TITLE: GCN CIRCULAR NUMBER: 44082 SUBJECT: EP260321a: VLT imaging and spectroscopy show the variable source to be at z = 0.0343 DATE: 26/03/22 19:50:53 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> N. R. Tanvir (Leicester), L. Izzo (INAF/OACN), A. J. Levan (Radboud and Warwick), D. Xu (NAOC), G. Corcoran (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), K. Wiersema (Hertfordshire), report on behalf of the Stargate collaboration: We observed the field of EP260321a (Huang et al., GCNs 44068, 44075) with the ESO VLT UT1 (Antu) and UT4 (Yepun), obtaining R-band imaging with the FORS2 instrument (mid-time 00:44 UT on 2026-02-22; 12.23 hr after the EP trigger), and integral field spectroscopy with the MUSE instrument on UT4 (mid-time 05:29 UT on 2026-02-22; 16.98 hr after the EP trigger). Within the refined EP/FXT error circle (Huang et al., GCN 44075), the blue, historically variable source coincident with the outskirts of a galaxy at z = 0.0343, noted by Lee et al. (GCN 44070), is prominent. We measure a magnitude of R = 19.5, which is within the range of archival values, but is significantly brighter than reported by Lee et al. (GCN 44070). The source itself is consistent with being marginally resolved in 0.7" seeing imaging, though this may be due to contamination from the underlying galaxy. Our spectroscopy shows the variable source (covering the wavelength range 4750 - 9300 Å) to be characterised by a blue power-law continuum, with very strong narrow emission lines at z = 0.0343 (e.g., [O III] 5008 EW ~ 500 Å). No absorption features are detected at either z = 0 or z = 0.0343. While the emission lines are visible across the entire galaxy, they are strongly spatially concentrated at the location of the blue variable object, with fluxes approximately twenty times brighter than at the galaxy nucleus. This makes it unlikely for the source to be a random foreground superposition, suggesting that it lies itself within the galaxy at z = 0.0343. At this redshift, our measured flux corresponds to an absolute magnitude of approximately M_R = -16.8, and the past variability (Lee et al., GCN 44070) appears to be consistent with previous outbursts to a comparable flux level. It is possible that past variability was due to LBV-like outbursts from a star embedded within a dense, luminous cluster (as evidenced by the strong emission line component). The short-term variability following the outburst (Lee et al., GCN 44070; Aryan et al., GCN 44081), coupled with the location within the revised FXT error circle, suggest that this source is the optical counterpart of EP260321a. It is unclear if the current outburst is related to a further LBV flare or the terminal collapse of the star, as may be suggested by the possible shock breakout properties of the X-ray outburst (Huang et al., GCN 44075). Under this interpretation, this would make EP260321a the first example of an X-ray transient progenitor detected prior to explosion. We encourage further monitoring of this object at all wavelengths in order to clarify its enigmatic nature. We acknowledge excellent support of the ESO observing staff at Paranal, in particular Abel de Burgos Sierra, Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Enrico Congiu, Israel Blanchard, and Rodrigo Palominos. We also thank the visitor observers on UT4, Silvia Piranomonte and Eleonora Parlanti, for generously allowing the execution of our observation. View this GCN Circular online at https://gcn.nasa.gov/circulars/44082. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...