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GCN Circular 22391

Subject
GRB 180205A: Further GROND Observations reveal Afterglow is still bright
Date
2018-02-06T02:28:56Z (6 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
J. Bolmer (ESO/MPE) and D.A. Kann (HETH/IAA-CSIC) report:

We once more observed the field of GRB 180205A (Swift trigger 808625; 
P.A. Evans et al., GCN 22381) simultaneously in g'r'i'z'JHK with GROND 
(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope 
at ESO La Silla Observatory (Chile).

Observations started at 01:28 UT on February 06, 2018, 0.8765 days after 
the GRB trigger. They were performed at an average seeing of 1".1 and at 
an average airmass of 1.68.

Based on total exposures of 1320 seconds in g'r'i'z' and 1200 seconds in 
JHK at a midtime of 01:40 UT, 0.8854 days after the burst, we measure 
the following preliminary magnitudes (AB magnitude system):

g' = 20.29 +/- 0.03 mag,
r' = 20.03 +/- 0.03 mag,
i' = 19.81 +/- 0.03 mag,
z' = 19.66 +/- 0.03 mag,
J  = 19.47 +/- 0.12 mag,
H  = 19.16 +/- 0.19 mag, and
K  > 17.6 mag.

These magnitudes are significantly brighter than the extrapolation of 
our GROND data from the first night (Bolmer & Kann, GCN 22383). We point 
out that the Nickel/KAIT light curve (E. Falcon, GCN 22390) shows a 
plateau phase setting in beyond 10ks, at a time we were not able to 
observe with GROND anymore. This plateau would explain why the afterglow 
remains bright.

Magnitudes and are calibrated against SDSS and 2MASS field stars and are 
not corrected for the Galactic foreground extinction corresponding to a 
reddening of E_(B-V)= 0.03 mag in the direction of the burst (Schlegel 
et al. 1998).

Further observations are encouraged.

We thank Markus Rabus for the excellent support from La Silla.
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