[vsnet-grb-info 28457] GRB 210730A: TNG optical observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30560
SUBJECT: GRB 210730A: TNG optical observations
DATE:    21/07/31 02:05:32 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <pda.davanzo at gmail.com>

P. D'Avanzo (INAF-OAB), V. D'Elia (ASI-SSDC), M. Della Valle (INAF-OACN) A. Melandri (INAF-OAB), A. Rossi (INAF-OAS), 
G. Tagliaferri (INAF-OOAB), A. Harutyunyan, D. Carosati (INAF-TNG) on behalf of the CIBO collaboration report:

We observed the field of GRB 210730A (Sonbas et al., GCN Circ. 30548; Prasad et al., GCN Circ. 30555; Wood et al., GCN Circ. 30577) 
with the Italian 3.6m TNG telescope equipped with the optical camera DOLORES. A series of images were obtained with the r-sdss filter 
on 2021-07-30 from 21:41:57 UT to 22:08:38 UT (i.e. about 17 hours after the burst).  

At the edge of the XRT error circle (https://www.swift.ac.uk/xrt_positions/01062150/) we detect a faint optical source at the following 
position (J2000): 
RA: 09:58:21.96
Dec: +69:41:23.6
(+/- 1.0"). 


With preliminary photometry, carried out using a small aperture to minimise contamination from a nearby bright star, we derive the following 
magnitude:
r = 22.7  +/- 0.2 

(AB; calibrated against the SDSS catalogue). 
In our co-added image we reach a limiting magnitude of r ~ 23.5 (AB, 3sigma c.l.). 

We note that the same source is visible in archival Pan-STARRS images. Using a common set of reference stars and carrying out differential 
photometry on TNG and Pan-STARRS images we find no evidence for a change in magnitude of this source. 

Therefore, this source is likely not related to GRB 210730A. Alternatively, it can be the GRB host galaxy.