HEAD 2000, November 2000
Session 29. X-Ray Binaries II
Oral, Wednesday, November 8, 2000, 1:15-2:45pm, Pago Pago Ballroom

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[29.08] Effects of Frame-Dragging on X-ray Emission from Black Holes and Neutron Stars

D. Markovic, F. K. Lamb (UIUC)

In previous work on the effects of frame-dragging on viscous accretion disks around black holes and neutron stars, we discovered gravitomagnetically precessing global modes localized near the inner edge of the Keplerian flow. The highest-frequency modes of this type precess in the prograde direction with frequencies slightly lower than the Lense-Thirring precession frequency at the mean radius of the mode. Contrary to what had been expected since the pioneering work of Bardeen & Petterson, these modes are very weakly damped (Q \approx 20--50) and are therefore likely to be excited. We present analytical arguments that shed further light on the nature of these modes and allow a better understanding of the symmetry-breaking pattern of viscous dissipation they create.

The two highest-frequency modes tilt the innermost part of the Keplerian flow, redirecting gas crossing the sonic radius. As we discuss, these modes are likely to be the most easily excited and may be excited by magnetoturbulence in the disk or interaction of the disk with a stellar magnetic field. They may modulate X-ray emission from black holes and neutron stars by periodically altering the inspiral of gas from the Keplerian disk, by periodically obscuring emission from the inner disk or star, or by creating a rotating pattern of enhanced emission. This modulation is expected to create power spectral peaks at ~1--10 Hz in black hole sources and at ~10--40 Hz in the kilohertz QPO sources. It may also produce sidebands on the kilohertz QPOs, separated from the main peaks by ~10--40 Hz. Detecting and measuring the frequencies of these modes would provide valuable new information about the strongly curved, twisting spacetime expected near spinning neutron stars and black holes. This research was supported in part by the NSF and NASA.

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