X-ray (Swift and NICER, left panel) and optical (LCO, right panel) light curves of the four outbursts (solid lines). In both panels, the photometric epochs used in this work are indicated by symbols (crosses, dots, triangles and stars for 2011, 2017, 2019, and 2021 outbursts, respectively), with smaller markers for epochs with no significant DRPs (#1 and #7 in 2021, see Sect. 3.3). The dates of first detections are given in Table 2. The grey band indicates an optical magnitude interval (17.1–17.3) with peculiar DRP evolution (see Fig. 8, and Sect. 4.2.1).