TITLE:   GCN CIRCULAR
NUMBER:  30983
SUBJECT: GRB 210919A: FORS2/VLT observations
DATE:    21/10/25 00:51:58 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. A. Kann (HETH/IAA-CSIC), D. B. Malesani (Radboud U. Nijmegen), V. 
D'Elia (ASI/SSDC, INAF/OAR), A. de Ugarte Postigo (HETH,DARK/NBI), A. 
Rossi (INAF-OAS) and C. C. Thoene (HETH/IAA-CSIC) report on behalf of 
the Stargate consortium:

We observed the field of the short GRB 210919A (Tohuvavohu et al., GCN 
#30846) at late times with FORS2 mounted on the ESO VLT at Cerro Paranal 
Observatory, Chile. We obtained 6 x 90 s exposure each in V, Rc, and Ic, 
and 12 x 68 s in z_special, under excellent conditions (seeing 0".9 to 
0".65 from V to z), on October 2.28 UT, 2021, at a midtime of 13.26 days 
after the GRB.

Our results are in agreement with the LDT/LMI observations obtained by 
O'Connor et al. (GCN #30934). At the position of the potential afterglow 
(Kann et al., GCN #30883, #30884), we do not detect any source. 
Calibrating against three nearby stars from the Pan-STARRS catalog 
(converted to Johnson-Cousins filters for V, Rc, Ic, following the 
Lupton transformations, then converted back to AB magnitudes), we derive 
an upper limit of Ic > 25.8 mag, with the V, Rc observations reaching a 
comparable depth and the z observation being shallower.

The nearby source reported by O'Connor et al. (GCN #30934) is detected 
in all filters, and we find that it is marginally extended. We measure, 
again in AB magnitudes, Ic = 23.83 +/- 0.06 mag in an aperture 
corresponding to the stellar PSF, it being somewhat brighter using a 
larger aperture. Therefore, this object is a host galaxy candidate for 
GRB 210919A, in agreement with O'Connor et al. (GCN #30934).

At the eastern edge of the XRT error circle (Goad et al., GCN #30850), 
we find a further, fainter source at RA = 5:21:01.35, Dec. = +1:18:41.9 
(J2000) with an error of 0".5. For this source, we measure Ic = 25.01 
+/- 0.19 mag. As the source is faint, we can't make any definite 
statements about its extension. It is also a host galaxy candidate, 
although its chance association probability is quite large.