TITLE:   GCN CIRCULAR
NUMBER:  9150
SUBJECT: GRB 090418: ROTSE-III Confirmation of Optical Counterpart
DATE:    09/04/18 12:10:59 GMT
FROM:    Fang Yuan at ROTSE  <yuanfang@umich.edu>

F. Yuan (U Mich), B. E. Schaefer (Louisiana State), W. Rujopakarn  
(Steward), report on behalf of the ROTSE collaboration:

ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB  
090418 (Swift trigger 349510; Mangano et al, GCN 9149) in twilight.  
The first image was at 11:07:59.2 UT, 19.0 s after the burst (4.2 s  
after the GCN notice time). The unfiltered images are calibrated  
relative to USNO B1.0.
We detect the optical counterpart at the KAIT and UVOT position  
(Chornock et al. GCN 9148;  Mangano et al, GCN 9149) at the following  
magnitude and limit:

start UT    	    end UT       mag      magerr    mlim(of image)
--------------------------------------------------------------------------------
11:07:59.2     11:09:06.3     15.9       0.1         17.0
11:09:19.6     11:14:02.1     15.9       0.1         16.8
11:14:11.3     11:25:31.5      -             -             16.7