TITLE:   GCN CIRCULAR
NUMBER:  45683
SUBJECT: EP260920b: 2.2m CAHA optical afterglow detection and 10.4m GTC redshift confirmation
DATE:    26/09/21 18:29:38 GMT
FROM:    I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>

I. Perez-Garcia, A.J. Castro-Tirado (IAA-CSIC), Y.-D. Hu (GXU), M.D. Caballero-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), D. Xu (NAOC), J. F. Agui Fernandez, A. Fernandez Martin (CAHA), S. Geier, I. Yanes Rizo (IAC, GRANTECAN), on behalf of a larger collaboration, report:

Following the detection of EP260920b (Ni et al., GCN 45664; Yang et al., GCN 45670), we observed the optical counterpart (Liu et al., GCN 45668; García-García et al., GCN 45669; Li et al., GCN 45671; Eyles-Ferris et al., GCN 45673) with the 2.2m telescope at Calar Alto Observatory in Almeria (Spain), equipped with CAFOS.

At the position of the optical counterpart we measure r = 20.96 +/- 0.21 (AB) on Sep 20 at 21:54 UT (7.8 h after the EP trigger) on a 900s coadded image. The object is also detected in the i-band, but not in the z-band (z > 20.5, 3-sigma).

We also obtained a nearly simultaneous spectrum with the 10.4m GTC equipped with the OSIRIS+ spectrograph, starting on 20 Sep 2026 at 22:00 UT (7.9h after the EP trigger), covering the wavelength range 3800-7750 AA, with a total exposure time of 3x500 s in the R1000B grism.

Superimposed on a strong continuum emission, we detect Si II, C II, Si IV and C IV absorption lines as well as the Lyman-alpha absorption trough and the Lyman forest with Lyman-beta, -gamma, and Lyman-limit features at slightly lower S/N, all under a common redshift of z = 3.580. This confirms the redshift of EP260920b reported by the VLT (Schneider et al. GCN 45675). We also detect the intervening system at z = 1.625.

We thank the staff at both CAHA and GTC for their excellent support.