Spectroscopic Confirmation of Gaia21efs as a Microlensing Event

ATel #15043; Kenta Taguchi, Junpei Ito (Kyoto University), Claudio Balcon (Belluno A. O., ISSP), Keisuke Isogai (Kyoto University)
on 17 Nov 2021; 18:41 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report our spectra of Gaia21efs = AT 2021ziw. We performed spectroscopy on 2021-11-02,847, 2021-11-04.818, and 2021-11-08.864 UT using the FOSC-ES32 spectrograph attached to a 0.2m telescope at Belluno A. O., and on 2021-11-07.392 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 Gaia Alerts, this object was alerted at 15.39 mag (Gaia G band) on 2021-09-10.715 UT. Before the alert, this object had an average magnitude of 15.76(2) mag (Gaia G band). According to ASAS-SN Sky Patrol, this object was the brightest at 13.756 g-mag on 2021-11-02.2651567, to which time the light curve is almost symmetry with respect.

All of our spectra are very similar to each other and also to the one (posted to TNS) taken by R. Leadbeater on 2021-11-04.769. They show the Hβ, and Na_ID absorption lines and the MgH absorption band. Also, the spectrum by Seimei Telescope shows the Hα, Ca I, and Fe I absorption lines and the CH absorption bands. Such spectra look like that of a late K star. According to ASAS-SN Sky Patrol, this object has dimmed about one magnitude during our observing epoch, but the shape of our spectra is unchanged.

Such evolutions in the light curve and the spectra are consistent that this object is a microlensing event. Taichi Kato reported that this object has shown 0.15-mag flickering, and hence suggests that this object is a posible CV (vsnet-alert 26370 and 26384). However, our spectra and the profile indicate that this object is not a CV.

Our Spectra