TITLE: GCN CIRCULAR NUMBER: 38359 SUBJECT: GRB 241127A: LCOGT (40-cm) optical counterpart detection DATE: 24/11/28 04:18:13 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias ipf@iac.es

I. Ortega-Casas, B. Armas-Chinea, F. Dobrindt, P. Escudero-Coca, G. Fernández-Rodríguez, Á. García Lozano, A. Huertas Ferrer, C. Méndez-Lapido, M. Torreiro Martínez, G. Villa (all ULL), S.R. Berlanas (IAC and ULL), and I. Pérez-Fournon (IAC and ULL)

We report on optical follow-up observations of GRB 241127A, detected by Swift BAT, XRT, and UVOT (Salvaglio et al., GCN circular 38354) and by MASTER (Buckley et al., GCN circular 38352 and Francile et al., GCN circular 38358).

We observed the field of GRB 241127A with the two Las Cumbres Observatory Global telescope network (LCOGT) Planewave Delta Rho 350 telescopes, equipped with QHY600 CMOS detectors, located at the LCOGT node at the Cerro Tololo Inter-American Observatory (Chile) in the SDSS r', i', and g' filters, with 600 sec exposures in each of the filters. We detect the optical counterpart at a position consistent with the Swift UVOT (Salvaglio et al., GCN circular 38354) and MASTER (Buckley et al., GCN circular 38352 and Francile et al., GCN circular 38358) detections.

We measure the following magnitudes, calibrated using stars from the catalog of Gaia DR3 synthetic photometry generated from the Gaia BP/RP mean spectra (Gaia collaboration 2022) and without corrections for Milky Way extinction:

r' = 19.42 +/- 0.08, on 2024-11-28 00:58:30 UT, 3.25 hours after the Swift trigger

i' = 19.27 +/- 0.15, on 2024-11-28 01:04:49 UT, 3.36 hours after the Swift trigger

g' = 19.70 +/- 0.07, on 2024-11-28 01:10:03 UT, 3.45 hours after the Swift trigger

In the Legacy Surveys DR10 catalog there is a faint galaxy close to the position of the GRB optical counterpart at RA, Dec (J2000, deg) = 327.8851, -56.6653 and with magnitudes g = 25.08, r = 24.70, i = 24.38, and z = 23.57, that is the likely GRB host galaxy.

These results are based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation and are part of a course on Astrophysical Techniques of the Master in Astrophysics of the Universidad de La Laguna, Tenerife, Spain (LCOGT observing programme IAC2024B-010, ULL-ASTRO-MASTER).

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