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

Abstract

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 Hα and He I λ6678 lines to monitor the state of the Be star’s circumstellar disk. The Hα line profiles show evidence for V/R variability, which we revealed by fitting the Hα 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 Hα line profiles show single-peaked profiles in most cases. This is consistent with the previously reported orbital inclination of i = 37° ±2°. Unlike the Hα lines, the He I λ6678 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 Hα and He I λ6678 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 → II → III → IV in the (B − V) color index versus V-band magnitude diagram constitute a cycle. From the behavior of the V/R ratio of Hα lines, and the variability of the V band, we believe that the precession of the density perturbation inside the disk is retrograde.