Tracking optical variability and outflows across the accretion states of the black hole transient MAXI J1820+070
M. C. Baglio, K. Alabarta, D. M. Russell, N. Masetti, M. M. Messa, T. Muñoz Darias, F. M. Vincentelli, S. K. Rout, P. Saikia, A. Gabuya, V. Chavushyan, T. Al Qaissieh, A. Palado
TL;DR
This paper presents a comprehensive, multi-wavelength study of MAXI J1820+070 during its 2018 outburst and subsequent reflares, focusing on minute-timescale optical variability and optical wind signatures. By combining high-cadence optical photometry from LCO and Al Sadeem with simple X-ray hardness diagnostics and low-resolution spectroscopy, the authors test the internal shock jet model as the primary driver of rapid optical variability in the hard state, while showing jet quenching and disc-dominated variability in the soft state. The results show a clear rms–hardness correlation, wavelength dependence favoring jet-dominated synchrotron emission, and wind signatures in hard states that are suppressed in soft states, reinforcing an accretion–ejection coupling in BH-LMXBs and extending jet variability signatures to optical wavelengths. Taken together, the findings constrain the jet–wind interplay and highlight residual cyclo-synchrotron emission as a potential contributor in the faint hard states.
Abstract
We present a study of the minute-timescale optical variability and spectroscopic outflow signatures in the black hole X-ray binary MAXI J1820+070 during its 2018 outburst and re-brightenings. Minute-cadence, multi-filter optical light curves were obtained with the Las Cumbres Observatory network and the Al Sadeem Observatory (UAE) over 2018-2020, complemented by X-ray data from Swift/BAT, XRT, and MAXI. We also acquired contemporaneous low-resolution optical spectra with the 2.1 m OAN San Pedro Martir and OAGH Cananea telescopes (Mexico) and the 1.5 m G.D. Cassini telescope at Loiano (Italy). The optical fractional rms peaks in the hard state and is dominated by short-timescale flickering that strengthens toward longer wavelengths, suggesting that the variability is jet-driven. In this scenario, inflow fluctuations inject velocity perturbations at the jet base (internal shock model). The variability is quenched in the soft state, with any residual signal likely linked to accretion-flow fluctuations. This behaviour supports the accretion-ejection coupling in black hole binaries and shows that the jet's variability signature extends to optical wavelengths in all hard states. In the faintest hard states, residual optical variability may instead trace cyclo-synchrotron emission from the hot flow. The spectra show double-peaked emission lines and tentative cold-wind signatures during the hard state. Such winds were reported during the main 2018 outburst; here we find evidence of their presence also in later re-brightenings. Their absence in the soft state likely reflects stronger disc ionisation from the higher X-ray flux, suppressing low-ionisation optical features.