TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4214
SUBJECT: GRB 051109A: ROTSE-III Decline rate for Optical Counterpart
DATE:    05/11/09 02:38:04 GMT
FROM:    Brad Schaefer at LSU  <schaefer@grb.phys.lsu.edu>

B. E. Schaefer (Louisiana State), E.S. Rykoff (U Mich), W. Rujopakarn (U 
Mich), F. Yuan (U Mich), R. Quimby (U Texas), K. Alatalo (Berkeley), T.A. 
McKay (U Mich), C. Akerlof (U Mich), report on behalf of the ROTSE 
collaboration:

A fast fading optical transient has been discovered inside the error
circle for GRB 051109A (Swift trigger 163136) by the ROTSE-IIIb telescope,
located at McDonald Observatory (cf. Rykoff et al. GCN 4211). The source
faded as a power law. At the central time of UT 01:16:05 (228 seconds
after the Swift trigger), the magnitude was 16.21. (Magnitudes are
unfiltered and calibrated against R-band magnitudes for nearby comparison
stars.) The source is continuing to fade, but accurate magnitudes will
have to await a more detailed analysis due to nearby faint stars.

With these magnitudes, the power law index for the decline is -0.4. If 
this decline continues unbroken, then at a time around two hours after the 
burst (UT 03:12), the magnitude should be 17.7. Eight hours after the 
burst (UT 09:12), a magnitude of 18.3 might be expected.