2025A&A...701A.224B

The nuclear X-ray transient eRASSt J012026.5-292727 (J012026 hereafter) was discovered in the second SRG/eROSITA all-sky survey (eRASS2). The source appeared more than one order of magnitude brighter than the eRASS1 upper limits (peak eRASS2 0.2-2.3 keV flux of 1.14 × 10^–12 erg cm^–2 s^–1) and with a soft X-ray spectrum (photon index of Γ = 4.3). Over the following months, the X-ray flux started decaying and demonstrated significant flaring activity on both short (hour) and long (year) timescales. By inspecting the multiwavelength light curves of time-domain wide-field facilities, we detected a strong mid-infrared flare, which evolved over two years, and a weaker optical counterpart, with possible hints of a rise > 3 years prior to the X-ray discovery. Follow-up optical spectroscopy revealed transient features, including redshifted Balmer lines (FWHM of ∼1500 km s^–1), strong Fe II emission, He II, Bowen fluorescence lines, and high-ionization coronal lines such as [Fe X] and [Fe XIV]. One spectrum displayed a triple-peaked Hβ line, consistent with emission from a face-on elliptical accretion disk. The spectroscopic features and the slow evolution of the event place J012026 within the nuclear-transient classifications of Bowen fluorescence flares (BFFs) and extreme coronal line emitters (ECLEs). BFFs have been associated with rejuvenated accreting supermassive black holes, although the mechanism triggering the onset of the new accretion flow is yet to be understood, while ECLEs have been linked to the disruption and accretion of stars in gas-rich environments. The association of J012026 with both classes, combined with the X-ray, multiwavelength, and spectroscopic information, supports the idea that the BFF emission could be, at least in some cases, triggered by tidal disruption events (TDEs) perturbing gaseous environments. The observed short- and long-term X-ray variability, uncommon in standard TDEs, adds complexity to these families of nuclear transients. These results highlight the diverse phenomenology of nuclear accretion events and demonstrate the value of systematic X-ray surveys, such as eROSITA and Einstein Probe, for uncovering such transients and characterizing their physical origin.