Optical photometry and spectroscopy of Nova AT 2025qmf in M31

ATel #17366; A. S. Vinokurov (SAO RAS), O. I. Spiridonova (SAO RAS), A. N. Tarasenkov (INASAN, SAI MSU), R. Fidrich (HAA/VSS, Budakeszi, Hungary), T. Tordai (HAA/VSS, Budapest, Hungary), D. V. Oparin (SAO RAS), V. P. Goranskij (SAO RAS), E. O. Dedov (SAO RAS)
on 31 Aug 2025; 19:57 UT
Credential Certification: Alexander Vinokurov (vinokurov@sao.ru)

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We present the results of the photometric monitoring and three spectroscopic observations of Nova AT 2025qmf carried out between 29 July and 29 August 2025. The data were obtained with seven telescopes: the 0.3m Newtonian reflextor of T. Tordai; the 0.43m CDK reflector, Alnitak Remote Observatory at Nerpio; the 0.5m RC500 telescope of the Institute of Astronomy of the Russian Academy of Sciences (RAS); the 0.51m CDK reflector (iTelescope.net network), Utah Desert Remote Observatory at Great Basin Desert; the 0.6m Planewave CDK telescope of Saint Mary's University Burke-Gaffney Telescope; and the 1m Zeiss-1000 and the 6m BTA telescopes of the Special Astrophysical Observatory of the RAS.

Analysis of the photometric data indicates that AT 2025qmf remains a bright source in the optical range, with an average magnitude of B=18.8, V=18.6, and Rc=18.2. Over the past month, the nova has exhibited several episodes of brightness increase and decline with an amplitude of approximately 0.7 mag. The spectral energy distribution constructed from the Zeiss-1000 UBVRcIc photometry (on the night of 27/28 August) shows a strong deviation from a blackbody spectrum (χ²/d.o.f.=49 for the best-fit model), which is in sharp contrast to the satisfactory approximation obtained for a similar dataset in late July (ATel #17314). The observed excesses in the U, Rc, and Ic bands suggest a significant contribution from Hα emission and free-bound radiation.

The first two spectroscopic observations were performed with the Zeiss-1000/MAGIC using the VPHG600@500 grism (spectral range ~3700-7300A, resolution 10A) on 29 and 30 July. For each epoch, only two 600s spectra were obtained, which were co-added to increase the signal-to-noise ratio (S/N). The object's brightness remained constant within the measurement errors during this period, Rc = 17.9-18.0 mag. The most prominent emission features in the combined spectrum are the hydrogen Balmer lines Hα and Hβ, and the lines of ionized iron FeII 4924,5018,5169. No clear absorption components were found, as were observed in the P Cyg profiles of the same spectral lines a few days later (TNS Classification Report No. 2025-3060). The width of the most prominent line after correction for the instrumental profile is FWHM(Hα)~800 km/s; its equivalent width is EW(Hα)~-300A.

The third spectrum was acquired on the night of 21/22 August with the SCORPIO-2 spectrograph on the 6-m SAO RAS telescope. The VPHG1200@540 grism with a 1" slit was used, covering the 3650-7250A range with a spectral resolution of about 5A. The total exposure time was 600s. In this spectrum, the Hα emission component is broader (~1100 km/s), and its equivalent width reaches nearly -600A. The high S/N of the spectrum allowed the detection of absorption components in all hydrogen lines up to Hε. The ejecta velocity, measured from the blue-edge absorption component relative to the emission peak of the hydrogen lines, is estimated to be 1700-1900 km/s. The spectrum also exhibits numerous FeII lines, some of which also show well-defined P Cyg profiles; the expansion velocity derived from these lines is 1400-1700 km/s. In addition to the aforementioned lines, there are notable features in the spectrum such as the sodium NaI D doublet and the forbidden oxygen lines [O I] 5577,6300,6363. The presence of the latter indicates that the nova has entered the nebular phase.

The work was performed as part of the SAO RAS government contract approved by the Ministry of Science and Higher Education of the Russian Federation. Observations with the SAO RAS telescopes are supported by the Ministry of Science and Higher Education of the Russian Federation. The renovation of telescope equipment is currently provided within the national project "Science and Universities". The observations made using the Alnitak Remote Observatories robotic telescope network, were supported by ARTA program.