SALT high resolution spectroscopy and MeerLICHT photometry of the LMC transient ASASSN-23ii (AT 2023ygp)

ATel #16369; D. A. H. Buckley (SAAO, UCT, UFS), E. Aydi (MSU), P. Groot (Radboud, SAAO, UCT), N. Erasmus (SAAO), C. Johnston (Radboud, MPA), P. M. Vreeswijk (Radboud), O. Mogawana (UCT, SAAO) J. Mikolajewska (NCAC), M. Orio (U. Wisconsin/INAF)
on 7 Dec 2023; 15:01 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)

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We report on high-resolution optical spectroscopy and additional multi-colour photometry of the optical transient ASASSN-23ii (AT 2023ygp), which was discovered by the All-Sky Automated Survey for SNe (ASAS-SN, Shappee et al. 2014, ApJ, 788, 48) on 2023-11-21.98 UT at a g-magnitude of 14.3. The transient, which is located in the LMC, reached a peak brightness of g=12.4 on 2023-11-23.23, before declining rapidly by more than 2 magnitudes in less than 5 days. The ASAS-SN light curve can be found here.

Combined MeerLICHT and BlackGEM-4 (Bloemen et al., Proc. SPIE, 9906, id. 9906642016, 2016) multi-colour photometry shows the source was at quiescent levels at u~20.5, q~20.5 and i~18-20, with considerable photometric variability between Jan 2019 and Apr 2023. After re-emerging from the solar shadow, BlackGEM observations show a significant brightening between Sep 14 and Sep 24, 2023, to levels of u~18.3, i~17.5 and q~17.2, where the source stayed until Nov 19, when the outburst reported by ASAS-SN started, with a peak flux around the same time as recorded by ASAS-SN.

We obtaining a low resolution (~16 Å) spectrum using the Mookodi spectrograph on the SAAO 1.0-m Lesedi telescope on 4 Dec, which showed a strong emission line source. A follow-up 1500 s spectrum was then obtained the same night (on 2023-12-04.9) using the High Resolution Spectrograph (HRS; Crause et al. 2014, Proc. SPIE, 91476), mounted on the 11m Southern African Large Telescope, as part of the SALT Large Science Program on Transients. The observations were taken in the LR mode of HRS, which covers the spectral range of 3800-8900 Å at a resolving power of R=14000 (~0.4Å resolution). The data were reduced with the SALT HRS MIDAS pipeline (Kniazev et al. 2016, MNRAS 459, 3068). The spectrum shows strong emission lines of Balmer, He I, He II, Fe II, N II, and O I. The line centers are redshifted by more than 250 km/s, consistent with the transient being in the LMC. The FWZI of Hα is more than 5000 km/s. The spectrum is consistent with that of an LMC nova during the decline from optical peak, in agreement with the optical light curve. We detect weak [O III] nebular lines at 5007 and 4959 Å, meaning that the nova might be approaching the nebular phase. The Balmer lines show P Cygni features on top of the broad emission at a velocity of around -70 km/s, which is likely originating in dense circumbinary material. These are common features observed in novae taking place in symbiotic systems (a nova with a giant secondary). The He I lines show evidence for satellite emission components at +/- 800 km/s, likely originating from bipolar outflows, which is also commonly seen in novae in symbiotic systems.

The symbiotic nature of AT2023ygp is further supported by its close coincidence (0.11 arcsec) with the C-rich Mira variable, OGLE-LMC-LPV-07945 (Bhardwaj et al. 2019, ApJ 884, 20B). The long ~480-day pulsation period of the Mira and its very red colours (J-K ~ 3.5 - 4, Cutri et al. 2006, , Kato et al. 2007, PASJ 59, 615) locate it among the extreme symbiotic Miras such as, e.g., RX Pup and SS73 38 (Gromadzki et al. 2009, AcA 59, 169).

Follow up observations in all bands are encouraged.