AstroNote 2024-210

We observed the transient AT2024qfm discovered by ZTF (Foster et al. TNS TR No. 219620), and highlighted to be a rapidly evolving transient by Fulton et al., (AstroNote 2024-206). Photometric follow-up observations were subsequently reported by Lai et al., (AstroNote 2024-208) and Perley et al., (AstroNote 2024-209). Here we report the results of our observations using the GMOS-N spectrograph at the Gemini-North observatory under the observing program ID GN-2024A-Q-128 (PI: M. Huber).

Our epoch of observation commenced approximately ~8 days after the first significant detection in ZTF (MJD 60515.4). We obtained 4x1200s exposures with the R400 grating, sampling a wavelength range of ~4200-9200 angstroms.

The reduced spectrum of AT2024qfm is mostly featureless and blue, and we find some matches (with SNID and GELATO) to superluminous and broad-lined Ic SNe, as well as AT2018cow, all at redshifts of 0.20-0.23. The exact classification is uncertain and requires further data. We also extract the spectrum of the host galaxy, and identify H-alpha, H-beta, [O III], [N II] and [S II] lines, all at a common redshift of z=0.227.

With this accurate redshift constraint, we determine that AT2024qfm has an absolute r-band magnitude (at peak) of -21.0 (AB). From our multi-telescope follow-up campaign (LT, LOT, Pan-STARRS), we note that the transient is continuing to decline rapidly, with the latest estimate for decline rate being ~0.3 (AB) mag per day in r-band. Despite the uncertainty in spectroscopic classification, AT2024qfm has all the characteristics of a Luminous Fast Blue Optical Transient (LFBOT).

We thank the staff of the Gemini Observatory for rapidly approving and executing these observations.