Discovery of a Featureless Tidal Disruption Event at z ∼ 1 with the Wide Field Survey Telescope

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Abstract We report the discovery of the tidal disruption event (TDE) WFST250820mmsw/AT2025wet by the 2.5 m Wide Field Survey Telescope (WFST). It exhibits a blue nuclear flare throughout the observed evolution with a g -band peak magnitude ∼ 22, which is about 3 mag brighter than its host galaxy. A Keck/LRIS spectrum taken near the optical peak reveals a featureless blue continuum, with no discernible emission lines. However, its redshift can be accurately determined to be 1.037 by its host galaxy absorption lines. Blackbody fits to the multiband spectral energy distribution (SED) of AT2025wet yield a constant temperature of ∼19,000 K and a peak luminosity of ( 8.2 7 − 0.71 + 0.92 ) × 1 0 44 erg s − 1 , while actually the SED likely peaks at a much shorter wavelength than a 19,000 K blackbody. The SED modeling of the host galaxy implies a stellar mass of ∼10 11.2 M ⊙ and an estimated central black hole mass of ∼10 8 M ⊙ , with no evidence of significant active galactic nucleus activity prior to the flare. All of these observations are well consistent with a featureless TDE scenario, making it the highest-redshift nonjetted TDE known to date. TDEs at such high redshift provide us a unique opportunity to explore the intrinsic SEDs of TDEs, particularly to test whether they peak in the extreme-UV regime, thereby addressing the missing energy puzzle and the origin of optical emission in TDEs. Ongoing surveys represented by WFST and the Legacy Survey of Space and Time are expected to discover an increasing number of TDEs at higher redshifts, which will extend our census of supermassive black holes across redshift space and help unravel the mysteries of optical TDEs through direct probes of their UV emission.

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