Multiwavelength observations of Swift J0243.6+6124 from 2017 to 2022

¹ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China
e-mail: weiliu@pmo.ac.cn
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
³ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
⁴ Physics Department, University of Crete, 71003 Heraklion, Greece
⁵ Physics Department/Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
⁶ Beijing Planetarium, Beijing Academy of Sciences and Technology, Beijing 100044, PR China

Received: 27 April 2022
Accepted: 12 July 2022

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.