A Photometric Analysis of the Luminous Red Nova (LRN) AT2025abao with Amateur Tools
Object: AT2025abao (Luminous Red Nova)
Host Galaxy: Andromeda (M31)
Current Status: Post-Maximum Decline Phase
Abstract
AT2025abao is a Luminous Red Nova (LRN) located in M31, the Andromeda Galaxy. Following its discovery in October 2025 and a subsequent peak in late 2025, the object is now transitioning through its decline phase. This article provides photometric data and technical analysis of the event’s evolution, utilizing V-band filtered CCD/CMOS observations to track its luminosity changes.
1. Stellar Mergers and Luminous Red Novae
Luminous Red Novae (LRN) are rare astronomical transients resulting from the merger of two stars in a binary system, a process scientifically known as Common Envelope Evolution. As the stellar orbits decay, the stars eventually coalesce into a single object. During this merger, a significant portion of the shared stellar envelope is ejected, creating a rapidly expanding and cooling photosphere. This physical evolution causes the object’s characteristic shift toward longer, redder wavelengths and produces a distinct light curve that differs significantly from both the thermonuclear explosions of classical novae and the core-collapse events of supernovae.
Discovery and Early Observations
AT2025abao (also cataloged as MASTER OT J003848.65+404607.5) was first identified on October 17, 2025, by the prolific Japanese astronomer Koichi Itagaki. Located in the southwest region of the Andromeda Galaxy (M31), the transient was initially detected at a faint magnitude of approximately 17.8.
Following the initial detection, the object underwent a notable brightening phase. By late October 2025, it reached a peak visual magnitude of approximately 14.6, becoming one of the brightest extragalactic transients visible in the northern hemisphere at that time. This brightness made it an accessible target for amateur telescopes and a subject of intense study for professional observatories like the Zwicky Transient Facility (ZTF).
Current Scientific Understanding
LRNe like AT2025abao are considered “intermediate-luminosity optical transients. They are brighter than standard novae but generally fainter than supernovae. Archival data from surveys such as Pan-STARRS suggest that the progenitor of this event was a faint, red stellar system around magnitude 21, which is consistent with a binary system undergoing a “merge-burst.”
As of early 2026, the object is in a slow decline. Because LRNe often produce vast amounts of dust as their ejected shells expand, they frequently show a secondary “plateau” or even a resurgence in infrared brightness as the visible light fades. Tracking the current fade rate is essential for determining the total energy released and the mass of the ejected material.
2. Observation Logs & Data Acquisition
The following observations were conducted under average-to-good seeing conditions, with the target positioned near the zenith to minimize atmospheric extinction.
| Parameter | Specification |
| Optics | 60mm Achromatic Refractor (measured focal length: 488mm) |
| Sensor | ToupTek G3M178M (IMX178 CMOS) |
| Filters | Bessel V-Band + UV/IR Cut |
| Integration | 42 x 60s (42 minutes total) |
| Calibration | Darks, Bias, Flats applied in Siril |
3. Photometric Analysis: January 7, 2026
Differential photometry was performed using AstroImageJ (AIJ). Comparison stars were selected from the AAVSO database, prioritizing targets with B-V color indices between 0.60 and 0.77 to align with the expected spectral profile of the LRN and mitigate potential color-dependent errors inherent in achromatic optics.
Results of Measurement (2026-01-07):
- Mean Magnitude: 15.08 V
- Statistical Error: ± 0.03 mag (including catalog uncertainty)
- Limiting Magnitude: 16.5 V
4. Evolutionary Trend
Initial reports in October 2025 placed the object at magnitude 17.8. Historical data from the Transient Name Server (TNS) indicate that AT2025abao reached a peak brightness of approximately 14.6 V in late 2025.
Our current measurement of 15.08 V confirms the object has begun its descent from the maximum. The current fade rate provides critical data for modeling the expansion velocity and cooling rate of the ejected stellar envelope.
5. Observations & Color Index
(Updated: 2026-01-12)
Our dual-filter monitoring campaign is now active. By combining V-band (Visual) and a calibrated “Synthetic Red” (Rsynth), we are tracking the expanding and cooling photosphere of the merger remnant. The following data highlights a clear transition: while the object is brightening overall, it is doing so much more rapidly in the red end of the spectrum.
Recent Photometry Data
| Date (UT) | Magnitude (V) | Magnitude (Rsynth) | Color Index (V−R) | Phase / Notes |
| 2026-01-07 | 15.08 ± 0.03 | — | — | Initial Obs. |
| 2026-01-10 | 15.05 ± 0.03 | 14.16 ± 0.02 | +0.89 | |
| 2026-01-11 | 14.99 ± 0.02 | 14.07 ± 0.02 | +0.92 | Expanding & Reddening |
Analysis: Physical Trend vs. Measurement Noise
A common challenge in amateur photometry is distinguishing between real stellar variability and “scatter” caused by seeing or sky transparency. To ensure data integrity, we implemented the following:
- Differential Photometry: By measuring the Nova against a stable “trio” of AAVSO-calibrated comparison stars (000-BNN-443, -444, and -463) within the same field of view, atmospheric extinction and thin clouds are mathematically cancelled out.
- 16-bit Linear Pipeline: We transitioned to a 16-bit unsigned integer workflow. This ensures that the high Signal-to-Noise Ratio (SNR) of our stacked images is preserved, providing a Zero-Point accuracy of ~0.02 mag.
- Statistical Significance: Between Jan 10 and 11, the Red-band brightened by 0.09 mag. This shift is nearly 5 times larger than our calculated system scatter (a “5-sigma” detection), confirming that the brightening is a real physical event.
Scientific Conclusion: The “Growing Giant”
The observed trend – simultaneous brightening in both bands with a widening V-R index – is a classic signature of a Luminous Red Nova (LRN) expansion.
As the binary merger remnant expands, its surface area increases, which causes the total brightness to rise (the “Luminous” part). Simultaneously, the gas is cooling, shifting the peak of its light emission toward the red (the “Red” part). The jump in color index from +0.89 to +0.92 is direct evidence of this rapid cooling phase.
References
- AAVSO Variable Star Index (VSX)
- Transient Name Server (TNS) – AT 2025abao
- Sonnensystem.com Technical Archive