The Evolution of Rapid Optical/X-ray Timing Correlations in the Initial Hard State of MAXI J1820+070

TL;DR

This study investigates the evolving optical and X-ray timing correlations in the black hole binary MAXI J1820+070 during its 2018 outburst, revealing complex relationships and proposing a dual-component synchrotron emission model.

Contribution

It provides the first detailed multi-epoch analysis of optical/X-ray timing correlations in MAXI J1820+070, suggesting the presence of two distinct synchrotron-emitting components and their changing significance.

Findings

Detection of a growing anti-correlation between optical and X-ray lightcurves.

Identification of a steady positive correlation at a 0.2 s optical lag across epochs.

Observation of a positive correlation at negative optical lags in the softest epoch, with wavelength-dependent effects.

Abstract

We report on a multi-epoch campaign of rapid optical/X-ray timing observations of the superbright 2018 outburst of MAXI J1820+070, a black hole low-mass X-ray binary system. The observations spanned 80 days in the initial hard-state, and were taken with NTT/ULTRACAM and GTC/HiPERCAM in the optical (ugriz filters at time resolutions of 8--300 Hz) and with ISS/NICER in X-rays. We find (i) a growing anti-correlation between the optical and X-ray lightcurves, (ii) a steady, positive correlation at an optical lag of 0.2 s (with a longer lag at longer wavelengths) present in all epochs, and (iii) a curious positive correlation at \textit{negative} optical lags in the last, X-ray softest epoch, with longer wavelengths showing a greater correlation and a more negative lag. To explain these we postulate the possible existence of two synchrotron-emitting components; a compact jet and a hot flow. In our model, the significance of the jet decreases over the outburst, while the hot flow remains static (thus, relatively, increasing in significance). We also discuss a previously discovered quasi-periodic oscillation and note how it creates coherent optical time lags, stronger at longer wavelengths, during at least two epochs.