A black hole X-ray binary at ̃100 Hz: multiwavelength timing of MAXI J1820+070 with HiPERCAM and NICER

Paice, J. A.; Gandhi, P.; Shahbaz, T.; Uttley, P.; Arzoumanian, Z.; Charles, P. A.; Dhillon, V. S.; Gendreau, K. C.; Littlefair, S. P.; Malzac, J.; Markoff, S.; Marsh, T. R.; Misra, R.; Russell, D. M.; Veledina, A.
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Monthly Notices of the Royal Astronomical Society

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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 (ugrizs) with HiPERCAM/GTC (Optical) at frame rates over 100 Hz, together with NICER/ISS observations (X-rays). Intense (factor of 2) red flaring activity in the optical is seen over a broad range of time-scales down to ̃10 ms. Cross-correlating the bands reveals a prominent anticorrelation on time-scales of ̃seconds, and a narrow sub-second correlation at a lag of ≈ +165 ms (optical lagging X-rays). This lag increases with optical wavelength, and is approximately constant over Fourier frequencies of ̃0.3-10 Hz. These features are consistent with an origin in the inner accretion flow and jet base within ̃5000 Gravitational radii. An additional ̃+5 s lag feature may be ascribable to disc reprocessing. MAXI J1820+070 is the third black hole transient to display a clear ̃0.1 s 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.
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