Multiwavelength observation of 1A 0535+262=HD 245770 from 2010 to 2021

Context. 1A 0535+262 is a high-mass X-ray binary that went into a giant X-ray outburst in 2020. During this event, the X-ray luminosity reached the highest value measured over the last 30 years. Aims. Our aim is to study the long-term variability of 1A 0535+262 before and after the 2020 major X-ray outburst and to uncover the mechanism that led to the X-ray outburst. Methods. We used the long-term photometric light curve and the equivalent widths of the H$\alpha$ and He I $\lambda$6678 lines to monitor the state of the Be star’s circumstellar disk. The H$\alpha$ line profiles show evidence for V/R variability, which we revealed by fitting the H$\alpha$ spectral line profiles with two Gaussian functions. In addition, we divided our data into four periods according to the intensity of the X-ray, optical, and infrared emission. Results. The H$\alpha$ line profiles show single-peaked profiles in most cases. This is consistent with the previously reported orbital inclination of $i$ = ${37}^{\circ }\pm 2^{\circ }$. Unlike the H$\alpha$ lines, the He I $\lambda 6678$ lines show a maximal intensity in October 2020, which is one month before the giant X-ray outburst in 2020. Based on the behavior of the equivalent widths of the H$\alpha$ and He I $\lambda 6678$ lines, and the $V$-band magnitude, we find two mass ejection processes from the Be star to the Be disk on MJD 55820 and MJD 56600. The V/R quasi-period is about two, years during 2011–2015, which is different from 1994 to 1995. Furthermore, the periods I$\to$II$\to$III$\to$IV in the $(B-V)$ color index versus $V$-band magnitude diagram constitute a cycle. From the behavior of the V/R ratio of H$\alpha$ lines, and the variability of the $V$ band, we believe that the precession of the density perturbation inside the disk is retrograde.