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GCN Circular 29753

Subject
GRB 210402A: LBT observations
Date
2021-04-03T18:51:46Z (3 years ago)
From
Andrea Rossi at INAF <andrea.rossi@inaf.it>
A Rossi (INAF-OAS) report on behalf of the CIBO collaboration:

We observed the field of GRB 210402A (Page et al., GCN 29743) 
simultaneously in the r' and z' bands with the LBC imager mounted on LBT 
(Mt Graham, AZ, USA). We obtained 20min of imaging on 2021-04-03 at 
mid-time 05:33:00 UT, 0.305 days days after the swift GCN. Observations 
were performed under an average seeing of ~1.4".

Within the refined XRT position (Evans etal., GCN 29750) we detect an 
extended source in both bands at position (J2000)
RA =13:13:02.04
DEC=+44:30:52.7
and preliminary magnitudes (AB system),
r=23.9+-0.2
z=23.1+-0.3
calibrated against SDSS field stars.

We note that a second source with r=23.4+-0.1 lies at position (J2000)
RA =13:13:02.46
DEC=+44:30:50.3
which is at south-east and outside the XRT error circle, and thus 
unlikely associated eith the GRB.

Moreover, this second source is already present (but it is very faint) 
in archival Pan-STARRS images, while the first one is beyond the survey 
limits. We note that, assuming that both are galaxies, the first one is 
more likely to be the host galaxy because of its brightness and distance 
which result in a smaller probability of chance association.

In both cases we cannot exclude that the afterglow is still contributing 
to the brightness of this sources. Further observations are necessary to 
assess their variability.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, 
particularly B. Rothberg, J. Williams, and F. Cusano, in obtaining these 
observations.
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