A Black Hole X-ray Binary at $\sim$100 Hz: Multiwavelength Timing of MAXI J1820+070 with HiPERCAM and NICER

TL;DR

This study presents simultaneous optical and X-ray timing observations of the black hole candidate MAXI J1820+070, revealing complex variability, anti-correlations, and wavelength-dependent lags that shed light on accretion and jet processes.

Contribution

It provides the first detailed measurement of sub-second optical lags and their wavelength dependence in a black hole X-ray binary, offering new constraints on accretion and jet models.

Findings

Detected intense optical flaring activity on timescales down to 10 ms.

Identified a consistent optical lag of about 165 ms behind X-rays, increasing with wavelength.

Observed a potential 5 s lag attributed to disc reprocessing.

Abstract

We report on simultaneous sub-second optical and X-ray timing observations of the low mass X-ray binary black hole candidate MAXI J1820+070. The bright 2018 outburst rise allowed simultaneous photometry in five optical bands ($ugriz_s$) with HiPERCAM/GTC (Optical) at frame rates over 100 Hz, together with NICER/ISS observations (X-rays). Intense (factor of two) red flaring activity in the optical is seen over a broad range of timescales down to $\sim$10 ms. Cross-correlating the bands reveals a prominent anti-correlation on timescales of $\sim$seconds, and a narrow sub-second correlation at a lag of $\approx$+165 ms (optical lagging X-rays). This lag increases with optical wavelength, and is approximately constant over Fourier frequencies of $\sim$0.3-10 Hz. These features are consistent with an origin in the inner accretion flow and jet base within $\sim$5000 Gravitational radii. An additional $\sim$+5 s lag feature may be ascribable to disc reprocessing. MAXI J1820+070 is the third black hole transient to display a clear $\sim$0.1s optical lag, which may be common feature in such objects. The sub-second lag $variation$ with wavelength is novel, and may allow constraints on internal shock jet stratification models.