The AT2025ndr transient was reported on TNS as discovered by ZTF (internal name ZTF25aatpeel) on 2025-05-28 UT. It was discovered at an r-mag of 20.619 (AB). It was also observed by ATLAS and GOTO on 2025-06-04 and 2025-06-29 respectively. It reached a peak mag of 19.41 in the g-band and 19.29 in the r-band ~ 25 days from the first observation.
The nearest extended source (separation of 0.085”) in Legacy Survey DR10 has a z-band magnitude of 19.55 mag.
The transient was flagged as an interesting lensed supernova candidate owing to its brightness and redder colours that g-r of 0 at 2025-06-27. Interest in follow-up grew after the transient started to rebrighten by ~0.4 mag after 2025-06-27 over the period of a week. Follow-up spectroscopy of AT2025ndr was obtained with the SOAR Goodman spectrograph on 2025-06-22 (program ID SOAR2025A-028, PI Andreoni), on 2025-07-16 (program ID SOAR2025A-028, PI Andreoni), and on 2025-07-18 (program ID SOAR-2025A-013, PI Dhawan). From the SNID (Blondin & Tonry 2007) fit to the data, we conclude that it is spectroscopically a post-peak (~ +15 - +22 depending on the phase of the spectra) Type Ia supernova (SNIa), with some of the top matches including peculiar SNIa, at redshift z =0.185.
Based on this redshift and the peak apparent magnitude, this implies a peak absolute magnitude of M_R = -20.58 mag - brighter than a normal Type ia supernova by ~ 1 mag, suggesting possible magnification by lensing. The lightcurve does not appear consistent with any subtype of Type Ia supernova (including Ia-CSM) owing to both a longer than typical (~19d) rise time and the flattening / rebrightening of the LC post peak.. When fitting with a multiply-imaged SALT (Kenworthy+2021) model, we do not see that both g and r bands are well-fit simultaneously. We also inferred spectral typing from NGSF (Goldwasser+2022) forcing the template types to SLSN. We do not find a good spectral fit, the closest match is to LSQ14qmo at z = 0.2.
Therefore, while the lightcurve and spectra look SN-like any further subclassification is unclear at this time as no suggestions from the spectra are consistent with the lightcurve.
We urge the community to follow-up, specially high-resolution imaging to confirm or refute whether this is a lensed SN.
| Catalog | Name | Reported RA | Reported DEC | Reported Obj-Type | Reported Redshift | Host Name | Host Redshift | Remarks | TNS RA | TNS DEC | TNS Obj-Type | TNS Redshift |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| TNS | 2025ndr [ZTF25aatpeel] | 20:25:19.322 | -02:29:59.80 | SN | 0.185 | 20:25:19.322 | -02:29:59.80 | SN Ia-CSM | 0.182 |
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