Spectroscopic and Photometric follow-up of MASTER OT J030227.28+191754.5: a Possible "Narrow-lined He Nova" or Large-amplitude Dwarf Nova

ATel #15072; Kenta Taguchi, Masaaki Shibata, Yamanaka Masayuki, Keisuke Isogai, Yusuke Tampo, Naoto Kojiguchi, Junpei Ito, Taichi Kato (Kyoto University)
on 29 Nov 2021; 04:07 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report our spectra of MASTER OT J030227.28+191754.5 (#15067, #15069) = AT 2021afpi = PNV J03022732+1917552, which was reported as a possible counterpart to IceCube-211125A (GCN #31126). We obtained a low-resolution (R ∼ 500) spectrum from 2021-11-28.5323 UT to 2021-11-28.5362 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.

As reported to Atel #15067, this object was discovered by MASTER-Tavrida auto-detection system at 15.1 mag (unfiltered) on 2021-11-26.82568 UT. As is reported to CBAT, it was also independently discovered by Yuji Nakamura at 11.7 mag (unfiltered) on 2021-11-28.4689. According to consequent observations by MASTER-Kislovodsk, this object brightened to 11.7 mag on 2021-11-27.70436 UT. There are no past such outbursts recorded in the ASAS-SN light curve.

Our spectra confirm the result of #16069. They are dominated by a blue continuum. The Balmer lines (Hα, Hβ, Hγ, and Hδ), the He I lines(4388, 4471, 4713, 4922, 5016, 5048, 5876, 6678, 7066, 7281 Å), the He II lines (4542, 4686, 5412 Å), the Bowen blend, and the O I line (7773 Å) are detected. All lines above show single-peaked emission profiles. We also performed moderately high resolution (R ∼ 2000) spectroscopy using KOOLS-IFU from 2021-11-28.5392 to 2021-11-28.5503 and confirmed that the Hα line has a single-peaked emission profile whose FWHM is ∼ 10 Å (∼ 500 km/s). Such a narrow emission line profile is unusual for a nova. We also performed time-series photometry using the 40 cm-telescope at Kyoto University. The object brightened by 0.2 mag in 5 hours from 2021-11-28.656 UT.

We found that the spectroscopic characteristics are similar to those of M31N 2007-10a, a "narrow-lined He nova (He/Nn)" proposed by Shafter et al. 2011. The large amplitude (∼ 10 mag, #15067) is also consistent that this object is a nova. Because of the narrow Hα line, which is unusual for novae, there is a possibility that this object is a WZ Sge-type dwarf nova with a very large amplitude. The characteristics of our spectra are also similar to those of (the WZ Sge-type dwarf nova) V455 And (Nogami et al. 2009). In any case, the rising rate of ∼ 3 mag/d (#15067) is slower than those of typical classical/dwarf novae. Follow-up observations are needed.

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