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

John Paice, Gandhi Poshak, Tariq Shahbaz, Alexandra Veledina, Julien Malzac, David Buckley, Phil Charles, Kaustubh Rajwade, Vikram S. Dhillon, Stuart P. Littlefair, Tom R. Marsh, Phil Uttley, Federico Vincentelli, Ranjeev Misra

Research output: Contribution to journalArticlepeer-review


We report on a multiepoch 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 d in the initial hard state and were taken with NTT/ULTRACAM and GTC/HiPERCAM in the optical (usgsrsiszs filters at time resolutions of 8–300 Hz) and with ISS/NICER in X-rays. We find (i) a growing anticorrelation between the optical and X-ray light curves, (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 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.
Original languageEnglish
Pages (from-to)3452–3469
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 27 May 2021


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