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GCN Circular 28022

Subject
GRB 200623A: OAJ deep multi-color imaging, XRT #2 is pre-detected
Date
2020-06-24T01:07:31Z (4 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), M. Blazek, C. C. Thoene, J. F. Agui Fernandez (all 
HETH/IAA-CSIC), V. Tilve, A. Moreno Signes, and H. Vasquez Ramio (all 
CEFCA) report:

We observed the error circle of the short GRB 200623A (Fermi GBM 
detection and analysis: Fermi GBM Team, GCN #28011, Veres & Meegan, GCN 
#28019; Swift BAT/GUANO localization: DeLaunay et al., GCN #28013) with 
the T80 0.8m telescope at the Observatorio de Javalambre (Teruel, 
Spain). Observations consisted of 3 x 500 s in g', 3 x 500 s in r',  8 x 
180 s in i', and 8 x 180 s in z', at midtimes 0.816779, 0.798439, 
0.779694, and 0.755722 days after the GRB, respectively.

At the position of XRT source #1, the known quasar from the Veron-Cetty 
& Veron 2006 catalog is clearly detected.

At the position of XRT source #2, we clearly detect the source proposed 
by Belkin et al. (GCN #28020) as a candidate afterglow. Measured against 
three nearby SDSS comparison stars, we find (AB mags):

g' = 23.70 +/- 0.30 mag;
r' = 22.55 +/- 0.15 mag;
i' = 21.54 +/- 0.15 mag;
z' = 20.74 +/- 0.10 mag,

indicating this is a very red source. We assume that the observations of 
Belkin et al. were performed without a filter, this would explain the 
brighter magnitude they find (their use of comparison stars from the 
USNO catalog may also play a role).

These very red colors are untypical for a GRB afterglow. Furthermore, 
comparison with images form PanSTARRS clearly reveals a pre-detection of 
the source at a similar magnitude and color, see:

https://www.iaa.csic.es/~deugarte/GRBs/200623A/200623A_color2.jpg

for a comparison. We therefore believe it is unlikely that this source 
is associated with GRB 200623A (additionally, the XRT detection is of 
low significance for now).

At the position of XRT source #3, we detected multiple sources in the 
vicinity also seen in PanSTARRS imaging. The faint source detected by 
the TNG (D'Avanzo et al., GCN #28021), may be vaguely detected in our r' 
and z' images, but it is unclear how much this could be due to the PSF 
of a nearby bright star. We present a second comparison image here:

https://www.iaa.csic.es/~deugarte/GRBs/200623A/200623A_color.jpg

No other obviously bright afterglow candidate is detected in the BAT 
error circle, however, a second epoch of comparable depth would be 
needed for a more detailed analysis.
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