TITLE:   GCN CIRCULAR
NUMBER:  23243
SUBJECT: GRB 180914B: Continued RATIR Optical and NIR Observations
DATE:    18/09/17 07:20:02 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>

Eleonora Troja (GSFC), Nat Butler (ASU), Alan M. Watson (UNAM),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer
(UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB),
Antonino Cucchiara (UVI), , Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jesus Gonzalez (UNAM), Carlos Roman-Zuniga (UNAM),
Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.),
and Vicki Toy (UMD) report:

We re-observed the field of GRB 180914B (Ursi, et al., GCN  23226)
with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the
Observatorio Astronomico Nacional on Sierra San Pedro Martir from
2018/09 17.12 to 2018/09 17.28 UTC (56.53 to 60.22 hours after the
burst), obtaining a total of 2.49 hours exposure in the r and i
bands and 1.04 hours exposure in the Z, Y, J, and H bands.

At the position of the optical/nIR counterpart (Zheng & Filippenko,
GCN 23237; Troja, et al., GCN 23238), in comparison with the
SDSS DR9 and 2MASS catalogs, we obtain the following detections:

r = 20.46 +/- 0.01
i = 20.08 +/- 0.01
z = 19.74 +/- 0.02
Y = 19.57 +/- 0.02
J = 19.33 +/- 0.02
H = 19.08 +/- 0.03

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

Compared to our first night of observations (Troja, et al., GCN 23238),
the source significantly faded at an approximate decay rate of t^(-1.4),
shallower than the value found by De Ugarte Postigo, et al. (GCN 23239)
at earlier times.

Further observations are planned.

We thank the staff of the Observatorio Astronomico Nacional in San Pedro
Martir.