TITLE: GCN CIRCULAR NUMBER: 28381 SUBJECT: GRB 200906A: LBT near-infrared upper limit DATE: 20/09/07 18:02:35 GMT FROM: Andrea Rossi at INAF A. Rossi (INAF-OAS) reports on behalf of the CIBO collaboration: We observed the location of the X-ray afterglow (Beardmore et al., GCN 28373) of the Swift GRB 200906A (Page et al., GCN 28371) simultaneously in the H and Ks bands with the LUCI near-infrared imager and spectrograph mounted on the Large Binocular Telescope (LBT, Mt Graham, AZ, USA). Observations started at 03:57 UT on 2020-09-07, and allowed us to obtain 18 min of exposure in each band. Unfortunately, observations were performed under variable sky conditions with passing clouds. Inspection of the combined H-band images (mid-time of 0.628 days after the GRB), does not reveal any source down to the limiting magnitude H<21.8 mag (AB system). A preliminary comparison of the simultaneous Swift/XRT flux of 1.06e-07 Jy at a mid-frequency 3.8E+17 Hz (1.6 keV), gives a X-ray to infrared slope-index b_OX<0.5, confirming the dark nature of this GRB. This XRT flux has been obtained modelling the XRT lightcurve after 200 seconds with a power-law with decay-index 0.60+-0.04. We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly B. Rothberg, F. Cusano, S. Paiano and D. Paris, in obtaining these observations.