Pre-discovery measurements of probable symbiotic nova AT 2025abao in M31
ATel #17527; Juan Fabregat (OAUV), Vicent Peris (OAUV), Alicia Lozano, Juan Conejero (PixInsight)
on 5 Dec 2025; 15:36 UT
Credential Certification: Juan Fabregat (juan.fabregat@uv.es)
Subjects: Optical, Cataclysmic Variable, Nova, Transient
We announce pre-discovery measurements of probable symbiotic nova in M31 AT 2025abao (ATEL #17476). Our images were acquired on the nights of October 13 to 14 and 14 to 15 at AstroCamp telescope hosting in Spain with a Planewave DeltaRho 350 telescope (35 cm f3, 29 cm effective aperture). The images show the nova already exploding in luminance and H-alpha filters on Oct. 13 and RGB and Johnson I on Oct. 14.
Being a photographic telescope, the nova was imaged with photographic filters. We used the APASS catalog to calibrate the photometry in RGB filters, using the catalog magnitudes closer to our filter bandpasses:
B -> Johnson B
G -> Johnson V
R -> SDSS r'
To calibrate the photometry in the luminance filer we used the Gaia BP filter.
We also took 9x10 min exposures in H-alpha on the Oct. 13 night. Using a 60-hour exposure image taken at the Observatorio de Aras de los Olmos in Spain with a 14 cm f2.8 telescope in 2021, we have been able to measure the increment in brightness in the H-alpha band from the nova baseline state. To measure it, we calculated the zero point of both H-alpha images using the APASS catalog SDSS r' magnitudes and compensated the bandwidth difference in both images (5 nm in the 2021 image vs 3.5 nm in the 2025 image). In Oct. 13, the nova presented a brightness increment in the H-alpha band of 0.54 magnitudes for a 5 nm bandwidth.
The average magnitudes and their associated uncertainties are below:
Oct. Day Bandpass Magnitude
13.886 Gaia BP 18.995 +-0.059
14.895 Johnson B 19.730 +-0.058
14.909 Johnson V 18.612 +-0.069
14.917 SDSS r' 18.068 +-0.069
15.107 Johnson I 17.235 +-0.065
For further analysis, we provide a table with all the measurement, the calibrated images from the AstroCamp and Aras de los Olmos observatories, and the filter transmission curves in the following link:
AT2525abao/AT2025abao_data.zip"> http://observatori.uv.es/AT2525abao/AT2025abao_data.zip
