Optical and X-ray timing analysis of the 2018–2020 outburst and rebrightening of the black hole transient MAXI J1820+070

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
³ Department of Physics and Astronomy, University of Padova, Via F. Marzolo 8, I-35131 Padova, Italy
⁴ INAF – Osservatorio Astronomico di Padova, Via dell’Osservatorio 8, I-36012 Asiago (VI), Italy
⁵ ANS Collaboration, c/o Astronomical Observatory, I-36012 Asiago (VI), Italy
⁶ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Karachai-Cherkesia 369167, Russia
⁷ Sternberg Astrophysical Institute, Moscow University, Universitetsky Ave., 13, Moscow 119992, Russia
⁸ Institute of Astronomy (Russian Academy of Sciences), Pyatnitskaya Str. 48, 119017 Moscow, Russia
⁹ Astronomical Institute of the Slovak Academy of Sciences, Tatranská Lomnica 059 60, The Slovak Republic
¹⁰ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate (LC), Italy

^⋆ Corresponding authors: michele.fiori@inaf.it, luca.zampieri@inaf.it

Received: 25 January 2025
Accepted: 5 April 2025

Abstract

We report the results of a comprehensive analysis of the multi-wavelength (in optical and X-rays) and multi-timescale (from months to tenths of a second) variability of the 2018–2020 outburst of the black hole transient MAXI J1820+070. A detailed analysis of the optical photometry revealed a periodicity that evolves over time during the first outburst episode and stabilises at a frequency of 1.4517(1) 1/d (∼0.5% longer than the orbital period). This super-orbital modulation is also seen in the X-rays for a few days shortly after the transition to the high-soft state. We also observed optical quasi-periodic oscillations (QPOs) that correspond to some of the QPOs observed in X-rays at three different epochs when the source was in the low-hard state. In two epochs, optical QPOs with a centroid consistent with half the frequency of the most prominent X-ray QPO can be seen. If the lowest modulation frequency is the one observed in the optical, the characteristic precession frequency of MAXI J1820+070 is lower than that inferred from the ‘fundamental’ QPO in the X-rays. Assuming that QPOs can be generated by Lense-Thirring precession, we calculated the spin of the black hole in the case where the fundamental precession frequency is tracked by the optical emission. We find a relatively slowly spinning black hole with a spin parameter of ≲0.15. The super-orbital optical and X-ray modulations observed after the disappearance of the QPOs may be triggered by the self-irradiation of the outer disc by a standard inner disc truncated at a few gravitational radii.