TITLE:   GCN CIRCULAR
NUMBER:  30883
SUBJECT: GRB 210919A CAHA 2.2m Observations: Afterglow/Host Galaxy Candidate
DATE:    21/09/26 22:58:23 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

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

We obtained observations of the enhance XRT position (Goad et al., GCN 
30850) of the Swift short GRB 210919A (Tohuvavohu et al., GCN #30846) 
with CAFOS mounted on the 2.2m telescope at CAHA, Almeria, Spain. We 
obtained 30 x 90 s images in the i' band, starting at 03:38 UT on 
September 19, under good observing conditions.

In the stacked image, we detect a faint source within the enhanced XRT 
error circle at:

RA (J2000)   =  05:21:01.098
Dec. (J2000) = +01:18:43.15

with an error of 0".5.

Compared to three nearby comparison stars from the PanSTARRS catalog, we 
measure:

i' = 24.14 +/- 0.30 mag (AB) @ 0.1525 days after the GRB.

We suggest this source to be either the afterglow or the host galaxy of 
GRB 210919A.

Other teams have reported upper limits so far:

CR > 18.6 @ 0.007824 d (deepest, Lipunov et al., GCN #30847)
CR > 17.5 @ 0.002975 d (Hu et al., GCN #30848)
Rc > 20.5, Ic > 18.7 @ 0.05083 - 0.06958 d (Strausbaugh et al., GCN 
#30849)
Rc > 22.1 @ 0.03194 d (Pankov et al., GCN #30851)
r' > 23.4 @ 0.13125 d, i' > 23.9 @ 0.14458 d (Perley et al., GCN #30852)
Rc > 20.5 @ 0.7208 d (Takamatsu et al., GCN #30855)
Rc > 21.716 @ 0.426 d (AB); r' > 20.16 @ 0.6558 d; i' > 19.107 @ 0.6595 
d (Kim et al., GCN #30856)
white > 20.8 @ 0.001826 d (Siegel et al., GCN #30857)
Kp > 21.5 @ 0.5542 d (AB) (Zhang et al., GCN #30858)
r' > 23.6 @ 2.4635 d (Gottlieb et al., GCN #30860)
i' > 24.1, z' > 23.6 @ 2.343 d (Pierel et al., GCN #30868)

None of these limits are as deep as our image, with the NOT imaging 
(Perley et al., GCN #30852) slightly earlier and the late DeCam imaging 
(Pierel et al., GCN #30868) coming closest. This leaves it unclear as to 
whether the source is fading, we can therefore not discern whether it is 
an afterglow candidate or a host-galaxy candidate.

Deeper imaging is needed, if the source is undetected to deeper limits, 
it is very likely the afterglow. If it is the host, it should be 
detected at higher significance.