GCN Circular 5894

J. Holtzman, T. Harrison, B. Mcnamara of New Mexico State University report:

The robotic NMSU 1m telescope at Apache Point Observatory responded 
to Swift trigger 240766 (Sbarufatti et al. GCN 5854
Not found
 
 
) shortly after the
burst announcement. Our first 10s I band image was started 47s
after the trigger (31s after the notification). This was followed
by images in R, V, B, and U; an optical source at the location of
the optical counterpart was clearly detected in all of the images
with the following brightness:

filter |  exptime |  midtime after burst  | magnitude +/- error

   I       10s          52s                  12.24 +/- 0.004
   R       10s          97s                  13.86 +/- 0.007
   V       20s         149s                  14.94 +/- 0.009
   B       40s         213s                  15.95 +/- 0.011
   U       60s         303s                  15.85 +/- 0.037

Our magnitudes were calibrated relative to a nearby star for which
transformed UBVRI magnitudes were obtained from the SDSS magnitudes
(Cool et al, GCN 5863
Not found
 
 
) using transformations given on the SDSS web site,
http://www.sdss.org/dr5/algorithms/sdssUBVRITransform.html, which were
taken from a paper by Jester et al. (2005). Error bars are statistical
only and do not include uncertainties in the SDSS photometry, the adopted
SDSS to UBVRI transformations, nor internal transformations; systematic
errors are probably on the order of several percent.

Ten cycles of UBVRI observations were continued for about an hour after
the burst. Analysis of temporal and spectral evolution is underway. Late
epoch imaging is also continuing.