Spectroscopic confirmation of NSV 11599 (=Gaia20fjv, AT2020aavq) as a dwarf nova having been misidentified with a W UMa star

ATel #14274; Keisuke Isogai, Masaaki Otsuka, Taichi Kato, Daisaku Nogami (Kyoto University)
on 17 Dec 2020; 12:53 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report on spectral features seen in the transient NSV 11599 (=Gaia20fjv, AT2020aavq) on 2020-12-09.376 UT obtained by using a fiber-fed integral field spectrograph (KOOLS-IFU; Matsubayashi et al. 2019) attached to one of the Nathmisth focuses of the 3.8-m telescope Seimei at Okayama Observatory of Kyoto University. In short, we conclude that this object would be a dwarf nova based on the following discussions.

NSV 11599 has been classified as an eclipsing binary (e.g. GCVS) and the light curve has shown W UMa-type variations. However, Gaia detected an unexplained outburst/flare from a W UMa star at G=14.18 mag on 2020-11-21.886 UT. It is of importance that five outbursts/flares at least have been detected by Gaia and ASAS-SN Sky Patrol (Shappee et al. 2014, Kochanek et al. 2017) (in 2014, 2015, 2017, 2019, 2020). From these data, the outburst/flare amplitude is about 2 mag, the duration is 3-4 weeks, and intervals are less than 2 years.

We estimated the orbital period to be 0.4294164(6) by using ASAS-SN Sky Patrol. This period is in good agreement with the value given in the ATLAS variable star catalog (P=0.429416 d) (Heinze et al. 2018).

We recently secured an optical spectrum around the end of the 2020 outburst using KOOLS-IFU. The spectrum clearly shows the strong Balmer emission lines and Na I D and Mg I b absorption lines. The appearance of these Balmer lines indicates that this object is not a W UMa star but a dwarf nova harboring an accretion disk. The metal absorption is thought to originate from the bright secondary star. Because eclipses were seen during past outbursts, this object is a deep-eclipsing dwarf nova.

This object is very similar to 1SWASP J162117+441254. The characteristics of 1SWASP J162117+441254 are low-amplitude, low-frequency, and long-duration outbursts and W UMa-like variations in quiescence. Kimura et al. 2018 discussed such objects and proposed that the characteristic behavior is caused by a low mass-transfer rate and/or a large disk. We conclude that NSV 11599 is a new member of them.

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