  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.
