AstroNote 2025-131

Here we present AT2025hbw/ZTF25aajjeon as a high redshift, extraordinarily luminous tidal disruption event (TDE) candidate. With a peak absolute magnitude of g = -23.7 and a redshift of z = 0.628, AT2025hbw is the highest redshift, optically-selected thermal TDE candidate to date. It was first detected by ZTF on 2025-03-19 at an apparent magnitude of g = 20.9. It was identified and reported as a high probability TDE candidate by NEEDLE (Sheng+2024) on 2025-04-11 (AstroNote 2025-111).

Early lower SNR observations show g - r ≈ 0, with the transient becoming bluer (g - r < -0.4) as it peaked. According to ZTF photometry, it peaked on 2025-04-07 and has plateaued with no evidence of cooling at an absolute magnitude of g = -23.6 for at least 25 days.

A spectrum of ZTF25aajjeon was observed with the Gemini Multi Object Spectrograph (GMOS) on the Gemini North Telescope on 2025-04-20 (PI: Hall), revealing an extremely blue continuum with no evidence of emission lines. Another spectrum was taken with the Low Resolution Imaging Spectrometer (LRIS) on the Keck-I Telescope on 2025-04-27 (PI: Ravi), revealing a similarly blue, featureless spectrum. A redshift of z = 0.628 was determined due to the Ca II H and K and Na I absorption lines present in both spectra, and Mg II absorption present in the LRIS spectrum. No evidence of any SLSNe lines were detected. The featureless nature of the optical spectra and the extraordinary luminosity suggest a possible association with the TDE-featureless class put forth by Hammerstein+2023.

Swift observations were triggered and observed on 2025-04-27. The source was not detected with XRT with an upper limit of 3.6 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.628, this gives a rough luminosity upper limit of 1.9 * 10⁴⁴ erg/s. The source was well-detected in all three UV filters, with uvw1 = 20.1 mag, uvm2 = 20.8 mag, and uvw2 = 21.0 mag. Further monitoring with Swift XRT/UVOT is ongoing.

The blue, featureless spectra, high luminosity, and high redshift are all indicative of a potentially unusual TDE. We also note that this transient may exhibit multiwavelength properties similar to ASASSN-15lh (Dong+2015). Currently, further multiwavelength observations are planned.