TITLE:   GCN CIRCULAR
NUMBER:  46164
SUBJECT: GRB 261104A: DECam Detection of an Optical Counterpart Candidate
DATE:    26/11/05 20:24:37 GMT
FROM:    Michael C. Davis at University of Minnesota <davi4614@umn.edu>

Michael C. Davis (UMN), Tomas Ahumada (NOIRLab), Michael Coughlin (UMN), Xander J. Hall (CMU), and  Anirudh Salgundi (UNC) report:

We observed the field of GRB 261104A, detected by SVOM/ECLAIRs (Bernardini et al., GCN 46150; SVOM burst-id sb26110402), using the Dark Energy Camera (DECam) on the 4-m Blanco telescope at CTIO. Observations began on 2026-11-05 at 01:57:06.43 UTC (approximately 5.34 hours after the trigger) under program 2026B-398365 (PI Davis).

Observations consisted of 1x300 s exposures in each of the SDSS r and i bands. The images were reduced using the NOIRLab DECam Community Pipeline and processed with the DESIRT difference-imaging pipeline (Hu et al. 2026). We additionally performed independent forced photometry and image subtraction using Legacy Surveys DR10 reference images.

We identify an optical transient candidate, AT 2026aigl, at the following coordinates:

RA (J2000): 6.336822 deg \
Dec (J2000): -54.412962 deg

The candidate lies approximately 3.17 arcmin from the SVOM/ECLAIRs localization center, within the reported 9.81 arcmin 90% confidence region. The DESIRT difference-imaging pipeline reports the following AB magnitudes:

|Start UT|T_mid - T0 (days)|Filter|Exposure (s)| Magnitude (AB)|
|:------------:|:------------:|:------------:|:------------:|:------------:|
|2026-11-05T01:57:06.43|0.2242|r|1x300|23.35 +/- 0.12|
|2026-11-05 02:02:35.34|0.2280|i|1x300|23.08 +/- 0.13|

We note that a faint, cataloged point source (magnitudes: r=24.04, i=23.64) is present in the archival Legacy Surveys DR10 images within approximately 0.16 arcsec of the candidate position. The archival source is classified as a point source by Tractor modelling at a photometric redshift of z_ph = 1.04 +/- 0.19. The archival source has no archival WISE photometry, nor is it in the Milliquas catalog.

Our detections are consistent with earlier optical observations reported non-detections by SVOM/VT (Li et al., GCN 46161) and LCO (Yin et al., GCN 46154). At present, no fading has been established, and the association of this candidate with GRB 261104A remains tentative.

Further time-resolved optical observations are encouraged to establish the nature of the candidate.