GCN Circular 41816
Subject
EP250908c: tentative VLT/FORS2 redshift z = 2.87
Event
Date
2025-09-12T16:10:17Z (10 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OACn and DARK/NBI), A. Saccardi (CEA/Irfu), J. An (NAOC), G. Corcoran (UCD), V. D’Elia (ASI/SSDC), M. Garnichey (LUX-Paris Obs.), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), A. Martin-Carrillo (UCD), E. Palazzi (INAF/OAS), G. Pugliese (API), D. Xu (NAOC), report on behalf of a larger collaboration:
We observed the optical counterpart (Malesani et al., GCN 41760; Ma et al., GCN 41761; Rau, GCN 41773) of the fast X-ray transient EP250908c (Zhang et al., GCNs 41754, 41763) using the ESO VLT UT1 equipped with the FORS2 spectrograph.
In the acquisition image (mid time 2025 Sep 9.134 UT, or 3.37 hr after the EP trigger) we measure a magnitude V = 21.4 +- 0.2 (calibrated using archival zeropoints).
A spectrum covering the wavelength range 3800-8600 AA was secured using grism 300V and no order-sorting filter, with a total exposure time of 3x600 s.
A faint trace is detected down to ~4790 AA, where a spectral break is observed. If due to Lyman alpha, this would correspond to a redshift of ~2.9. Consistently, a few, weak absorption features are detected, which can be interpreted as C II 1334, Si IV 1393,1402, Si II 1526, C IV 1548,1550, all at a common redshift of z = 2.87. Given the low S/N of the continuum, we caution that this redshift solution should be considered tentative.
Rectifying what was stated in our previous report (Malesani et al., GCN 41760), we note the presence of an extended object in the Legacy Survey underlying the afterglow position, which is a candidate host galaxy for EP250908c. With a Legacy Survey magnitude of r = 23.1, this corresponds to M(UV) = -22.4 (AB) at z = 2.87. No emission lines are visible in our spectrum to determine the redshift of this object.
We acknowledge excellent support from the observing staff in Paranal, in particular Julien Drevon, Jonathan Smoker and Marcela Espinoza.