusion and bubble coalescence in the highly nonlaminar accretion column resulting in photon bubble oscillations (PBOs). The relationship between the rms amplitude of the PBOs and the intensity is in qualitative agreement with observations. Our calculations also suggest that the observed high-frequency red-noise component with a -5/3 power-law index from 40 to 600 Hz is the first evidence of photon bubble turbulence in the accretion column of an X-ray pulsar. Recent RXTE observations of Sco X-l have found high-frequency QPOs at 1100 and 830 Hz. We show that PBOs at these frequencies are a natural consequence of photon bubble instabilities. We also show that the rms amplitudes of the calculated PBOs at these frequencies are consistent with the observations. We predict that further RXTE observations of Sco X-l should reveal additional PBOs at 2000 and 2600 Hz as well as a broadband continuum spectrum with a -5/3 power law, extending from 3000 to several times 10(4) Hz.