An Optically-Discovered Outburst from XTE J1859+226

TL;DR

In 2021, optical surveys detected a rare, low-luminosity outburst of the black hole X-ray transient XTE J1859+226, revealing new insights into its behavior at minimal accretion levels.

Contribution

This study reports the first optical detection of a low-luminosity outburst from XTE J1859+226 since 1999, demonstrating the effectiveness of optical surveys in identifying faint X-ray transient activity.

Findings

Outburst lasted less than 20 days with lower peak luminosity.

The source remained in the hard spectral state throughout.

X-ray lightcurve showed an exponential decline.

Abstract

Using the Zwicky Transient Facility, in 2021 February we identified the first known outburst of the Black Hole X-ray Transient XTE J1859+226 since its discovery in 1999. The outburst was visible at X-ray, UV, and optical wavelengths for less than 20 days, substantially shorter than its 320-day full outburst in 1999, and the observed peak luminosity was two orders of magnitude lower. Its peak bolometric luminosity was only $2\times 10^{35}$ erg s$^{-1}$, implying an Eddington fraction of about $3\times10^{-4}$. The source remained in the hard spectral state throughout the outburst. From optical spectroscopy measurements we estimate an outer disk radius of 10$^{11}$ cm. The low observed X-ray luminosity is not sufficient to irradiate the entire disk, but we observe a surprising exponential decline in the X-ray lightcurve. These observations highlight the potential of optical and infrared (O/IR) synoptic surveys to discover low-luminosity activity from X-ray transients.