GCN Circular 29318

D. A. Kann (HETH/IAA-CSIC), A. Rossi (INAF-OAS), A. de Ugarte Postigo 
(HETH/IAA-CSIC, DARK/NBI), C. C. Thoene (HETH), M. Blazek, J. F. Agui 
Fernandez (both HETH/IAA-CSIC), and J. I. Vico Linares (CAHA) report:

We observed the afterglow position (Troja et al., GCN #29233) of the 
bright Swift/Fermi GRB 210104A (Swift detection: Troja et al., GCN 
#29233; GBM detection: Fermi GBM Team, GCN #29232; Biltzinger et al., 
GCN #29234; Konus-Wind detection: Frederiks et al., GCN #29258; CALET 
detection: Cherry et al., GCN #29268; AstroSat CZTI detection: Nadella 
et al., GCN #29299) with CAFOS at the 2.2m telescope at Calar Alto, 
Almeria, Spain, in the Rc band. We obtained 6 x 600 s exposures, 
centered at 11.4624 days after the GRB, under good conditions but 
mediocre seeing.

After removing the halo of a nearby star, the afterglow (Xin et al., GCN 
#29235; Hu et al., GCN #29236; Hosokawa et al., GCN #29237; Kim et al., 
GCNs #29238,29265,29283; Horiuchi et al., GCN #29241; Breeveld et al., 
GCN #29247; Lipunov et al., GCN #29248; Zhu et al., GCN #29252; Paek et 
al., GCN #29254; Kumar et al., GCN #29257; Smartt et al., GCN #29262; 
Anandagoda et al., GCN #29273; Gokuldass et al., GCN #29274; Mao et al., 
GCN #29275; Moskvitin et al., GCN #29277; Belkin et al., GCN #29286; 
Nakamura et al., GCN #292291) is faintly but clearly detected.

Against three SDSS comparison stars (transformed to Rc band using the 
Lupton 2005 equations, then transformed back to AB magnitude), we 
measure Rc = 23.67 +/- 0.14 mag. This value is in agreement with the 
extrapolation of the light curve decay reported by Belkin et al., GCN 
#29286 (A. Pozanenko, priv. comm.). This implies there is no significant 
evidence for a supernova rise, and therefore the redshift of GRB 210104A 
is conservatively estimated to be z > 0.4 (e.g., SN 2012eb associated 
with GRB 120714B at z ~ 0.4 peaks at r' ~ 22.2 mag 12 days after 
trigger, Klose et al. 2019, A&A, 622, A138).

We thank Alexei Pozanenko for discussions.