Optical observations of candidate host galaxies of eight fast X-ray transients

Context. Fast X-ray transients (FXTs) are extragalactic flashes of X-rays with a typical duration of minutes to hours for which a variety of origins has been proposed and observed.

Aims. To decipher the origin of FXTs, particularly those lacking multi-wavelength counterparts, we aim to understand their energetics and environments.

Methods. We present deep, optical ground-based observations of the positions of eight FXTs in order to try and identify and characterise candidate host galaxies. We used their properties to discriminate between possible progenitor scenarios.

Results. We identify candidate host galaxies for Swift J050400.2+673405, XRT140507, XRT040610, XRT151121, XRT191127, and EP240708a. For each candidate, we infer the spectroscopic or photometric redshift, stellar mass, star formation rate, metallicity, and stellar population age by fitting our data with the spectral-energy-distribution fitting code BAGPIPES. We re-identify XRT191223 as a Galactic stellar flare.

Conclusions. For several FXTs, there are multiple candidate host galaxies, which complicates deriving constraints on the origin of the FXT. Assuming association with (one of) those candidates, all are consistent with a (non-)relativistic white dwarf–intermediate mass black hole tidal disruption event (WD-IMBH TDE) and a binary neutron star (BNS) merger. Two are consistent with a supernova shock breakout and only EP240708a with cocoon emission from a long γ-ray burst. We also discuss the possibility that the host galaxies remain undetected in our observations. We conclude that FXTs detected by Chandra and XMM-Newton are likely to arise from a variety of origins, and we discuss the fact that part of this population differs from FXTs detected by the Einstein Probe, many of which appear consistent with a collapsar scenario.