Expanding hot flow in the black hole binary SWIFT J1753.5-0127: evidence from optical timing

TL;DR

This paper analyzes the evolution of optical and X-ray timing properties during the outburst of the black hole binary SWIFT J1753.5-0127, proposing a model of hot flow expansion that explains observed correlations and oscillations.

Contribution

It introduces a comprehensive model explaining the evolution of optical/X-ray timing properties during the entire outburst through hot flow expansion.

Findings

The model reproduces the observed cross-correlation and phase lag spectra.

Optical and X-ray quasi-periodic oscillations are intrinsically connected.

The evolution of timing properties is explained by hot flow expansion.

Abstract

We describe the evolution of optical and X-ray temporal characteristics during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127. The optical/X-ray cross-correlation function demonstrates a single positive correlation at the outburst peak, then it has multiple dips and peaks during the decline stage, which are then replaced by the precognition dip plus peak structure in the outburst tail. Power spectral densities and phase lags show a complex evolution, revealing the presence of intrinsically connected optical and X-ray quasi-periodic oscillations. For the first time, we quantitatively explain the evolution of these timing properties during the entire outburst within one model, the essence of which is the expansion of the hot accretion flow towards the tail of the outburst. The pivoting of the spectrum produced by synchrotron Comptonization in the hot flow is responsible for the appearance of the anti-correlation with the X-rays and for the optical quasi-periodic oscillations. Our model reproduces well the cross-correlation and phase lag spectrum during the decline stage, which could not be understood with any model proposed before.