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We report on small-amplitude optical variability and recent dissipation ofthe unusually persistent broad emission lines in the blue compact dwarf galaxyPHL 293B. The galaxy's unusual spectral features (P Cygni-like profiles with$\sim$800 km s$^{-1}$ blueshifted absorption lines) have resulted inconflicting interpretations of the nature of this source in the literature.However, analysis of new Gemini spectroscopy reveals the broad emission hasbegun to fade after being persistent for over a decade prior. Precisedifference imaging light curves constructed with the Sloan Digital Sky Surveyand the Dark Energy Survey reveal small-amplitude optical variability of$\sim$0.1 mag in the g band offset by $100\pm21$ pc from the brightest pixel ofthe host. The light curve is well-described by an active galactic nuclei(AGN)-like damped random walk process. However, we conclude that the origin ofthe optical variability and spectral features of PHL 293B is due to along-lived stellar transient, likely a Type IIn supernova or non-terminaloutburst, mimicking long-term AGN-like variability. This work highlights thechallenges of discriminating between scenarios in such extreme environments,relevant to searches for AGNs in dwarf galaxies. This is the second long-livedtransient discovered in a blue compact dwarf, after SDSS1133. Our resultimplies such long-lived stellar transients may be more common inmetal-deficient galaxies. Systematic searches for low-level variability indwarf galaxies will be possible with the upcoming Legacy Survey of Space andTime at Vera C. Rubin Observatory.

The Curious Case of PHL 293B: A Long-Lived Transient in a Metal-Poor Blue Compact Dwarf Galaxy