L. Izzo (DARK/NBI), D. B. Malesani (DTU Space) and D. A. Kann (HETH/IAA-CSIC) report:
We observed the field of GRB 200922A (Moss et al., GCN #28471) with the Sinistro instrument mounted on the 1-m telescope of the LCO network located at Siding Spring Observatory, Australia. Observations started on September 22 at 13:53:10 UT (1.77 hours after the GRB trigger). We obtained a series of 5x120s images in r and i filters each.
Within the UVOT-enhanced XRT position at RA (J2000) = 19:47:48.87 (296.95364), Dec. (J2000) = -55:12:15.9 (-55.20442) with an error of 1".5 (improved compared to the one reported in Osborne et al., GCN #28473) we clearly detect the source that had also been seen in the early UVOT observation (Moss et al., GCN #28471) at a white magnitude of 14.4. In our stacked image we measure i = 17.31 +- 0.04 (AB mag), calibrated against nearby SkyMapper catalog stars. This value is brighter than the archival value i = 17.74 +- 0.01 mag from the SkyMapper Data Release 2 (Onken et al., 2019, PASA 36, 33). Our result suggests variability compared to the historic value, and likely fading compared to the earlier UVOT observation. Similar fading is also measured versus archival DECam field images (i = 17.79 +- 0.02 mag, NOIRLab Prop. ID 2017A-0388; PI: A. Zenteno).
The coordinates of the source in our images are (J2000.0):
RA = 19:47:48.9285
Dec = -55:12:16.421
At this stage, it is unclear if this GRB was caused by the likely stellar source, or if it is a background GRB that happens to have occurred very close to a foreground object. Further observations are encouraged.
This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 730890.