AstroNote 2023-177

We report continuing 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 (ZTF23aaeozpp, AstroNote #2023-93). Our first epoch of observations (as detailed in AstroNote #2023-101) were taken MJD 60057.98456.

Three subsequent epochs of multi-filter photometry (g/R/I) have since been obtained. The next epoch at MJD 60069.97992 was affected by high levels of sky background, but the transient was still clearly detected.  We estimate g~23.0 at this time.

In our third epoch from MJD 60079.98046 the transient is only marginally detected, and is strongly blended with the host galaxy.  Aperture photometry fixed at the transient location gives a magnitude of g~23.6, although a substantial fraction of this is likely from the host galaxy. 

Our fourth epoch was acquired on MJD 60107.99736 and shows no obvious detection of the transient in any filter.  Aperture photometry fixed at its location gives a nominal detection of approximately g~24.7, which is likely to mostly be host-galaxy flux.

Image subtraction between the third and fourth epochs shows a clear detection in the g-band but not in the other bands, implying that the transient colour remains blue throughout its late-time evolution.

The fast-fading lightcurve and blue colour further strengthen the likeness of AT2023fhn to previously identified LFBOTs such as 2018cow and 2020xnd.

We thank Henri Boffin and the VLT staff for their support with these observations.