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

TL;DR

This study presents four years of optical and infrared observations of Swift J0243.6+6124, revealing the evolution of its Be star disk, including formation, dissipation, and reformation phases, with detailed analysis of spectral line variability and disk dynamics.

Contribution

It provides a comprehensive long-term observational analysis of the Be star disk evolution in Swift J0243.6+6124, including phase division, timescales, and disk perturbation motion, which was not previously documented.

Findings

The complete disk formation and dissipation cycle takes about 1250 days.

A delay of 100-200 days exists between optical/infrared minima and Halpha line strength.

The disk's density perturbation exhibits prograde motion with a four-year V/R quasi-period.

Abstract

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 Halpha and He I 6678 lines were used to monitor the state of the Be star disk. The Halpha line profiles show evidence for V/R variability that was accounted for by fitting the Halpha 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. Phase I covers the rise and decay of the giant X-ray outburst that took place in October to November 2017. We interpret phase II as the dissipation of the Be star 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 about 100 to 200 days between the minimum of the optical or infrared intensity and the strength of the Halpha 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.