AstroNote 2023-101

We report Very Large Telescope (VLT) imaging observations using the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) of the candidate Luminous Fast Blue Optical Transient (LFBOT) AT2023fhn (Astronote #2023-93) from 2023-04-23/24 UT (MJD range 60057.98456-60058.00975). We took 3 exposures in the u and I bands, and 2 exposures in g and R, measuring AB magnitudes using aperture photometry of:

u = 21.474 ± 0.043

g = 21.504 ± 0.025

R = 21.730 ± 0.032

I = 22.012 ± 0.075

The transient has therefore faded by more than two magnitudes from the peak as reported by Ho et al. (Astronote #2023-93) over a span of just 12 days (observer-frame; equivalent to 9.6 days rest-frame).

After then correcting for Galactic extinction, fits of these observations to blackbody SEDs yield a pseudo-blackbody temperature and radius of:

T = 17.46 (+ 1.22 - 1.02) kK

R = 90.58 (+ 7.28 - 7.45) AU

The inferred temperature is very high for an extragalactic transient after peak, and comparable to the temperature of 19.32 (+ 0.7 - 0.83) kK of the archetypal FBOT AT2018cow at a similar phase (Perley et al. 2019, MNRAS, 484, 1031). Subtraction of the (relatively red) host galaxy was not carried out, and no correction for host extinction was applied, so the true colour of the transient may be even bluer. The measured temperature, alongside its blue colour, strengthens the association of this transient with past LFBOTs such as AT2020xnd and AT2022tsd, even in the absence of significant observed X-ray and millimeter emission (Astronote #2023-100).