GCN Circular 43934
Subject
EP260306a: Gemini-North redshift z = 4.773
Event
Date
2026-03-06T13:36:30Z (7 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
A. J. Levan (Radboud), P. G. Jonker (Radboud), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), F. E. Bauer (UTa), J. Chácon (PUC), L. Cotter (UCD), R. A. J. Eyles-Ferris (Leicester), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud), report on behalf of a larger collaboration:
We observed the optical counterpart (Li et al., GCN 43929; Globus et al., GCN 43931; Ma et al., GCN 43933) of the fast X-ray transient EP260306a (Huang et al., GCN 43930) using the Gemini-North telescope equipped with the GMOS-N spectrograph.
Four exposures of 600 s each were obtained, using grating R400, a slit width of 1 arcsec, and covering the wavelength range 5200-9900 Å. The observation mid time was 2026 March 6.489 UT (10.2 hr after the EP/WXT trigger).
Continuum is significantly detected over the red part of the spectrum, down to ~7020 Å, where a spectral break is apparent. This is due to a combination of the H I damping wing and the onset of the Lyman forest at z ~ 4.77. A number of absorption features, including fine-structure lines, are clearly visible, which we interpret as Si II, Si II*, C II, Si IV, C IV, Fe II, Al II, all at a common redshift z = 4.773, establishing it as the redshift of EP260306a.
We note that, given their colors and g-band detection, the two nearby objects noted by Globus et al. (GCN 43931) are both in the foreground of the transient.
We acknowledge the excellent support from the Gemini North staff in the execution of this challenging observation.