Spectroscopic classification of ASASSN-24by (AT 2024epj) as a classical nova in the LMC

ATel #16545; J. Merc (Charles University), T. Love, P. Velez, H. Barker (ARAS Group), S. Charbonnel, O. Garde, P. Le Du, L. Mulato, T. Petit (2SPOT Team)
on 21 Mar 2024; 08:31 UT
Credential Certification: Jaroslav Merc (jaroslav.merc@mff.cuni.cz)

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The transient ASASSN-24by (AT 2024epj) was discovered by the ASAS-SN survey (Kochanek et al. 2017, PASP, 129, 4502) on 2024-03-18.088 UT at g = 10.2 mag and was subsequently classified as a nova candidate in the LMC. Additional ASAS-SN observations obtained during the following two days (see the light curve here: https://asas-sn.osu.edu/sky-patrol/coordinate/224fda81-e1b3-45a9-af3a-87fa97972e43) and the LCOGT follow-up (ATel #16543) showed that the object is fading very rapidly (by about 2.5 mag in two days). At 2024-03-20.087 UT, it had a magnitude of g = 12.7 mag.

Our photometric observations are as follows: 2024-03-19.511 UT, B = 12.41, V = 12.20, R = 11.43, I = 11.38; 2024-03-20.476 UT, B = 13.20, V = 12.96, R = 12.12, I = 12.23.

We have also obtained a medium-resolution spectrum (R~11000) of the transient at 2024-03-20.026 UT using the 30-cm Ritchey-Chretien telescope with eShel spectrograph located in Chile, a spectrum of the Halpha region (R~2770) at 2024-03-20.475 UT using 32- cm Planewave CDK telescope equipped with UVEX spectrograph located in Australia, and a low-resolution spectrum (R~640) at 2024-03-20.856 UT using the 30-cm Ritchey-Chretien telescope with Alpy600 spectrograph located in New Zealand. Eight exposures of 1200 s each, four exposures of 600 s each, and six exposures of 600 s each were summed to obtain the final spectra, respectively. While the signal-to-noise ratio is not very high, our data confirms the classical nova classification. Namely, very broad emission Balmer lines of hydrogen are present in the spectra (FWHM of Halpha = 6600 km/s, with the wings suggesting the expansion velocity of ~4500 km/s), with possible hints of He I and N III emission. The position of emission lines is consistent with the location of the source in the LMC. The peak brightness (of about -8.9 mag in g for the LMC distance) is consistent with this classification as well.

The evolution of the nova reminds the fast LMC nova ASASSN-23hd detected in October 2023 (ATel #16294).

The data are continually submitted to the ARAS database.