SWIFT J1753.5-0127: a surprising optical/X-ray cross-correlation function

TL;DR

This study presents the first clear measurement of the optical/X-ray cross-correlation function in SWIFT J1753.5-0127, revealing optical leads and energy-dependent variations, challenging previous models of such systems.

Contribution

It provides the first detailed optical/X-ray cross-correlation function measurement for SWIFT J1753.5-0127, showing unique features and energy dependence.

Findings

Optical leads X-rays by a few seconds with a broad negative peak.

The correlation function varies significantly with X-ray energy.

This behavior differs from similar systems like XTE J1118+480.

Abstract

We have conducted optical and X-ray simultaneous observations of SWIFT J1753.5-0127 with RXTE and ULTRACAM, while the system persisted in its relatively bright low/hard state. In the cross-correlation function (CCF), we find that the optical leads the X-rays by a few seconds with a broad negative peak, and has a smaller positive peak at positive lags. This is markedly different from what was seen for the similarly interesting system XTE J1118+480, and the first time such a correlation function has been so clearly measured. Furthermore, there appears to be a significant variation of the correlation with X-ray energy. We suggest a physical scenario for its origin.