An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

TL;DR

This study measures a 0.1-second optical delay relative to X-ray emission in a black hole system, revealing the jet base is located approximately 10^3 Schwarzschild radii above the black hole, providing new constraints on jet formation models.

Contribution

It provides the first direct measurement of the optical emission zone's elevation in a black hole jet, linking optical variability to jet launching physics and unifying models across different systems.

Findings

Optical flux variations lag X-rays by ~0.1 seconds.

The optical emission zone is located within ~10^3 Schwarzschild radii.

Similar optical lags observed in different black hole binaries.

Abstract

Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub-second optical variations has hitherto been controversial. Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of <~10$^3$ Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems.