TITLE:   GCN CIRCULAR
NUMBER:  44686
SUBJECT: GRB 260511A: iTelescope T-75 optical detection
DATE:    26/05/21 22:55:39 GMT
FROM:    Takanori Sakamoto at AGU <tsakamoto@phys.aoyama.ac.jp>

H. Ban, T. Kato, K. Hotta, Y. Kawakubo, M. Serino, T. Sakamoto (AGU)

We observed the field of GRB 260511A detected by
the Fermi/GBM (Fermi GBM Team, GCN Circ. 44529),
the SVOM/ECLAIRs (Godet et al., GCN Circ. 44532),
the NuSTAR CsI anti-coincidence shields (Waratkar et al., GCN Circ. 44555),
the Konus-Wind (Ridnaia et al., GCN Circ. 44585),
the Glowbug (Woolf et al., GCN Circ. 44586), and
the GRID (Li et al., GCN Circ. 44664)
with the iTelescope.Net (http://www.itelescope.net) T75 (ASAN250) telescope
located at Rio Hurtado Valley, Chile.

We took 20 images of 60 sec exposures in the Red filter starting from 
May 11 at 06:37:27 (UT), about 24 minutes after the Fermi/GBM trigger 
and stopped at 07:07:42.

The afterglow (Jiang et al., GCN Circ. 44533; Akl et al., GCN Circ. 44534;
Saccardi et al., GCN Circ. 44535; Zheng et al., GCN Circ. 44536;
Wu et al., GCN Circ. 44538; Harper et al., GCN Circ. 44545; 
Brivio et al., GCN Circ. 44562; Li et al., GCN Circ. 44575; 
Fu et al., GCN Circ. 44579; Zhu et al., GCN Circ. 44584) is detected
in the stacked image of the first eight images (total exposure of 480 s) 
collected between 06:37:27 and 06:49:12. The afterglow magnitude is 
19.3 +- 0.6 using the USNO-B1 catalog. No afterglow was detected in 
the following twelve images collected between 06:49:44 and 07:07:42.
The 5-sigma upper limit of the stacked image of the last twelve images 
(total exposure of 720 s) is 18.3 mag.