TITLE:   GCN CIRCULAR
NUMBER:  11062
SUBJECT: GRB 100805A: Swift/UVOT Observations of the Optical Afterglow
DATE:    10/08/05 19:02:30 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <Stephen.T.Holland@nasa.gov>

S. T. Holland (CRESST/GSFC/USRA) and E. A. Hoversten (PSU)
report on the behalf of the Swift UVOT team:

      The Swift/UVOT observed the field of GRB 100805A starting 112 s
after the BAT trigger (Hoversten, et al., 2010, GCN Circ. 11047).
Settled observations started at 130 s.  We detect the optical
afterglow (Hoversten, et al., 2010, GCN Circ, 11047) in all filters
except uvw2.  The refined UVOT position is

     RA (J2000) =  19:59:30.51  =  299.87713 (deg)
    Dec (J2000) = +52:37:40.1   =  +52.62783 (deg)

with an estimated uncertainty of 0.43 arcsec (radius, 90% confidence,
statistical + systematic).  This is 1.31 arcsec west of the
UVOT-enhanced XRT position (Goad, et al., 2010, GCN Circ. 11053).

     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)      130      280       147     18.10 0.08
    u (fc)      288      538       246     18.32 0.11

    v           619     5984       432     19.80 0.23
    b           544   10,831      1213     21.22 0.20
    u           288     6600       659     18.55 0.09
 uvw1           668     6395       432     20.78 0.39
 uvm2           643     6189       432     20.68 0.45
 uvw2           594   12,479      1149    >21.6  3-sigma UL
white           130   11,744      1319     19.42 0.07
-----------------------------------------------------------

     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} = 0.19 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 uvw2 filter, and the observed UVOT
spectral energy distribution at 4895 s, is consistent with GRB 100805A
having a redshift of approximately z = 1.3, although we can not rule
out the non-detection in uvw2 being due to extinction in the host
galaxy.  The white light curve exhibits a power-law decay with an
index of alpha = -0.71 +/- 0.03, consistent with what was observed by
Cenko, et al. (2010, GCN Circ. 11051).