Evidence for enhanced mass transfer in the disc preceding the transition to the soft state in MAXI J1820+070

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 NL-9700 AV Groningen, The Netherlands
² Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
³ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences Kunming 650216, PR China
⁴ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁵ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, (LC), Italy
⁶ School of Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
⁷ Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, Taoyuan 32001, Taiwan
⁸ Laboratory for Space Research, The University of Hong Kong, Cyberport 4, Hong Kong

^⋆ Corresponding authors: pyang@astro.rug.nl, zhangguobao@ynao.ac.cn, mariano@astro.rug.nl

Received: 8 April 2025
Accepted: 20 June 2025

Abstract

We investigate the 2018–2019 main outburst and the subsequent mini-outbursts of the black hole low-mass X-ray binary MAXI J1820+070 using optical/ultraviolet data from the Las Cumbres Observatory (LCO), the American Association of Variable Star Observers (AAVSO), and Swift/UVOT, as well as X-ray data from Insight-HXMT and Swift/XRT. Given the high-cadence observations, we identify a broad dip-like feature in both the optical and X-ray light curves preceding the transition to the soft state, with the X-ray dip lagging the optical dip by approximately 10 days. We propose that the dip is caused by a brief decrease followed by an increase in the mass accretion rate as it propagates through the disc, ultimately triggering the transition to the soft state. This might be a potential tool to predict impending hard-to-soft state transitions, although such a dip has not yet been observed in many sources. Additionally, we find that the optical colour (g′−i′) becomes bluer and less variable before the transition to the intermediate state, preceding a dramatic change in the hardness ratio. This appears to be an unusual case, differing from the typical scenario in which the optical colour changes along with the transition to the soft state. Finally, we explore the implications of the complex evolution of optical/X-ray correlation during both main outbursts and mini-outbursts. In particular, we find a loop-like evolutionary track before the transition to the soft state, which is linked to optical/X-ray dips in the light curves.