The Curious Case of PHL 293B: A Long-Lived Transient in a Metal-Poor Blue Compact Dwarf Galaxy
Creators: name: Burke, Colin J.
name: Baldassare, Vivienne F.
name: Liu, Xin
name: Foley, Ryan J.
name: Shen, Yue
name: Palmese, Antonella
name: Guo, Hengxiao
name: Herner, Kenneth
name: Abbott, Tim M. C.
name: Aguena, Michel
name: Allam, Sahar
name: Avila, Santiago
name: Bertin, Emmanuel
name: Brooks, David
name: Rosell, Aurelio Carnero
name: Kind, Matias Carrasco
name: Carretero, Jorge
name: da Costa, Luiz N.
name: De Vicente, Juan
name: Desai, Shantanu
name: Doel, Peter
name: Eifler, Tim F.
name: Everett, Spencer
name: Frieman, Josh
name: Garcia-Bellido, Juan
name: Gaztanaga, Enrique
name: Gruen, Daniel
name: Gruendl, Robert A.
name: Gschwend, Julia
name: Gutierrez, Gaston
name: Hollowood, Devon L.
name: Honscheid, Klaus
name: James, David J.
name: Krause, Elisabeth
name: Kuehn, Kyler
name: Maia, Marcio A. G.
name: Menanteau, Felipe
name: Miquel, Ramon
name: Paz-Chinchon, Francisco
name: Plazas, Andres A.
name: Sanchez, Eusebio
name: Santiago, Basilio
name: Scarpine, Vic
name: Serrano, Santiago
name: Sevilla-Noarbe, Ignacio
name: Smith, Mathew
name: Soares-Santos, Marcelle
name: Suchyta, Eric
name: Swanson, Molly E. C.
name: Tarle, Gregory
name: Tucker, Douglas L.
name: Varga, Tamas Norbert
name: Walker, Alistair R.
name: Collaboration, DES
Publisher: name: arXiv
Year: 2020
Subjects: subject: Astrophysics of Galaxies (astro-ph.GA)
subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report on small-amplitude optical variability and recent dissipation of the unusually persistent broad emission lines in the blue compact dwarf galaxy PHL 293B. The galaxy's unusual spectral features (P Cygni-like profiles with $\sim$800 km s$^{-1}$ blueshifted absorption lines) have resulted in conflicting interpretations of the nature of this source in the literature. However, analysis of new Gemini spectroscopy reveals the broad emission has begun to fade after being persistent for over a decade prior. Precise difference imaging light curves constructed with the Sloan Digital Sky Survey and 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 of the host. The light curve is well-described by an active galactic nuclei (AGN)-like damped random walk process. However, we conclude that the origin of the optical variability and spectral features of PHL 293B is due to a long-lived stellar transient, likely a Type IIn supernova or non-terminal outburst, mimicking long-term AGN-like variability. This work highlights the challenges of discriminating between scenarios in such extreme environments, relevant to searches for AGNs in dwarf galaxies. This is the second long-lived transient discovered in a blue compact dwarf, after SDSS1133. Our result implies such long-lived stellar transients may be more common in metal-deficient galaxies. Systematic searches for low-level variability in dwarf galaxies will be possible with the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory.
description: 15 pages, 6 figures, 1 table. Accepted for publication in ApJ Letters