Fast infrared variability from a relativistic jet in GX 339-4

TL;DR

This study reports the detection of rapid infrared and X-ray correlated variability in GX 339-4, revealing that the infrared emission likely originates from a relativistic jet near the black hole, with implications for jet physics.

Contribution

It provides the first measurement of infrared/X-ray correlation with sub-second resolution in a black-hole transient, indicating a mildly relativistic jet near the launch region.

Findings

Infrared emission lags X-ray by 100 ms

Infrared emission likely from a jet near the black hole

Jet Lorentz factor estimated to be greater than 2

Abstract

We present the discovery of fast infrared/X-ray correlated variability in the black-hole transient GX 339-4. The source was observed with sub-second time resolution simultaneously with VLT/ISAAC and RXTE/PCA in August 2008, during its persistent low-flux highly variable hard state. The data show a strong correlated variability, with the infrared emission lagging the X-ray emission by 100 ms. The short time delay and the nearly symmetric cross-correlation function, together with the measured brightness temperature of ~2.5 x 10^6 K, indicate that the bright and highly variable infrared emission most likely comes from a jet near the black hole. Under standard assumptions about jet physics, the measured time delay can provide us a lower limit of Gamma > 2 for the Lorentz factor of the jet. This suggests that jets from stellar-mass black holes are at least mildly relativistic near their launching region. We discuss implications for future applications of this technique.