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The evolution of rapid optical/X-ray timing correlations in the initial hard state of MAXI J1820+070

Abstract : 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 (u_sg_sr_si_sz_s 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.
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https://hal.archives-ouvertes.fr/hal-03281126
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Submitted on : Tuesday, August 16, 2022 - 9:06:28 AM
Last modification on : Wednesday, August 17, 2022 - 3:03:01 AM

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J.A. Paice, P. Gandhi, T. Shahbaz, A. Veledina, J. Malzac, et al.. The evolution of rapid optical/X-ray timing correlations in the initial hard state of MAXI J1820+070. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2021, 505 (3), pp.3452-3469. ⟨10.1093/mnras/stab1531⟩. ⟨hal-03281126⟩

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