Multiwavelength observations of Swift J0243.6+6124 from 2017 to 2022
Liu, Wei; Wang, Xiaofeng; Sai, Hanna; Yan, Shengyu; Zhang, Xinhan; Yan, Jingzhi; Reig, Pablo; Xiao, Guangcheng; Lin, Han; Chen, Zhihao; Liu, Qingzhong
China, Greece
Abstract
Context.Swift J0243.6+6124 is a high-mass X-ray binary that went into a giant X-ray outburst in 2017. During this event, the X-ray luminosity reached the highest value ever measured in a galactic Be/X-ray binary.
Aims: Our aim is to study the long-term variability of Swift J0243.6+6124 after the 2017 major X-ray outburst.
Methods: We have obtained optical spectroscopy and photometry data during four years after the event. The long-term photometric light curve and the equivalent widths of the Hα and He I λ6678 lines were used to monitor the state of the Be star's circumstellar disk. The Hα line profiles show evidence for V/R variability that was accounted for by fitting the Hα spectral line profile with two Gaussian functions. We divided our data into three phases according to the intensity of the X-ray, optical, and infrared emission.
Results: Phase I covers the rise and decay of the giant X-ray outburst that took place in October-November 2017. We interpret phase II as the dissipation of the Be star's equatorial disk and phase III as its recovery. The timescale of a complete formation and dissipation process is about 1250 days. The epoch when the dissipation process stopped and the reformation period began is estimated to be around MJD 58530. We find a delay of ∼100-200 days between the minimum of the optical or infrared intensity and the strength of the Hα line after the X-ray outburst, which may indicate that the dissipation of the disk begins from the inner parts. The motion of the density perturbation inside the disk is prograde, with a V/R quasi-period of about four years. The source shows a positive correlation in the (B − V) color index versus V-band magnitude diagram, which implies that the system is seen at a small or moderate inclination angle.
Conclusions: Despite the super-Eddington X-ray luminosity during the outburst, the subsequent pattern of long-term optical and IR variability of Swift J0243.6+6124 is typical of Be/X-ray binaries.