TITLE:   GCN CIRCULAR
NUMBER:  11420
SUBJECT: GRB 101117B: Swift/UVOT Observations of the Optical Afterglow
DATE:    10/11/22 15:25:34 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <Stephen.T.Holland@nasa.gov>

S. T. Holland (CRESST/GSFC/USRA) and
C. W. Wolf (PSU)
report on the behalf of the Swift UVOT team:

      The Swift/UVOT observed the field of GRB 101117B starting 63 s
after the BAT trigger (Wolf, et al., 2010, GCN Circ. 11411).  Settled
observations started at 82 s.  We detect the optical afterglow
(Holland, 2010, GCN Circ. 11412) in the U, B, and white filters.  The
refined UVOT position is


     RA (J2000)  11:32:00.54  =  173.00225 (deg)
    Dec (J2000) -72:39:45.9   =  -72.66275 (deg)

with an estimated uncertainty of 0.43 arcsec (radius, 90% confidence,
statistical + systematic).  This is 1.16 arcsec northwest of the
UVOT-enhanced XRT position (Goad, et al., 2010, GCN Circ. 11413).

     Preliminary magnitudes, and 3-sigma upper limits for detecting a
source in the finding charts and in the co-added images, are

Filter      T_start   T_stop    Exp(s)      Mag   Err
-----------------------------------------------------------
white (fc)       82      232       147     17.81 0.08 
    u (fc)      295      545       246     18.68 0.15

    v           626      646        20    >17.9
    b           551      571        20     18.22 0.27
    u           700     1325        59    >19.1
 uvw1           675     1300        78    >19.3
 uvm2           825     1275        58    >18.7   
 uvw2           601     1051        58    >19.0
white           575      595        20     19.20 0.34
-----------------------------------------------------------

     The quoted magnitudes and upper limits have not been corrected
for the expected Galactic extinction along the line of sight
corresponding to a reddening of E_{B-V} = 0.38 mag (Schlegel, et al.,
1998, ApJS, 500, 525).  All photometry is on the UVOT photometry
system described in Poole et al. (2008, MNRAS, 383, 627).

     The non-detection in the UVOT uvw1 filter, and the observed UVOT
spectral energy distribution at 600 s, is consistent with GRB 101117B
having a redshift of approximately z <= 3, although we can not rule
out the non-detection in the ultraviolet being due to extinction in
the host galaxy.  The white light curve exhibits a power-law decay
with an index of alpha = -0.8 +/- 0.1 between 82 and 6005 s after the
BAT trigger.