AT 2023txn (Gaia23cse): A new symbiotic star undergoing a puzzling outburst

Since it began in 2022, the outburst of AT 2023txn continues to unfold via multiple maxima and deep minima, bringing the system well below quiescence brightness. It has been monitored with B V R I g r i photometry, low- and high-resolution spectroscopy, and Swift XRT+UVOT observations. AT 2023txn has turned out to be a previously unknown symbiotic star, harboring an M6III red giant of great photometric stability during quiescence. A large and cold accretion disk around the white dwarf companion provided about 3/4 of the system brightness at blue wavelengths during quiescence; however, it was unable to power a significant emission-line spectrum, as implied by the 2004 observations from the IPHAS r iHα survey and Gaia DR3 BP RP spectrum. The outburst begun toward the end of 2021, passed through a first unnoticed maximum in March 2022, and was eventually announced as transient Gaia23cse ∼19 months later (in September 2023) when the object was peaking at a second and brighter maximum. Close to that epoch, we measured 1500 L⊙ as the luminosity radiated by the outbursting component over the 2000–9000 Å interval, with an upper limit of < 1 L⊙ to the X-ray luminosity over the 0.3−10 keV range. The emission-line spectrum has been characterized by persistent low ionization conditions, with FeII and Balmer series being the dominating species and varying their integrated flux in phase with the brightness evolution during the outburst. Three deep minima separated by ∼866 days brought the system Δg ∼ 1.5 mag below the brightness in quiescence, but they have no counterpart prior to the start of the outburst season. Interpreting them as eclipses of the hot component by the red giant encounters severe difficulties; thus, alternative explanations have been considered within the framework of a massive accretion disk similar to T CrB transferring mass to the WD only episodically via a collapse, which affects the inner radii.