TITLE:   GCN CIRCULAR
NUMBER:  22985
SUBJECT: GRB 180720B: OSN detection, fading slower?
DATE:    18/07/21 02:51:06 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. A. Kann, L. Izzo (both HETH/IAA-CSIC), V. Casanova (IAA-CSIC), and A. de
Ugarte Postigo (HETH-IAA/CSIC and DARK/NBI) report on behalf of HETH:

We observed the position of the extremely bright GRB 180720B (Swift-BAT 
detection: Siegel et al., GCN #22973, GCN #22975; Fermi-LAT detection: 
Bissaldi and Racusin, GCN #22980; Fermi-GBM detection: Roberts et al., 
GCN #22981) with the T90 telescope of the Observatorio Sierra Nevada 
(OSN) near Granada, Spain.

The optical afterglow (Martone et al., GCN #22976; Sasada et al., GCN 
#22977; Reva et al., GCN #22979; Itoh et al., GCN #22983) is clearly 
detected in single 180 s images. For an Rc image at mid-time 0.455188 
days after the INTEGRAL SPI/ACS trigger (14:21:40 UT), we derive Rc = 
17.53 +/- 0.03 mag.

We note that this is only ~0.4 mag fainter than the value found by Reva 
et al. at 0.23 days after the GRB, indicating the decay may have slowed 
down. This feature also seems to be seen in the XRT light curve (Page et 
al., GCN #22984).

Further observations, even with smaller telescopes, are strongly 
encouraged.

Magnitudes were obtained against SDSS standard stars transformed 
following the equations of Lupton (2005).

[GCN OPS NOTE(21jul18): Due to a processing problem with some
in-line UTF formatting characters, the "slower?" was deleted
from the SUBJECT-line in the distributed circulars.  It has been
replaced in these archived copies.]

[GCN OPS NOTE(21jul18): Per author's request, A. de U.P. was added
to the author list.]