AstroNote 2026-20

AT 2025alrm (RA=08:13:00.60, Dec=+21:09:10.05) was discovered by the WFST survey on 2025-11-19 (MJD=60998.92). At the first epoch, the AB magnitudes are 21.45 ± 0.16 mag in the WFST u band and 21.99 ± 0.21 mag in the g band. The light curve peaks on 2025-12-16 (MJD 61025), reaching u = 20.17 ± 0.06, g = 19.98 ± 0.02, and r = 20.27 ± 0.04, followed by a subsequent decline. Its optical light curve shows a persistently blue color (g - r ~ -0.3), which is consistent with the photometric behavior of optically selected TDEs. We triggered optical spectroscopic observations of AT2025alrm using the BINOSPEC spectrograph mounted on the 6.5m Multiple Mirror Telescope (MMT) on 2026-01-14 (MJD 61054). We determine a redshift of z=0.179 from the Balmer absorption lines in the spectrum. The spectrum exhibits a prominent broad emissions at the wavelength of He II λ4686 and NIII λ4640. These spectral characteristics are consistent with those of tidal disruption events (TDE-H+He).

We immediately triggered Swift ToO observations of AT2025alrm following its spectroscopic classification based on MMT. The first epoch was observed on 2026-02-03 (MJD=61074.96). No significant X-ray emission was detected, and a 3-sigma upper limit of 1.767e-13 erg s⁻¹ cm⁻² was derived for the 0.3–10 keV flux. In contrast, the source is clearly detected in the UVOT UV bands, with AB magnitudes of UVW2 = 19.99 ± 0.13 mag, UVM2 = 20.11 ± 0.15 and UVM1=20.27 ± 0.16, after correcting for Galactic extinction and subtracting the host-galaxy contribution based on SED fitting. It is important to note that the AB magnitudes in the UV bands are brighter than those in the g and r bands. Such a blue color further indicates that AT2025alrm is consistent with a TDE.

We tentatively classified AT2025alrm as a TDE-H+He, based on its consistently blue color and potential broad He II λ4686 and NIII λ4640 emissions.