A.J. Levan (Radboud), S. Campana (INAF-OAB), D.A. Kann (HETH/IAA-CSIC), P. D'Avanzo (INAF-OAB) report:
���We note that the Swift XRT localisation (D���Ai et al. GCN 30379) of GRB 210704A detected by Fermi-LAT, Fermi-GBM (Kunzweiler et al. GCN 30369, Malacaria et al. GCN 30380), AGILE (Ursi et al. GCN 30722) and AstroSat (Prasad et al. GCN 30378) lies in a significant galaxy overdensity. Of particular note, the source localisation overlaps the galaxy cluster 400d J1036+5713 at z=0.203. Although there are no apparent galaxies within the XRT localisation visible in the PanSTARRS 3pi imaging of the field, there are numerous likely members of this cluster within a few arcseconds.
Furthermore, an even brighter foreground galaxy WISEA J103604.24+571327.7 lies offset approximately 30��� from the XRT position and has a reported redshift of z=0.0817. This galaxy has I=15.962 +/- 0.007 (and absolute magnitude around -22). Its projected physical offset from the burst position is approximately 45 kpc. Although this offset is significant, the probability of chance alignment remains small.
The burst itself appears dominated by a short-spike, with a fainter tail of emission, and these possible host galaxies are consistent with the hosts and locations of short GRBs (and not of long GRBs). At z=0.203 (or z=0.0817) the isotropic equivalent energy would be E_iso ~ 2e51 erg (or E_iso ~ 3e50 erg at z=0.0817), consistent with SGRBs.
Observations to search for the optical afterglow or possibly rising kilonova emission are strongly encouraged. ���
[GCN OPS NOTE(02Aug21): In the SUBJECT the "200704A" was corrected to "210704A".
In the author list, it should be "P. D'Avanzo", not "P D'Avanzo".
Both z-values given in the 3rd paragraph were listed as 0.0891, but they should be 0.0817.]