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,
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.