{
  "bibcode": "2020GCN.28022....1K",
  "body": "D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, \nDARK/NBI), M. Blazek, C. C. Thoene, J. F. Agui Fernandez (all \nHETH/IAA-CSIC), V. Tilve, A. Moreno Signes, and H. Vasquez Ramio (all \nCEFCA) report:\n\nWe observed the error circle of the short GRB 200623A (Fermi GBM \ndetection and analysis: Fermi GBM Team, GCN #28011, Veres & Meegan, GCN \n#28019; Swift BAT/GUANO localization: DeLaunay et al., GCN #28013) with \nthe T80 0.8m telescope at the Observatorio de Javalambre (Teruel, \nSpain). Observations consisted of 3 x 500 s in g', 3 x 500 s in r',  8 x \n180 s in i', and 8 x 180 s in z', at midtimes 0.816779, 0.798439, \n0.779694, and 0.755722 days after the GRB, respectively.\n\nAt the position of XRT source #1, the known quasar from the Veron-Cetty \n& Veron 2006 catalog is clearly detected.\n\nAt the position of XRT source #2, we clearly detect the source proposed \nby Belkin et al. (GCN #28020) as a candidate afterglow. Measured against \nthree nearby SDSS comparison stars, we find (AB mags):\n\ng' = 23.70 +/- 0.30 mag;\nr' = 22.55 +/- 0.15 mag;\ni' = 21.54 +/- 0.15 mag;\nz' = 20.74 +/- 0.10 mag,\n\nindicating this is a very red source. We assume that the observations of \nBelkin et al. were performed without a filter, this would explain the \nbrighter magnitude they find (their use of comparison stars from the \nUSNO catalog may also play a role).\n\nThese very red colors are untypical for a GRB afterglow. Furthermore, \ncomparison with images form PanSTARRS clearly reveals a pre-detection of \nthe source at a similar magnitude and color, see:\n\nhttps://www.iaa.csic.es/~deugarte/GRBs/200623A/200623A_color2.jpg\n\nfor a comparison. We therefore believe it is unlikely that this source \nis associated with GRB 200623A (additionally, the XRT detection is of \nlow significance for now).\n\nAt the position of XRT source #3, we detected multiple sources in the \nvicinity also seen in PanSTARRS imaging. The faint source detected by \nthe TNG (D'Avanzo et al., GCN #28021), may be vaguely detected in our r' \nand z' images, but it is unclear how much this could be due to the PSF \nof a nearby bright star. We present a second comparison image here:\n\nhttps://www.iaa.csic.es/~deugarte/GRBs/200623A/200623A_color.jpg\n\nNo other obviously bright afterglow candidate is detected in the BAT \nerror circle, however, a second epoch of comparable depth would be \nneeded for a more detailed analysis.",
  "circularId": 28022,
  "createdOn": 1592960851000,
  "email": "kann@iaa.es",
  "subject": "GRB 200623A: OAJ deep multi-color imaging, XRT #2 is pre-detected",
  "submitter": "Alexander Kann at IAA-CSIC  <kann@iaa.es>",
  "eventId": "GRB 200623A"
}