DRAFT
2026-02-25 09:31:39
Type: Object/s-Data/Analysis
Bibcode: 2026TNSAN..45....1A
Spectroscopic Observations of AT 2025agpz, a Long-Rising Hydrogen-Rich Transient in Rubin-LSST Commissioning Data
Authors: C. R. Angus (QUB), H. Stevance (Oxford), M. Nicholl (QUB), K. W. Smith, S. Smartt (Oxford/QUB), D. R. Young, M. Fulton, M. McCollum, T. Moore, J. Weston, X. Sheng, A. Aamer, D. Magill, A. J. Smith (QUB), P. Ramsden (QUB/Birmingham), L. Shingles (GSI/QUB), S. Srivastav, J. Gillanders, A. J. Cooper, F. Stoppa, J. Tweddle, L. Eastman (Oxford), L. Rhodes (TSI/McGill), L. Denneau, J. Tonry, H. Weiland, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), A. Jordan, V. Suc (UAI, Obstech), M. R. Alarcón, J. Licandro, P. Nichita (IAC), A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard), J. Sommer (LMU), B. P. Schmidt (ANU)
Source Group: OxQUB
Abstract:
We report spectroscopic observations of AT 2025agpz (ATLAS25pny), a luminous, long-rising hydrogen-rich transient discovered by ATLAS and independently detected in Rubin-LSST Deep Drilling Field commissioning data. Rubin detections (at r~23.0 +/- 0.1) prior to the ATLAS discovery reveal that the source has a rise time of over 75 days in the rest frame. Two VLT/FORS spectra obtained on 2026-01-20 and 2026-02-07 show blue continua with strong, narrow Balmer and He I emission at z = 0.147 and exhibit minimal evolution between epochs. The luminosity, slow rise, and persistent narrow emission places AT 2025agpz at the boundary between hydrogen-rich interacting supernovae and long-duration nuclear transients, including ambiguous nuclear transients and AGN turn-on events. Continued spectroscopic and photometric monitoring is encouraged to confirm its classification.

We report spectroscopic observations of AT 2025agpz (ATLAS25pny), a long rising transient with narrow hydrogen and helium emission at z = 0.147.

AT 2025agpz was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018, PASP, 130, 064505) on MJD 61019.92 (2025-12-10.92) at c = 19.43 +/- 0.19 mag. The transient is offset by 0.02″ north and 0.10″ west of a faint host galaxy (r = 23.75 mag) in the DESI catalogue (DESI Collaboration et al. 2025, DOI:10.48550), consistent with an approximately nuclear location. 

AT 2025agpz lies within the Euclid Deep Field South (EDFS) Rubin-LSST Deep Drilling Field, which was observed during commissioning between MJD 61003 and MJD 61095. Transient alerts from these observations were ingested in the Lasair alert broker (Williams et al. 2024, RASTI, 3, 362) during the commissioning and science verification period and discovered during testing of the Lasair Virtual Research Assistant (based on Stevance et al. 2025, ApJ, 990, 20; further details see documentation). Scanning of forced LSST photometry in Lasair showed a first detection of AT 2025agpz on MJD 61004.30 (2025-11-25.30) at  r=23.07 +/ 0.10. The smoothly rising light curve peaked at  r=18.78 +/-  0.01 on MJD 61090.1 (2026-02-19.1), such that the event rose for almost 86 days in the observer frame (75 days in the rest frame). The LSST light curve is available at https://lasair.lsst.ac.uk/objects/313761043604045880/.

We obtained an initial spectrum with VLT/FORS (PI: Angus) on 2026-01-20 (27 days before peak in the rest frame). The spectrum shows a blue continuum with strong Balmer and He I emission at z = 0.147. At this redshift, the transient peaked at an absolute magnitude of M_r = -20.4 +/- 0.2 mag (distance modulus m-M = 39.23; Galactic extinction A_g = 0.04, A_r = 0.03). A second FORS spectrum obtained on 2026-02-07 (11 days before peak) shows little spectral evolution. 

The high luminosity, long rise time, minimal spectral evolution, and position consistent with the nucleus of its faint host make AT 2025agpz consistent with both hydrogen-rich interacting supernovae (including SLSN-II) and long-duration nuclear transients such as Ambiguous Nuclear Transients (e.g. Wiseman et al. 2025, MNRAS, 537, 2024) or the ignition of a previously dormant AGN (e.g. Ridley et al. 2024, MNRAS, 531, 1905). Continued spectroscopic and multi-band photometric monitoring will be essential to distinguish between these scenarios.

ATLAS forced photometry indicates the transient currently remains bright at o = 18.87 +/- 0.19 mag.

Show current TNS values
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 2025agpz [ATLAS25pny] 04:07:51.568 -48:42:47.65 04:07:51.557 -48:42:48.40
File Comments
2025agpz_FORS_20012026_0.txt VLT/FORS spectrum 20/01/2026
2025agpz_FORS2_spectra_0.png Spectral evolution of AT 2025agpz

Comments