Hi Roger,
I saw a comment suggesting a Be star on the TNS discovery entry where it is AT2021ziw. I uploaded my spectrum there to eliminate that suggestion
https://www.wis-tns.org/object/2021ziw
but at the time the gaia team uploaded their discovery there it had only brightened by around 0.4 mag so I would have said the possibilities were pretty wide open at that point. That’s why a spectrum is useful.
Not sure why a brightening Be star would be singled out as a likely possibility, (there must be many mechanisms that can produce such an upward drift in brightness) but my first spectrum clearly eliminated that possibility and that given it looked like a pretty average K class star, probably many other alternatives. The shape and symmetry of the light curve now pretty much seals it as microlensing and my second spectrum supports that. (Only a microlensing event or eclipsing binary would be expected to leave the spectrum unchanged through a significant change in brightness)
I see there is now another spectrum in TNS, taken using the 3.8m Seimei telescope. Although higher resolution the features are the same as in my spectra. (There is a bit of difference in continuum slope, possibly down to my choice of reference star.)
Cheers
Robin