Multi-wavelength observations of the luminous fast blue optical transient AT 2023fhn

Up to ∼200 days post-explosion

¹ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
³ Department of Physics, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, UK
⁴ Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁵ South African Astronomical Observatory, P.O. Box 9 7935 Observatory, South Africa
⁶ Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author; ashley.chrimes@esa.int

Received: 19 June 2024
Accepted: 21 October 2024

Abstract

Context. Luminous fast blue optical transients (LFBOTs) are a class of extragalactic transients notable for their rapid rise and fade times, blue colour, and accompanying luminous X-ray and radio emission. Only a handful have been studied in detail since the prototypical example AT 2018cow. Their origins are currently unknown, but ongoing observations of previous and new events are placing ever stronger constraints on their progenitors.

Aims. We aim to put further constraints on the LFBOT AT 2023fhn, and LFBOTs as a class, using information from the multi-wavelength transient light curve, its host galaxy, and local environment.

Methods. Our primary results were obtained by fitting galaxy models to the spectral energy distribution of AT 2023fhn’s host and local environment, and by modelling the radio light curve of AT 2023fhn as due to synchrotron self-absorbed emission from an expanding blast wave in the circumstellar medium.

Results. We find that neither the host galaxy nor circumstellar environment of AT 2023fhn are unusual compared with previous LFBOTs, but that AT 2023fhn has a much lower X-ray to ultraviolet luminosity ratio than previous events.

Conclusions. We argue that the variety in ultraviolet-optical to X-ray luminosity ratios among LFBOTs is likely due to viewing angle differences, and that the diffuse, yet young local environment of AT 2023fhn – combined with a similar circumstellar medium to previous events – favours a progenitor system containing a massive star with strong winds. Plausible progenitor models in this interpretation therefore include the mergers of black holes and Wolf-Rayet stars or failed supernovae.