TITLE:   GCN CIRCULAR
NUMBER:  42918
SUBJECT: EP251130a: GTC OSIRIS+ redshift z = 4.035 and rebrightening
DATE:    25/12/01 09:30:53 GMT
FROM:    Agnes van Hoof at Radboud University <agnes.vanhoof@ru.nl>

J. N. D. van Dalen (Radboud), A. P. C. van Hoof (Radboud), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud), P. G. Jonker (Radboud), D. Mata-Sanchez (IAC and ULL), N.C. Rodríguez (GTC), A. Cabrera-Lavers (GTC), C. Hernandez (GTC), D. Gonzalez (GTC) report on behalf of a larger collaboration:

We observed the counterpart (Dornic et al., GCN 42904; Evans et al., GCN 42908; He et al., GCN 42914; Gupta et al., GCN 42916; Zheng et al., GCN 42917) of EP251130a (Wu et al., GCN 42903; Zhang et al., GCN 42915) with the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. Three 1200 s exposures were obtained using the R1000R grating and a 1 arcsec slit. Observations began on 2025 Dec 1 at 01:28 UT, approximately 0.6 days after the EP trigger time.

From the 60 s acquisition image taken in the i band, we measure a magnitude of i = 19.54+/-0.03 AB (calibrated against nearby Pan-STARRS objects). We find a rebrightening of the optical counterpart by ~1 mag in ~10 hr as compared to He et al. (GCN 42914).

In the spectrum, a broad trough centered around 6120 AA is detected, which we interpret as due to a DLA. Several metal absorption lines are also detected on top of the bright continuum, which match Si II, Si II*, O I, C II, Si IV, C IV, Fe II, Al II, all at a common redshift of z = 4.035.