TITLE: GCN CIRCULAR NUMBER: 39743 SUBJECT: GRB 250314A: GTC z-band upper limit and updated photo-z ~ 7.27 DATE: 25/03/16 10:32:24 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS adeugartepostigo@gmail.com

N. A. Rakotondrainibe (LAM), A. de Ugarte Postigo (LAM), L. Izzo (INAF/OACN), D. B. Malesani (DAWN/NBI and Radboud), J. F. Agui Fernandez (CAHA), J. P. U. Fynbo (DAWN/NBI), S. Geier (GTC), G. Lombardi (GTC), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), C. C. Thoene (AbAO), and G. Gomez Velarde (GTC) report on behalf of a larger collaboration:

We observed the optical counterpart (Malesani et al., GCN 39727; see also Kennea et al., GCN 39734; Turpin et al., GCN 39739) of the SVOM/ECLAIRs GRB 250314A (Wang et al., GCN 39719), using the Gran Telescopio Canarias equipped with the OSIRIS+ camera. A sequence of 30 exposures of 30 s each (15 min in total) was secured in the z band, with mid time 2025 March 15.24 UT (16.8 hr after the GRB).

In a preliminary reduction, no source is detected down to a 3-sigma limiting magnitude z > 24.4 (AB), calibrated against nearby stars from the Legacy Survey.

Adding our z-band limit to the nearly simultaneous YJH photometry provided by VLT/HAWK-I (Malesani et al., GCN 39732) allows us to derive a more refined photometric redshift of z = 7.27 +0.14 -0.19 (1 sigma c.l.), assuming a power-law afterglow model with no dust extinction.

The lack of optical detection is consistent with, and provides further evidence for, the high redshift of this GRB (Malesani et al., GCN 39732).

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