TITLE: GCN CIRCULAR NUMBER: 28022 SUBJECT: GRB 200623A: OAJ deep multi-color imaging, XRT #2 is pre-detected DATE: 20/06/24 01:07:31 GMT FROM: Alexander Kann at IAA-CSIC 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.