GCN Circular 30391

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), C. Thoene, M. Blazek, J. F. Agui Fernandez (all 
HETH/IAA-CSIC), and P. Martin-Fernandez (CAHA) report:

We observed the afterglow of GRB 210704A (BALROG localization: 
Kunzweiler et al., GCN #30369; Fermi/LAT detection: Berretta et al., GCN 
#30375; Fermi/GBM detection: Malacaria & Meegan, GCN #30382; AGILE/MCAL 
detection: Ursi et al., GCN #30372; AstroSat/CZTI detection: Prasad et 
al., GCN #30378; Konus-Wind detection: Ridnaia et al., GCN #30388; 
Swift/XRT afterglow detection: D'Ai et al., GCN #30379) with CAFOS 
mounted at the 2.2m Calar Alto telescope (Almeria, Spain), under good 
conditions but at high airmass. We obtained 5 x 300 s images in i'.

The optical afterglow (Kim et al., GCN #30384, D'Avanzo et al., GCN 
#30385) is faintly detected in the stacked image. Against four 
Pan-STARRS comparison stars, we derive a preliminary measurement of i' = 
21.84 +/- 0.13 mag at 1.09286 ���d after the trigger.

Compared to contemporary observations (D'Avanzo et al., GCN #30385), the 
r' - i' color is unremarkable, indicating the afterglow likely does not 
suffer from high extinction, and is therefore intrinsically faint. If it 
is associated with the surrounding galaxy cluster (Levan et al., GCN 
#30381), this would add evidence to the merger-induced ("short") nature 
of the event.