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

J. A. Paice, P. Gandhi, T. Shahbaz, P. Uttley, Z. Arzoumanian, P. A. Charles, V. S. Dhillon, K. C. Gendreau, S. P. Littlefair, J. Malzac, S. Markoff, T. R. Marsh, R. Misra, D. M. Russell, A. Veledina

Published 2019-10-09Version 1

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.