TITLE:   GCN CIRCULAR
NUMBER:  30511
SUBJECT: GRB 210724A: Afterglow candidate from OSIRIS/GTC
DATE:    21/07/25 16:50:29 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

A. de Ugarte Postigo (HETH/IAA-CSIC,DARK/NBI), C. C. Thoene, D. A. Kann, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), J. P. U. Fynbo (DAWN/NBI), L. Izzo (DARK/NBI), N. R. Tanvir (U. Leicester), N. Castro-Rodriguez, and A. Marante (GRANTECAN, IAC) report:

We observed the field of GRB 210724A (D'Avanzo et al. GCN 30497) with OSIRIS mounted on the 10.4 m GTC telescope at  the Roque de los Muchachos observatory (La Palma, Spain). Observations started at 22:07 UT (1.88 hr after the burst). Observations were affected by strong calima and the full Moon, which resulted in bad transparency and very high background. In a series of short images in r', i' and z' bands we detect a new source, not present in the PanSTARRS catalogue images. The object is located at (J2000 +/- 0.5"):

RA: 15:09:39.44
Dec.: -06:17:26.0

This object had a magnitude of r' = 22.00 +/- 0.15 mag as compared to PanSTARRS field stars. We note that this source is within the early XRT alert error circles but outside the refined one (D'Avanzo et al. GCN 30497). There is no new source detected within this refined error circle. Due to the fact that this source is significantly brighter than the PanSTARRS limit, we propose it as the optical counterpart of GRB 210724A.