Optical spectroscopy of the 2025 outburst of recurrent nova M31N 2008-12a
ATel #17521; Dipankar P. K. Banerjee (PRL, India), Saurabh Sharma (ARIES, India), Vishal Joshi (PRL, India), Kartik Ghanshyam Gokhe (ARIES, India), Ayush Rana (PRL, India)
on 2 Dec 2025; 16:17 UT
Credential Certification: Vishal Joshi (vjoshi@prl.res.in)
We obtained an optical spectrum (3500-8500A) of the recurrent nova M31N 2008-12a during its current 2025 outburst reported in the CBAT Transient Object Follow-up reports by K. Itagaki 2025.11.30.3634 (also see ATel #17511, ATel #17514, ATel #17520). The spectrum was obtained on 2025 November 30.723UT (mid-observing time). using the ARIES Devasthal Faint Object Spectrograph and Camera (ADFOSC; Omar et al., 2019, Current Science, 116, 1472) at the 3.6-meter Devasthal Optical Telescope (DOT). We used Grism 676R (3500-8950A), a 2 arcsec slit giving a spectral resolving power of R ~ 600 and a 3600s exposure time.
The spectrum comprises of emission lines of H, He I, He II, NII and NIII. The double-peaked Halpha line sits on a broad pedestal, with a remarkable unconvolved Full Width at Zero Intensity (FWZI) of ~ 15000 Km/s. The pedestal is reminiscent of the Halpha profile seen by Darnley et al. (2016, ApJ, 833, 149) on day 0.67 p.m. 0.02 past discovery of the 2015 eruption, which had a base width of ~13000 km/s and which was attributed to collimated flows. However, the extended red wing of the broad Halpha emission that we see is partially due to the presence of the He I 6678 A line. He II 4686A is also detected, merged with the Bowen blend (4640-4650 A). There is a prominent emission feature blueward of H Alpha in the region covering the NII 6346A line and the red coronal line of [FeX] 6375A. If part of this feature is indeed coronal [Fe X], then its early occurrence suggests it is likely due to shock heating of the giant secondary's wind by interaction with the high velocity nova ejecta. Photoionization, as the causative mechanism, is unlikely as the super soft X-ray phase (generally starting on days 5 - 6) is yet to begin. Further observations are encouraged.
DPKB expresses his thanks to Director, ARIES for the allotment of Director's Discretionary time for the observations. He also thanks Sudanshu Barway and Shatakshi Chamoli of IIA, Bengaluru for alerting us early, as well as Varun Bhalerao of IIT, Mumbai. We also thank the Observatory staff of DOT for their support.
