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GCN Circular 9462

GRB 090531B: ROTSE-III Optical Limits
2009-05-31T19:20:55Z (15 years ago)
Brad Schaefer at LSU <>
B. E. Schaefer (Louisiana State), T. Guver (U Arizona), S. B. Pandey (U 
Mich), report on behalf of the ROTSE collaboration:

ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, 
responded to GRB 090531B (Swift trigger 353728; J. R. Cummings et al., GCN 
9461), producing images beginning 7.3 s after the GCN notice time. An 
automated response took the first image at 18:36:22.3 UT, 26.1 s after the 
burst, and during the gamma-ray emission, under excellent conditions. We 
took 10 5-sec, 10 20-sec and 50 60-sec exposures. These unfiltered images 
are calibrated relative to USNO A2.0 (R). Imaging is on going.

Comparison to the DSS (second epoch) reveals no new sources within the 
3-sigma Swift/BAT error circle, for both single images and coadding into 
sets of 10; however, we are limited as the field is somewhat crowded. 
Individual images have limiting magnitudes ranging from 15.3-16.0; we set 
the following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
18:36:22.3   18:36:27.3         5     15.3           26.1       N
18:47:31.3   18:52:11.6       280     16.3          695.1       Y

[GCN OPS NOTE(31may09):  Per author's request, the "B" was added
to the burst name.]
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