GCN Circular 45965
Subject
GRB 261011A: 1.3m DFOT optical upper limit
Event
Date
2026-10-16T16:49:50Z (6 hours ago)
From
Yash Sharma at Aryabhatta Research Institute of Observational Sciences <yashandyash30@gmail.com>
Via
Web form
Yash Sharma, Amit K. Ror, Shashi B. Pandey (ARIES) report:
We observed the field of GRB 261011A detected by Swift (Cenko et al., GCN 45896) and SVOM (Huang et al., GCN 45895) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India. The observations were started on 2026-10-14 at 18:09:17 UT, i.e., ~ 3.18 days after the Swift/BAT trigger (T0 = 2026-10-11 13:46:41 UT). We obtained multiple frames with an exposure time of 300 s each in the R filter. The images were aligned and stacked.
We do not detect any optical afterglow emission down to the 3-sigma background limit at the optical afterglow position reported by Swift/UVOT (Cenko et al., GCN 45896) and refined by SOAR (Cheng et al., GCN 45922), nor within the 2.9 arcsec error radius of the enhanced Swift-XRT position (Page et al., GCN 45933; Melandri et al., GCN 45934).
We obtain the following 3-sigma upper limit from the stacked image:
Date Start_UT T_start-T0 (days) Filter Exp time (s) Magnitude (3-sigma)
===================================================================================
2026-10-14 18:09:17 ~ 3.18 R 12 x 300 s > 22.0
The non-detection of the optical counterpart is consistent with the rapid fading of the afterglow at z = 2.407 (Pérez-García et al., GCN 45931; Monsalves et al., GCN 45920) and later-epoch optical/NIR upper limits reported by REM (Brivio et al., GCN 45926), SYSU (Wen et al., GCN 45942), and SAO RAS (Moskvitin et al., GCN 45951).
Photometric calibration was performed against secondary standard stars from the Pan-STARRS (PS1) catalog with Zero Point = 4.081 ± 0.015 mag. The reported limiting magnitude is in the AB system and has not been corrected for the Galactic extinction. This circular may be cited.