AstroNote 2021-195

AT 2021seu (ZTF21abjciua) was discovered by the Zwicky Transient Facility in the nucleus of a z=0.06 galaxy on July 5, and it is currently rising. An archival SDSS spectrum of the host galaxy, SDSS J142134.55+375434.7, taken in 2004-03-20, classifies the host as a starburst galaxy, with prominent Balmer emission lines.

We obtained an optical spectrum with the FLOYDS instrument mounted on the Las Cumbres Observatory 2m telescope in Helakala, Hawaii (through our nuclear transients classification program, PI: Arcavi) at  2021-07-10 07:56:41 UT. The FLOYDS spectrum shows a bluer continuum compared to the SDSS archival spectrum, and a new emission feature around rest-frame 4680A, consistent with a blend of HeII 4686A and NIII 4640A emission lines. These characteristics have been previously associated with Bowen Fluorescence Flares (BFFs) - a class of enhanced accretion events around supermassive black holes (Trakhtenbrot et al. 2019, Nature Astronomy, 3, 242), likely driven by intense EUV flares that lead to the excitation of the Bowen Fluorescence cascades. However, here we do not detect the OIII 3341A, 3429A and 3444A lines seen in other BFFs. OIII 3133A is possibly detected, however it is right on the edge of the wavelength coverage of the spectrum.

We are pursuing additional followup observations of this event.