GCN Circular 23018

I. Carrasco (Univ. de Malaga), Y. Hu, E. Fernandez-Garcia, J. C. Tello 
and A. J. Castro-Tirado (IAA-CSIC),
D. Hiriart and W. H. Lee (UNAM), S. Jeong and I. H. Park (SKKU), M. D. 
Caballero-Garcia (ASU-CAS, CZ) and
R. Cunniffe (Inst. of Physics, CZ) on behalf of a larger collaboration, 
report:

The 60cm BOOTES-5/JGT robotic telescope at Observatorio Astronomico 
Nacional in San Pedro Martir (Mexico)
automatically responded in 21s (and 53s after the GRB onset) to the 
Swift trigger of GRB 180721A (Sbarufatti
et al., GCNC 22994). The first image (10s exposure, r-band) was obtained 
at 11:07:10 UT. At the position of
the Swift X-ray afterglow (Goad et al. GCNC 22997), no optical afterglow 
is detected down 18.1 mag. Furthermore,
on the co-add of the images obtained until 11:33:52 UT no afterglow is 
detected down to 21 mag, consistent with
the limit reported by Guidorzi et al. (GCNC 22995).

We thank the staff at Observatorio Astronomico Nacional in San Pedro 
Martir for its excellent support.