Evidence for a Link between Fe Kα Emission-Line Strength and Quasi-periodic Oscillation Phase in a Black Hole

Abstract

In X-ray binaries, the frequencies revealed in X-ray quasi-periodic oscillations (QPOs) are often interpreted as characteristic frequencies in the inner accretion disk, although the exact oscillation mechanism is unknown at present. Broadened Fe Kα lines are also excellent diagnostics of the inner accretion flow, if their broadening is indeed due to inner disk reflection. Here we present two cases where the flux and equivalent width of the Fe Kα emission lines in spectra of the Galactic black hole GRS 1915+105 vary with the phase of strong 1 and 2 Hz QPOs in the X-ray flux. These results provide strong evidence that both QPOs and the Fe Kα lines originate in the inner disk. If the 1 Hz QPO is only a Keplerian orbital frequency, the QPO comes from a distance of RSchw from the black hole; the 2 Hz QPO implies a radius of RSchw. At these radii, relativistic shaping of a disk line is inevitable. Moreover, the link holds in radio-bright and radio-faint phases, signaling that in systems like GRS 1915+105, the Fe Kα line is a disk line and not a jet line as per SS 433. A particularly interesting possibility is that a stable warp in the inner disk, e.g., due to Lense-Thirring precession, may produce the observed QPOs and line modulations. More broadly, the Fe K-QPO link provides an unprecedented mechanism for revealing the inner accretion flow and relativistic regime in accreting systems, in that it gives two measures of radius: for a given disk QPO model, the frequency translates into a specific radius, and relativistic line models yield radii directly.