Spectroscopic Confirmation of ASASSN-21gq = AT 2021klf = ZTF21aanfcmk as a gravitational micro-lensing event

ATel #14575; Kenta Taguchi, Keisuke Isogai, Naoto Kojiguchi, Yusuke Tampo(Kyoto University)
on 26 Apr 2021; 07:47 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report our spectra of the transient ASASSN-21gq = AT 2021klf = ZTF21aanfcmk. We obtained the spectra from 2021-04-25.773 UT to 2021-04-25.788 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 TNS, this object is discovered by ASAS-SN at g = 15.4 AB mag on 2021-04-24.260. There is a PS1 counterpart of g = 18.50, r = 17.77, i = 17.42, z = 17.24, and y = 17.07. Before the discovery by ASAS-SN, ZTF detected it at r ∼ 17.26 on 2021-02-24.54. After one month, ZTF detected it again at r = 16.80 on 2021-03-27.465 and g = 17.70 on 2021-03-27.503. Then, this object has been gradually rising for 2.5 mag, during which the g - r color remained almost constant (the last record of ZTF is r = 14.34 on 2021-04-24.416 and g = 15.18 on 2021-04-24.482).

Our spectra show a red continuum and Balmer (Hα and Hβ), the infrared Ca II triplet, Na I D, CH, and Fe I absorption lines. Such spectral features are consistent with a normal G-type star. The g - r color is also consistent with a normal G-type star. Based on our spectra, the almost-constant g - r color, and the profile of the light curve, we conclude that this object is a high-amplitude gravitational microlensing event.

Our Spectrum