GCN Circular 24587

N. P. M. Kuin (UCL-MSSL) and M. G. Bernardini (INAF-OAB)
report on behalf of the Swift/UVOT team:

In GCN Circ. 24494
Not found
 
 
 (Kuin and Bernardini) we described how we detect
the afterglow of the GRB, but that the photometry was affected by
an object located at a distance of 1.3 arcsec. Further Swift
observations show that the uv-optical afterglow was no longer
detected in white beyond 26ks after the trigger. Data that were taken
after that time are considered template observations that
provide the background contribution to the photometry.

The template net corrected count rates, with instrumental effects
removed, are found to be as follows for a 3" radius circular aperture
at the location of the GRB as reported in GCN 24494
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:
White = 1.066 +/- 0.031 c/s; u = 0.149 +/- 0.016 c/s;
b = 0.509 +/- 0.061 c/s; uvw2 =  0.011 +/- 0.011 c/s;
v = 0.272 +/- 0.042 c/s; while uvm2 and uvw1 were not observed
until after the afterglow became too faint.

After removal of the template contribution, the following preliminary
detections and 3-sigma upper limits were determined from the count rates
using the zeropoints in the UVOT photometric system (Poole et al. 2008,
MNRAS 383, 627; Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for
the image mode exposures:

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white              131          281          147          15.96 +/- 0.05
u                  290          385           93          19.62 +/- 0.42
b                 5684         5884          197          > 20.13
white             5888         6088          197          20.61 +/- 0.37
uvw2              6094        11850          518          > 17.81
v                17150        17450          300          > 19.35

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.09 in the direction of the burst
(Schlegel et al. 1998).