{
  "bibcode": "2020GCN.28492....1M",
  "body": "D. B. Malesani (DTU Space), L. Izzo (DARK/NBI), D. A. Kann (HETH, \nIAA/CSIC), A. de Ugarte Postigo (HETH, IAA/CSIC), A. J. Levan (Radboud \nUniv.), D. Xu (NAOC), P. D'Avanzo (INAF/OABr), and A. Rossi (INAF/OAS) \nreport on behalf of the Stargate collaboration:\n\nFor the recent GRB 200922A, several authors have noted the presence of a \nbright optical afterglow, initially detected by UVOT onboard the Neil \nGehrels Swift observatory (Moss et al., GCN 28471; Izzo et al., GCN \n28478; Siegel & Moss, GCN 28482), superimposed on a point-like object, \nseen e.g. in the DSS, DECam, and SkyMapper surveys.\n\nWe retrieved the white-band UVOT images (Siegel & Moss, GCN 28482). We \nfocused on the initial image (344 s exposure starting at 12:08:32 UT on \nSep 22) and on the image taken roughly one day after (274 s exposure \nstarting at 10:05:38 UT on Sep 23). We also retrieved for comparison the \narchival DECam i-band images of the same region (from 2017 August).\n\nWe remark that the position of the UVOT object is entirely consistent \nwith the latest XRT position of the afterglow (Evans et al., GCN 28483; \nsee also https://www.swift.ac.uk/xrt_positions/), strengthening the \nassociation between the optical transient and GRB 200922A.\n\nWe then cross-matched the astrometry between the UVOT and the archival \nDECam images. The scatter in the astrometric solution, computed on stars \nof brightness similar to the UVOT afterglow, is 0.06 and 0.10 arcsec for \nthe initial and late exposure, respectively.\n\nIn the initial UVOT image, we measure an offset of 0.62\" compared to the \nobject seen in the archival data. While small, this offset is \nsignificant. The same comparison carried out in the late time image, \nhowever, returns no measurable offset to within the errors. \nIndependently, we also checked that the proper motion of the star \n(catalogued in the Gaia DR2; Brown et al. 2018, A&A, 616, A1) is too \nsmall to play any effect in producing the offset.\n\nWe thus conclude that this is a rare case of near-perfect chance \nsuperposition between a background afterglow and a bright foreground \nstar. In the initial UVOT image, the light was dominated by the \nafterglow (thus yielding the offset in the position), which subsequently \nfaded to a level fainter than the foreground star. The light curve of \nthe X-ray afterglow (D'Ai et al., GCN 28476) is also consistent with \nthis interpretation, as it shows the typical behaviour of a cosmological \nGRB.\n\n\nA spectrum of the optical transient was taken using the ESO VLT1 UT1 \n(Antu) equipped with the FORS2 instrument. Observations were carried out \nusing the grisms 300V and 300I (900 s exposure each), covering the \nwavelength range 3500-9200 AA, and started on 2020 Sep 22.997 UT (11.8 \nhr after the trigger). In the acquisition image, using (old) archival \nzeropoints, we measure V = 18.0 (Vega). This value is roughly consistent \nwith the archival values of the star (for example the Gaia catalog gives \nG = 17.85), and indicates only little contribution from the afterglow at \nthe epoch of our spectrum (at least in the V band).\n\nThe spectrum we observe is consistent with the one of a G-type star. \nAbsorption features are observed at z = 0 from Ca H and K, Mg I, Na I D, \nHdelta, Hgamma, Hbeta, and Halpha. The ordinary type of the star \nsupports the lack of a connection with the GRB. A search was conducted \nfor potential emission lines from the background GRB host galaxy, but \nthe intense and spatially-variable glare from the star does not allow us \nto place quantitative limits. We are thus unfortunately unable to \nprovide constraints on the GRB redshift.\n\n\nWe welcome the reopening of the ESO observatory at Paranal, even if at \nreduced capacity. We thank the ESO staff for carrying out our \nobservations, in particular Steffen Mieske, Claudia Cid, and Romain \nThomas, as well as and the entire support team on- and off-site.",
  "circularId": 28492,
  "createdOn": 1600982128000,
  "email": "malesani@space.dtu.dk",
  "subject": "GRB 200922A: chance superposition between optical afterglow and archival star",
  "submitter": "Daniele B Malesani at DTU Space  <malesani@space.dtu.dk>",
  "eventId": "GRB 200922A"
}