AT2023fhn (the Finch): a Luminous Fast Blue Optical Transient at a large offset from its host galaxy

TL;DR

This paper reports the discovery of a luminous fast blue optical transient, AT2023fhn, located far from its host galaxy, challenging previous notions that such events occur only near star-forming regions.

Contribution

It presents multi-wavelength observations of AT2023fhn, showing it occurs at a large offset from its host galaxy, expanding the known environmental diversity of LFBOTs.

Findings

AT2023fhn is located more than 3.5 half-light radii from its host galaxies.

This is the first LFBOT observed at such a large offset.

LFBOTs can occur in a wider range of galactic environments than previously thought.

Abstract

Luminous Fast Blue Optical Transients (LFBOTs) - the prototypical example being AT2018cow - are a rare class of events whose origins are poorly understood. They are characterised by rapid evolution, featureless blue spectra at early times, and luminous X-ray and radio emission. LFBOTs thus far have been found exclusively at small projected offsets from star-forming host galaxies. We present Hubble Space Telescope, Gemini, Chandra and Very Large Array observations of a new LFBOT, AT2023fhn. The Hubble Space Telescope data reveal a large offset (greater than 3.5 half-light radii) from the two closest galaxies, both at a redshift of 0.24. The location of AT2023fhn is in stark contrast with previous events, and demonstrates that LFBOTs can occur in a range of galactic environments.