Spectroscopic and Photometric confirmation of MASTER OT J030227.28+191754.5 as a very large-amplitude WZ Sge-type dwarf nova

ATel #15074; Keisuke Isogai, Yusuke Tampo, Masayuki Yamanaka, Naoto Kojiguchi, Kenta Taguchi, Junpei Ito, Masaaki Shibata, Daisaku Nogami, Taichi Kato (Kyoto University)
on 29 Nov 2021; 16:59 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report the results of our further follow-up spectroscopy and photometry of the large outburst (~10 mag) transient MASTER OT J030227.28+191754.5 = AT 2021afpi = PNV J03022732+1917552. As already reported to ATel #15067, #15069, #15072, #15073, this object was reported as a possible counterpart to IceCube-211125A. Zhirkov et al. indicated that this object is an He/N-class nova based on their low-resolution spectrum (ATel #15067). We also reported our spectra and proposed that this object is not a usual classical nova but a possible "narrow-lined He nova" or large-amplitude dwarf nova (ATel #15072). We additionally obtained low and moderate resolution (R ∼ 500 and 1500) spectra from 2021-11-29.509 UT to 2021-11-29.521 UT using the fiber-fed integral field spectrograph (KOOLS-IFU; Matsubayashi et al. 2019) mounted on the 3.8 m Seimei telescope (Kurita et al. 2020) at Okayama Observatory of Kyoto University.

Our quick look spectra on 2021-11-29 show Balmer, He I, and He II emission lines. In comparison with our spectra obtained last night (ATel #15072), Bowen blend and O I 7773 were obscured, and He I/II lines were also weakened. Furthermore, the profiles of Hβ and He I/II lines changed from single to double-peaked. The peak separation of He I/II is about 500 km/s and that of Hβ is about 250 km/s. These features indicate that this object is not a classical nova but a dwarf nova.

We also performed time-series photometry for about 7 hours by using a 40-cm telescope. We detected double-peaked modulations with a period of 0.05935 +/- 0.0020 day and amplitudes of 0.05 mag. The characteristics are consistent with those of early superhumps. Thus, we conclude that this object is a WZ Sge-type dwarf nova having the largest outburst amplitude so far.

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