AstroNote 2026-213

We performed a systematic search for transient 3 GHz radio sources at the positions of reported optical transients, as a scaled-up version of the archival analysis that identified the nuclear transient AT 2020aboe (AstroNote 2026-209), and using a VLASS epoch-to-epoch selection comparable to the radio-selected TDE search of Somalwar et al. 2025 (ApJ 982, 163).

METHOD. All public TNS objects (about 187,000 with Dec above -40 deg) were crossmatched against the CIRADA VLASS Quick Look epoch 1 (v3.1) and epoch 2 (v2) component catalogs (counterpart radius 2.5 arcsec; quality cuts Duplicate_flag below 2, Quality_flag = 0, peak at least 1 mJy). A +1 deg shifted-position control yields about 10 chance associations per epoch versus about 2,500 real ones. Each radio-coincident transient was classified by the timing of its covering subtile observations relative to the optical discovery date (CIRADA subtile tables; per-candidate dates verified against CADC observation metadata): EMERGENT (undetected in epoch 1, detected in epoch 2, discovery inside the radio window) or FADER (the reverse). Candidates with FIRST or NVSS counterparts (5 arcsec) were rejected as pre-existing radio sources; survivors were characterized via Legacy Surveys DR10 (host Tractor offsets, photo-z), SDSS DR17 spectra, NEOWISE photometry, and ZTF/ALeRCE light curves, and ranked by similarity to AT 2020aboe (nuclear position, galaxy host, quiescent spectrum, flare-like optical morphology, L(3 GHz) of order 10^29 erg/s/Hz).

VALIDATION. The two top-ranked objects with known nature are AT 2020aboe itself and the repeating partial TDE AT 2020vdq (VT J1008; Somalwar et al. 2025, ApJ 985, 175), both recovered blind. The same object is the radio-selected TDE VT J1008 of Somalwar et al. 2025 (ApJ 982, 163), whose independently reported VLASS emergence (undetected 2019, detected 2021) matches our EMERGENT classification.

CANDIDATES. The 5 unclassified candidates (all listed in Related Objects below with positions and redshifts where available):

1. AT 2020xmt (ZTF20aclgesv): optical discovery 2020-10-14; radio emerged between 2019-04-22 and 2021-11-21: below ~0.6 mJy to 1.65 mJy, z=0.078, log L(3GHz) 29.4.
2. AT 2021ahvo (ZTF17aadqidc): optical discovery 2021-01-04; radio emerged between 2019-04-23 and 2021-12-03: below ~0.6 mJy to 2.15 mJy, z=0.087, log L(3GHz) 29.6.
3. AT 2020nbs (ZTF18acgxjuy): optical discovery 2020-06-16; radio emerged between 2017-11-01 and 2020-10-20: below ~0.6 mJy to 2.06 mJy, z(phot)=0.025, log L(3GHz) approx 28.5.
4. AT 2018jir (ZTF18acrdhcf): optical discovery 2018-11-25; radio emerged between 2017-11-21 and 2020-07-18: below ~0.6 mJy to 1.15 mJy, z=0.038, log L(3GHz) 28.6.
5. AT 2021syi: optical discovery 2021-07-08; radio emerged between 2019-04-25 and 2021-11-30: below ~0.6 mJy to 1.31 mJy, z(phot)=0.127, log L(3GHz) approx 29.8.

Fluxes are catalogued Quick Look values (systematically about 10-15 percent low; Gordon et al. 2021, 2025). These are candidates: optical light-curve morphology and difference-image stamps have been human-vetted, but spectroscopic classification exists for none of them. We encourage spectroscopy of the hosts and GHz-band monitoring. Methods and per-object evidence packages are available on request, and the pipeline is reproducible from public archives.

Related Objects rows (one per line: IAU name | RA | DEC | Obj-Type | redshift):