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AT2025agpz in Rubin commissioning data: distinguishing a luminous interacting supernova from nuclear transients in compact galaxies

This paper characterizes AT2025agpz, a luminous transient in a compact dwarf galaxy discovered during Rubin Observatory commissioning, as an interaction-powered supernova distinguished from ambiguous nuclear transients by its slow spectral evolution, specific emission-line morphology, and early-time light curve properties.

Original authors: C. R. Angus, M. Quilt, H. F. Stevance, M. Nicholl, S. J. Smartt, P. Wiseman, A. Möller, C. T. Murphey, P. J. Pessi, K. Auchettl, M. Dennefeld, M. Dominik, C. Frohmaier, P. Francis, M. Gromadzki, A. La

Published 2026-08-07

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CC BY 4.0

Original authors: C. R. Angus, M. Quilt, H. F. Stevance, M. Nicholl, S. J. Smartt, P. Wiseman, A. Möller, C. T. Murphey, P. J. Pessi, K. Auchettl, M. Dennefeld, M. Dominik, C. Frohmaier, P. Francis, M. Gromadzki, A. Lawrence, G. Leloudas, C. Lidman, D. Magill, I. Mandel, B. Martin, S. Mattila, G. Narayan, F. Onori, S. R. Oates, H. M. L. Perkins, L. Rauf, A. Rest, S. Romagnoli, R. Roy, S. Schulze, X. Sheng, K. W. Smith, K. de Soto, P. M. Veres, M. E. Verrico, A. Wasserman, R. Williams, Ł. Wyrzykowski, D. R. Young

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, chaotic stage where stars are born, live, and eventually die in spectacular explosions. For decades, astronomers have been trying to sort these cosmic fireworks into neat categories: some are standard supernovae (like a firecracker), others are super-luminous supernovae (like a massive flare gun), and some are nuclear transients, which are like sudden, bright flares from the hearts of galaxies caused by black holes eating stars. The problem is, the universe doesn't always follow the rulebook. Sometimes, an explosion happens so far away or looks so much like a black hole event that it's impossible to tell what's actually going on just by looking at the light. This is a big headache for scientists because if they misidentify the "actor" on stage, they can't understand the "script" of how stars die or how black holes behave. With new, incredibly powerful telescopes coming online that will find thousands of these events, figuring out how to tell them apart quickly and accurately has become one of the most urgent puzzles in modern astronomy.

Enter AT 2025agpz, a cosmic mystery that recently popped up in the data from the Rubin Observatory, a high-tech camera system currently being tested. This object is a bit of a chameleon. It's a brilliant, long-lasting flash of light that sits right in the middle of a faint, small galaxy. At first glance, it looks suspiciously like a "nuclear transient"—a sign that a supermassive black hole in the center of that galaxy might be feasting on a star. However, a team of astronomers led by C. R. Angus decided to take a closer look, using a combination of deep, high-speed snapshots and detailed spectroscopy (which breaks the light down into a rainbow to reveal its chemical makeup).

What they found is that AT 2025agpz is likely not a black hole eating a star, but rather a massive star that exploded and is now crashing into a thick shell of gas it shed before it died. Think of it like a runner (the exploding star) sprinting into a crowd of fans (the gas shell); the collision creates a massive, glowing shockwave that outshines the galaxy itself. The team measured that this explosion took a very long time to reach its peak brightness—about 77.9 days in its own time frame—and shone with a power of 6.3 × 10^43 erg s^-1. When they looked at the light through a high-resolution "prism," they saw specific patterns in the hydrogen gas that are characteristic of this kind of violent collision, rather than the steady glow of a black hole's accretion disk.

The paper suggests that while AT 2025agpz looks like a black hole event in many ways (it's bright, it's in the center of a galaxy, and it changes slowly), the evidence points toward it being a "super-luminous interacting supernova." The host galaxy is a small, star-forming dwarf, which is a common birthplace for these types of massive stellar explosions, whereas the black hole theory would require a tiny, hard-to-find black hole that doesn't quite fit the usual profiles. The authors note that this event highlights a growing "gray area" in astronomy where these two very different physical processes look almost identical from a distance.

One of the most exciting parts of the study is how they caught it. The Rubin Observatory, even while still in its testing phase, spotted the explosion days before it was officially discovered by other telescopes. This allowed the team to measure exactly how fast the light rose, finding a steep, power-law increase of 3.33. This is much faster than a standard fireball expansion, suggesting the explosion hit that dense gas shell almost immediately. The team also tested a computer program designed to automatically sort these events. The program was initially confused, flipping back and forth between guessing "black hole" and "supernova," which proves just how tricky these objects are.

Ultimately, the paper concludes that AT 2025agpz is most likely a supernova, but it serves as a warning: as we find more of these events, the lines between "star explosion" and "black hole feast" are blurring. It suggests that some of the mysterious "Ambiguous Nuclear Transients" we've been seeing might actually be these rare, gas-crashing supernovae in disguise. The authors emphasize that while they are fairly confident in the supernova interpretation based on the host galaxy and the specific shape of the light, the similarity to nuclear events is so strong that we need better tools and more data to be 100% sure. This discovery is a crucial step in learning how to tell these cosmic twins apart before the next generation of telescopes floods us with thousands of them.

Technical Summary: AT 2025agpz in Rubin Commissioning Data

Problem Statement
The upcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will discover unprecedented numbers of rare, long-lived optical transients. A significant challenge arises from the growing observational overlap between distinct physical classes: luminous interacting supernovae (specifically hydrogen-rich Superluminous Supernovae, SLSNe-IIn) and the recently identified population of Ambiguous Nuclear Transients (ANTs). Both classes exhibit long rise times, blue continua, and strong Balmer emission. Furthermore, the preference of SLSNe-IIn for faint dwarf galaxies, combined with the angular resolution limits of wide-field surveys, often results in transients appearing coincident with galactic nuclei, mimicking nuclear flares. With spectroscopic follow-up resources insufficient for the anticipated volume of LSST discoveries, robust photometric classification is critical. However, current classifiers struggle with "borderline" events that occupy the shared parameter space of these populations.

Methodology
The authors present a multi-faceted characterization of AT 2025agpz, a luminous transient discovered by ATLAS but extensively monitored during the commissioning phase of the Rubin Observatory in the Euclid Deep Field South (EDFS).

Photometry: The study utilizes deep, high-cadence difference imaging from Rubin (LSSTCam) and the Dark Energy Camera (DECam), alongside ATLAS and Las Cumbres Observatory (LCO) data. Rubin and DECam provided deep pre-discovery non-detections and early detections, tightly constraining the explosion epoch. The authors applied a power-law model to the early flux data to determine the rise index and explosion time.
Spectroscopy: Low-resolution spectra were obtained with VLT/FORS2, while higher-resolution kinematic structure was analyzed using VLT/X-shooter and ANU/WiFeS. The authors performed multi-component Gaussian decomposition of the H $\alpha$ profile to resolve narrow, intermediate, and broad velocity components.
Host Galaxy Analysis: Spectral Energy Distribution (SED) fitting was performed using Prospector on archival optical and near-infrared photometry (Euclid Q1 MER and DECam) to derive stellar mass, star formation rate, and metallicity.
Classification Testing: The authors tested the transient against the ANTEATER machine-learning classifier (a Random Forest model trained on simulated LSST light curves for ANTs, AGN, TDEs, and SNe IIn) under various data configurations (full LSST bands, ZTF-like depth, and band limitations) to assess photometric discriminability.

Key Results

Discovery and Rise: AT 2025agpz was detected significantly earlier by Rubin and DECam than by ATLAS. The deep pre-discovery imaging constrained the explosion epoch to MJD 60999.19, yielding a rest-frame rise time of $77.9 \pm 1.3$ days. A power-law fit to the early rise yielded a steep index of $n = 3.33^{+0.33}_{-0.28}$ , steeper than the canonical $n=2$ for homologous expansion.
Photometric Properties: The transient reached a peak bolometric luminosity of $6.3 \times 10^{43}$ erg s $^{-1}$ and radiated $\sim 7.2 \times 10^{50}$ erg over 191 days. The light curve showed a gradual reddening ( $g-r$ evolving from $-0.36$ to $\sim 0.16$ ) and a cooling blackbody temperature, consistent with interaction-powered supernovae but overlapping with some ANT candidates.
Spectroscopic Evolution: Spectra are dominated by narrow Balmer emission with broad electron-scattering wings, showing remarkably slow evolution over $\sim 100$ rest-frame days. High-resolution X-shooter data resolved the H $\alpha$ profile into a narrow core ( $\sim 70$ km s $^{-1}$ ), an intermediate component ( $\sim 320$ km s $^{-1}$ ), and a broad component ( $\sim 2300$ km s $^{-1}$ ), alongside a weak blueshifted absorption feature suggestive of a P-Cygni profile.
Host Galaxy: The host is a low-mass ( $\log(M_*/M_\odot) \approx 8.3$ ), moderately star-forming dwarf galaxy with sub-solar metallicity. While consistent with SLSN-II hosts, the location is projected to the nucleus (offset $0.16 \pm 0.19$ arcsec), a feature common to both SLSNe and ANTs.
Classification Ambiguity: The ANTEATER classifier initially struggled to distinguish AT 2025agpz from an SN IIn, eventually favoring an ANT classification after $\sim 50$ days due to the exceptionally long rise time. This highlights a bias in current training sets where extreme SLSNe-IIn are under-represented.

Conclusions and Significance
The authors conclude that AT 2025agpz is most likely a hydrogen-rich, interaction-powered superluminous supernova (SLSN-IIn). This interpretation is favored by the steep early rise index, the specific evolution of the H $\alpha$ line profile (broad wings and P-Cygni features), and the host galaxy properties (low mass, star-forming). However, the event's observational properties—long rise time, nuclear location, and spectral morphology—overlap significantly with the ANT population.

The significance of this work lies in demonstrating that luminous interacting supernovae can occupy the same observational parameter space currently used to define ANTs. The paper argues that without high-resolution spectroscopy, which will be unavailable for the majority of LSST transients, distinguishing these classes based solely on photometry or low-resolution spectra is extremely difficult. AT 2025agpz serves as a critical test case, illustrating that current classification frameworks may misidentify extreme interacting supernovae as nuclear transients. The authors emphasize that expanding the spectroscopic training sets for machine-learning classifiers to include such long-rising SLSNe-IIn is essential for the Rubin era to avoid sample contamination and to correctly identify the physical progenitors of these rare events.

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