TITLE:   GCN CIRCULAR
NUMBER:  30436
SUBJECT: GRB 210704A: OSIRIS/GTC Observations and Archival Detection of the Possible Host Galaxy 
DATE:    21/07/10 07:23:45 GMT
FROM:    Alan M Watson at UNAM  <alan@astro.unam.mx>

Alan M. Watson (UNAM), Eleonora Troja (GSFC/UMD), Rosa L. Becerra (UNAM), Rubén
Sánchez (INAF),  Aishwarya Thakur (INAF), Simone Dichiara (GSFC/UMD), Nat Butler
(ASU), William H. Lee (UNAM), Océlotl Lopez (UNAM), Margarita Pereyra (UNAM),
and Srihari Ravi (ASU) report:

We observed the field of GRB 210704A (Berretta et al., GCN Circ. 30375; D'Ai et
al., GCN Circ. 30379; Malacaria & Meegan, GCN Circ. 30380; Kim et al., GCN Circ.
30384) with the OSIRIS instrument on the 10.4 meter GTC telescope.

We observed at two epochs. The first epoch was at 2.10 days after burst and the
second at 5.10 days after burst.  At both epochs, we detect a source at the
position of the candidate optical afterglow (Kim et al., GCN Circ. 30384) at a
magnitude consistent with the TNG observations of r~23.3 at 4.1 days (D'Avanzo
et al., GCN Circ. 30432). We also resolve a second fainter (r~24.5 AB mag)
source (S2) located 0.7 arcsec E and 2.4 arcsec S of the OT position.

Our observations indicate only a marginal fading of about 0.2 magnitudes in the
r-band between our two epochs, which is unusually shallow for a GRB afterglow or
for a kilonova.

Archival pre-explosion images of the field reveal the presence of an underlying
faint source (S1), which might be the host galaxy. Based on a deep exposure
obtained in 2012 with the MegaPrime/MegaCam, we estimate a magnitude of r~25.3
+/- 0.2 AB mag for S1. The source S2 is also detected in archival images at a
magnitude consistent with our GTC observations.

Assuming no intrinsic variability, S1 would only partially contribute to the
observed optical light (~25%) and this by itself is not sufficient to explain
the slow fading of the optical emission. Given the ambiguous classification of
the gamma-ray emission, whose duration lies at the intersection between short
and long GRBs, we cannot exclude that the observed flattening marks the onset of
an associated supernova.

Further observations to monitor the evolution of this source are encouraged.

We thank David Garcia and Antonio Cabrera for assistance with these
observations.