GCN Circular 23256

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), L. Izzo, C. C. Thoene, M. Blazek, K. Bensch (all 
HETH/IAA-CSIC), and A. Sota (IAA/CSIC) report:

We observed the location of the extremely bright AGILE/Fermi GRB 180914B 
(Ursi et al., GCN #23226; Bissaldi et al., GCN #23232) with the T150 
telescope of the Sierra Nevada Observatory (OSN). We obtained 3 x 180 s 
images in the Ic band, centered 3.157 days after the GRB.

The afterglow (e.g., Zheng & Filippenko, GCN #23237; Troja et al., GCN 
#23238; de Ugarte Postigo et al., GCN #23239) is clearly detected, and 
we derive Ic (AB) = 20.28 +/- 0.06 mag vs. several SDSS comparison 
stars, using the transformation equations of Lupton (2005).

Using the photometry given in the GCNs so far (Zheng & Filippenko, GCN 
#23237; Troja et al., GCN #23238, #23243; de Ugarte Postigo et al., GCN 
#23239; Kuin, GCN #23241; Mazaeva et al., GCN #23244, #23249; Lipunov et 
al., GCN #23245; D'Avanzo et al., GCN #23246; Ramsay et al., GCN #23252; 
Watson et al., GCN #23253, Izzo et al., GCN #23255), we find the optical 
afterglow can be fit by an achromatic simple power-law with a decay 
slope alpha = 1.65 +/- 0.02 (taking host-galaxy magnitudes from SDSS 
into account). The SED (from u to H band) is well-fit with a small 
amount of SMC extinction, we find beta = 0.94 +/- 0.11, A_V = 0.09 +/- 
0.07 (assuming F_nu ~ t^(-alpha)*nu^(-beta) ). Note the earliest 
detection, from MASTER, lies significantly below the back-extrapolation 
of the later decay slope, indicating a rebrightening may have taken 
place.