Spectroscopic observation of the unusual variable ASASSN-21gk
ATel #14593; Keisuke Isogai, Taichi Kato, Kenta Taguchi (Kyoto University), Franz-Josef Hambsch (GEOS, BAV, VVS)
on 3 May 2021; 06:59 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)
Subjects: Optical, Binary, Transient, Variables
We report on spectral features seen in the unusual variable ASASSN-21gk on 2021-05-02.661 UT
obtained by using a 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 already reported to vsnet-alert 25757 by Patrick Schmeer,
the unusual activity was discovered at 2021-04-18.39 UT by ASAS-SN as a possible weird CV (Shappee et al. 2014).
According to the PS1, the optical counterpart is g = 13.78, r = 13.18, i = 12.96, z = 12.83, and y = 12.82.
The overall light curve of the ASAS-SN Sky patrol
shows that this object had been stable at least for several years but the brightness started to change from 2021-04-14
(Kochanek et al. 2017).
See also the enlarged light curve.
Then, we reported that this object has oscillated on a timescale of one day and indicated that similar activity was also seen in ASASSN-20da
(vsnet-alert25781,
25787).
ASASSN-20da had also been stable but brightened from g = 12.5 to 11.9 for several days in May 2020
(see the light curve of the ASAS-SN Sky Patrol).
This object also showed slow modulations with a possible period of 0.61571(8) d
(vsnet-alert 25790).
Aydi et al. reported the spectrum of the object (ATel #13573).
However, their spectral features imply a normal star, and hence the nature of the activity has been unclear.
Possibly, it may be worth noting that the amplitude of the modulations gradually decayed in the fading stage.
To compare with ASASSN-20da, we obtained a spectrum of ASASSN-21gk on 2021-05-02.661 UT.
Our spectrum shows
Balmer, Na DI, Fe I absorption lines.
These lines and the profile of the continuum suggest that this object is likely to be a normal (F or G-type) star in common with ASASSN-20da.
Although the nature is still unclear, these objects may represent a new type of variable stars.
Follow-up photometry and spectroscopy are encouraged.
Our Spectrum
