AstroNote 2025-142

We report the spectroscopic classification of the optical re-brightening observed towards the center of the galaxy SDSS J124000.57+010924.3 (z=0.07959) as a likely Supernova of type Ia-peculiar (SNIa-pec). The optical transient was discovered based on new Zwicky Transient Facility (ZTF) data in April 2025 and reported in Munoz-Arancibia+2025. The latest observations show that the source may still be rising. 

We obtained two independent spectroscopic observations using the low-resolution spectrograph #2 (LRS-2, Chonis+2016) on the 10m Hobby-Eberly Telescope (HET, Ramsey+1998, Shetrone+2007) on April 30th, 2025, and with the FLOYDS spectrograph on the 2m Faulkes Telescope North (Brown+2013) on May 3rd, 2025. The spectra display narrow emission lines coming from the host galaxy, in agreement with an archival 2001 SDSS spectrum (https://skyserver.sdss.org/DR18/en/get/SpecById.ashx?id=327727614946994176). In addition, deep and broad absorption features are detected around 6150Å, 5400Å, 4300Å, and 4700Å in the rest frame, characteristic of SNIa. We model the spectra with the Next Generation SuperFit python code (NGSF, Goldwasser+2022, based on the IDL routine of Howell+2005, https://github.com/oyaron/NGSF), and find the best fitting templates to be those of SNIa-pec. 

The optical transient was initially associated with AT2021gqu (ZTF21aapriwm), discovered by ZTF on March 20, 2021. The event was classified by the ALeRCE broker as a supernova candidate, and Aleo+2023 classified it as SN type Ibc based on photometry. However, no spectroscopical classification was reported. 

The SN classifications of the 2025 event and a spatial offset between the 2021 and 2025 ZTF detections of 0.6” strengthen the interpretation of AT2021gqu and the new transient being unrelated. Multi-wavelength follow-up observations for this source are ongoing.

This report is based on observations obtained with the Hobby-Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August Universität Göttingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing high-performance computing, visualization, and storage resources that have contributed to the results reported within this note. This report also made use of observations obtained with the FLOYDS spectrograph on the 2-meter Faulkes Telescopes, operated by Las Cumbres Observatory. We acknowledge Las Cumbres Observatory and its staff for their continued support of robotic observations.

References: Aleo, P.D., et al. 2023, ApJS, 266, 9A; Brown, T.M., et al. 2013, PASP, 125, 1031; Chonis, T.S., et al. 2016, Proc. SPIE, 9908, 99084C; Goldwasser, S., et al. 2022, TNS Astronomer’s Telegram, 191, 1G; Howell, A., et al. 2005, ApJ, 634, 1190H; Muñoz-Arancibia, A., et al. 2025, TNS Astronomer’s Telegram, 123, 1M; Ramsey, L.W., et al. 1998, Proc. SPIE, 3352, 34; Shetrone, M.D., et al. 2007, PASP, 119, 556