TITLE:   GCN CIRCULAR
NUMBER:  46103
SUBJECT: GRB 261029A: ULL-ASTRO-MASTER observations with LCO 1-m telescope at Siding Spring Observatory
DATE:    26/10/30 00:09:44 GMT
FROM:    Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>

L. Rodríguez-Torres, S. Abad Gómez, K.N. Alvez Báez, G. de la Mata Núñez, J. Díaz-Miguel Font, A. Fernández-Roselló, L. García Rodríguez, S. M. Holtham, R. Jiménez Rioja, H. Morín-González, L. Ortega-Barcenilla, C. Pasamontes-Duarte, M. Ríos-Jiménez, S.J. Riquelme-Salinas,  A. Rosendo Pato, E. Ruiz-Laguna, I. Ruiz-Pérez, D. Zazo-Callejas (all ULL), S. R. Berlanas (IAC and ULL), and I. Pérez-Fournon (IAC and ULL)

We report on observations of the long GRB 261029A detected by Fermi GBM (Fermi GBM team, GCN 46071; and Scotton et al., GCN 46098), SVOM/ECLAIRs (Goto et al., GCN 46072), Fermi-LAT (Longo et al., GCN 46085), EP-FXT (Kan et al., GCN 46086), AstroSat CZTI (Salunke et al., GCN 46089), the CALET Gamma-ray Burst Monitor (Marrocchesi et al., GCN 46095), and NuSTAR (Waratkar et al., GCN 46100) at a redshift of z = 3.27 (Cotter et al., GCN 46081).

We observed the field of GRB 261029A with a Las Cumbres Observatory (LCO) 1-m telescope, equipped with a Sinistro camera, at the LCO node at Siding Spring Observatory, New South Wales, Australia. The observations consisted of 300-sec exposures in each of the SDSS r', i' and g' filters that were obtained at a large airmass. The r´-band observation began on 2026-10-29 at 10:23:26 UT (about 8.94 hr after the Fermi trigger). The optical counterpart first reported by GOTO (O'Neill et al., GCN 46073; AT 2026ahaw, O'Neill et al., TNS Astronomical Transient Report No. 330168 ) is clearly detected in the three images. We measure the following AB magnitudes, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction:

Date (UT, mid-time)       | filter  |  mag   | error | 
------------------------------------------------------
2026-10-29 10:25:56           r'      19.61     0.05
2026-10-29 10:56:32           i'      19.33     0.07
2026-10-29 11:32:06           g'      20.78     0.10

These results are consistent with other optical and near-IR detections (O'Neill et al., GCN 46073; He et al., GCN 46078; Wang et al., GCN 46079; De et al., GCN 46080; Cotter et al., GCN 46081; Sanchez Alvarez et al., GCN 46083; Yin et al., GCN 46084; Shi et al., GCN 46088; Kang et al., GCN 46090; Ma et al., GCN 46092; De et al., GCN 46093; Novotny et al., GCN 46097; Zhao et al., GCN 46099; Dimple et al., GCN 46101; and Novotny et al., GCN 46102).

This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026B-010). 

These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain).

This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).