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

J.A. Paice , P. Gandhi , T. Shahbaz , A. Veledina , J. Malzac (1) , D.A.H. Buckley , P.A. Charles , K. Rajwade , V.S. Dhillon , S.P. Littlefair , T.R. Marsh , P. Uttley , F.M. Vincentelli , R. Misra

1 IRAP - Institut de recherche en astrophysique et planétologie

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.

Mots clés: accretion; accretion discs; X-rays: binaries; X-rays: individual: MAXI J1820+070; black holes

Domaines: Astrophysique [astro-ph]

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, 2021, 505 (3), pp.3452-3469. ⟨10.1093/mnras/stab1531⟩. ⟨hal-03281126⟩