TITLE:   GCN CIRCULAR
NUMBER:  46211
SUBJECT: GRB 261108D: DECam optical detection of candidate afterglow 
DATE:    26/11/10 04:32:14 GMT
FROM:    Michael C. Davis at University of Minnesota <davi4614@umn.edu>

Michael C. Davis (UMN), Tomas Ahumada (NOIRLab), and Michael Coughlin (UMN) report:

We observed the field of GRB 261108D, detected by SVOM/ECLAIRs (Zhang et al., GCN 46200), with the Dark Energy Camera (DECam) on the 4-m Blanco telescope at CTIO. Observations began on 2026-11-09 at 07:52:29 UTC (approximately 8.00 hours after the trigger) under program 2026B-398365 (PI Davis).

Observations consisted of 1x300 s exposures in each of the r and i bands. The images were reduced with the NOIRLab DECam Community Pipeline (Valdes et al. 2014). After performing PSF-matched image subtraction against pre-burst Legacy Surveys DR11 coadds (Dey et al. 2019), we detect the candidate counterpart reported by Passaleva et al., GCN 46210. Our difference-image position, RA, Dec (J2000) = 170.604125 deg, Dec = -77.631611 deg, is about 0.3" from the near-infrared position. Photometry is calibrated against Legacy Surveys DR11 field stars.

We measure the following AB magnitudes, not corrected for Galactic extinction:

|Start UT|T_mid - T0|Filter|Exposure|Magnitude (AB)|
|:------------:|:------------:|:------------:|:------------:|:------------:|
|2026-11-09T07:52:29|0.3350|r|1x300s|22.13 ± 0.09|
|2026-11-09T07:57:59|0.3388|i|1x300s|21.56 ± 0.11|

We encourage further monitoring to establish the source’s relationship to the burst. 

We thank the CTIO and NOIRLab staff for their support during these observations.