The 1-12 Hz QPO and dips in GRS 1915+105: tracers of Keplerian and viscous time scales?

TL;DR

This study links 1-12 Hz QPOs and spectral dips in GRS 1915+105 to viscous and Keplerian time scales, suggesting QPOs originate from boundary variations in the accretion disk.

Contribution

It proposes that QPO frequency variations are driven by changes in the boundary between the accretion disk and hot Comptonization region, related to viscous and Keplerian time scales.

Findings

QPO frequency correlates with hard spectral episodes.

The correlation matches viscous and Keplerian time scale relations.

QPOs are linked to boundary movements in the accretion disk.

Abstract

We analysed 9 RXTE/PCA observations of GRS 1915+105 in the flaring state, when the hardness of the source spectrum was changing on the time scales from few seconds up to 1000 seconds. The quasi periodic oscillations (QPOs) with the frequency varying between ~ 1 and 12 Hz are associated with the episodes of harder source spectrum. In each observation we found tight correlation between the duration of the hard episode and the characteristic QPO frequency. For a half of the observations this correlation matches the relation between the viscous time scale and the Keplerian frequency, when both quantities are evaluated for various radii in the radiation pressure dominated accretion disk. Assuming that the QPO frequency is proportional to the Keplerian frequency at the boundary between an optically thick accretion disk and a hot comptonization region, the changes of the QPO frequency can then be understood as due to variations of this boundary position on the viscous time scales.