Optical counterpart to Swift J0243.6+6124

TL;DR

This study characterizes the optical counterpart of Swift J0243.6+6124, revealing its spectral type, distance, and variability, and linking optical and X-ray emissions during the giant outburst.

Contribution

It provides the first detailed optical spectral and photometric analysis of the counterpart, including its spectral type, rotational velocity, and variability behavior.

Findings

Optical counterpart is an O9.5Ve star at ~5 kpc.

Optical and X-ray luminosities are correlated during outburst.

No short-term pulsations detected in optical data.

Abstract

Swift J0243.6+6124 was discovered during a giant X-ray outburst in October 2017. While there are numerous studies in the X-ray band, very little is known about the optical counterpart. We have performed an spectral and photometric analysis of the optical counterpart of this intriguing source. We find that the optical counterpart to Swift J0243.6+6124 is a V = 12.9, O9.5Ve star, located at a distance of $\sim5$ kpc. The optical extinction in the direction of the source is $A_V=3.6$ mag. The rotational velocity of the O-type star is 210 km s$^{-1}$. The long-term optical variability agrees with the growth and subsequent dissipation of the Be circumstellar disk after the giant X-ray outburst. The optical and X-ray luminosity are strongly correlated during the outburst, suggesting a common origin. We did not detect short-term periodic variability that could be associated with nonradial pulsations from the Be star photosphere.