eRASSt J074426.3+291606: prompt accretion disc formation in a 'faint and slow' tidal disruption event

Abstract

We report on multi-wavelength observations of the tidal disruption event (TDE) candidate eRASSt J074426.3+291606 (J0744), located in the nucleus of a previously quiescent galaxy at 𝑧 = 0.0396. J0744 was first detected as a new, ultra-soft X-ray source (photon index ∼ 4) during the second SRG/eROSITA All-Sky Survey (eRASS2), where it had brightened in the 0.3-2 keV band by a factor of more than ∼160 relative to an archival 3𝜎 upper limit inferred from a serendipitous Chandra pointing in 2011. The transient was also independently found in the optical by the Zwicky Transient Factory (ZTF), with the eRASS2 detection occurring only ∼20 days after the peak optical brightness, suggesting that the accretion disc formed promptly in this TDE. Continued X-ray monitoring over the following ∼400 days by eROSITA, NICER XTI and Swift XRT showed a net decline by a factor of ∼100, albeit with large amplitude X-ray variability where the system fades, and then rebrightens, in the 0.3-2 keV band by a factor ∼50 during an 80 day period. Contemporaneous Swift UVOT observations during this extreme X-ray variability reveal a relatively smooth decline, which persists over ∼400 days post-optical peak. The peak observed optical luminosity (absolute 𝑔-band magnitude ∼ −16.8 mag) from this transient makes J0744 the faintest optically-detected TDE observed to date. However, contrasting the known set of 'faint and fast' TDEs, the optical emission from J0744 decays slowly (exponential decay timescale ∼120 days), making J0744 the first member of a potential new class of 'faint and slow' TDEs.