Rapid optical and X-ray timing observations of GX 339-4: flux correlations at the onset of a low/hard state

TL;DR

This study reports rapid optical and X-ray flux correlations in GX 339-4's low/hard state, revealing insights into the emission mechanisms and inner accretion processes through simultaneous multi-wavelength timing analysis.

Contribution

It provides the first detailed rapid timing correlation analysis of optical and X-ray emissions in GX 339-4 during its low/hard state, challenging reprocessing models and supporting jet-corona interaction scenarios.

Findings

Optical lagging X-ray by ~150 ms with positive correlation.

Broad X-ray ACF compared to optical ACF, arguing against reprocessing.

Spectral hardening during X-ray flares without optical changes.

Abstract

We present the discovery of optical/X-ray flux correlations on rapid timescales in the low/hard state of the Galactic black hole GX 339-4. The source had recently emerged from outburst and was associated with a relatively-faint counterpart with mag V~17. The optical (VLT/ULTRACAM) and X-ray (RXTE/PCA) data show a clear positive cross-correlation function (CCF) signal, with the optical peak lagging X-rays by ~ 150 ms, preceded by a shallow rise and followed by a steep decline along with broad anti-correlation dips. Examination of the light curves shows that the main CCF features are reproduced in superpositions of flares and dips. The CCF peak is narrow and the X-ray auto-correlation function (ACF) is broader than the optical ACF, arguing against reprocessing as the origin for the rapid optical emission. X-ray flaring is associated with spectral hardening, but no corresponding changes are detected around optical peaks and dips. The variability may be explained in the context of synchrotron emission with interaction between a jet and a corona. The complex CCF structure in GX 339-4 has similarities to that of another remarkable X-ray binary XTE J1118+480, in spite of showing a weaker maximum strength. Such simultaneous multi-wavelength, rapid timing studies provide key constraints for modeling the inner regions of accreting stellar sources.