GCN Circular 14003

S. T. Holland (STScI) and
E. A. Helder (PSU)
report on behalf of the Swift/UVOT team:

     The Swift/UVOT began settled observations of the field of GRB
121123A starting 131 s after the BAT trigger (Helder et al., 2012,
GCNC 13982).  The refined UVOT position is

     RA (J2000)  20:29:16.30  =  307.31792 (deg)
    Dec (J2000) -11:51:35.6   =  -11.85989 (deg)

with an estimated uncertainty of 0.44 arcsec (radius, 90% confidence,
statistical + systematic).  We observe a slow decay of the optical
afterglow, consistent with that reported by Xu et al., (2012, GCNC
13986).  The afterglow is weakly detected in white at approximately 40
ks after the BAT trigger.  However, the UVOT photometry is affected by
a bright (B ~ 14 mag) star (USNO B1.0 0781-0714667) located
approximately 22 arcsec north of the afterglow, so the late-time
behavior of the optical afterglow's light curve requires further
analysis.  Preliminary UVOT photometry, and 3-sigma upper limits, for
the afterglow are presented below.

------------------------------------------------------
Filter      TSTART     TSTOP    Exposure     Mag   Err
------------------------------------------------------
white (fc)     131       281         147   19.43  0.15
u (fc)         289       539         246   19.35  0.23
white (fc)     869      1019         147   19.13  0.12
------------------------------------------------------
v              619      1070          58   18.29  0.33
b              546      1169          58   19.12  0.29
u              695      1145          39   18.58  0.32
uvw1           669    18,011        2137  >21.5
uvm2           645    17,104        1376  >21.3
uvw2           595    22,980        1376  >21.5
white          570       590          19  >19.7
------------------------------------------------------

    The detection in the UVOT u band, combined with the non-detection
in the UVOT uvw1 band, is consistent with GRB 121123A having a
redshift of approximately 1.5 < z < 3.4.

    The quoted magnitudes and upper limits have not been corrected for
the Galactic extinction along the line of sight to this burst of
E_{B-V} = 0.05 mag (Schlafly et al. 2011, ApJS, 737, 103).  The
photometry is in the UVOT photometric system (Breeveld et al. 2011,
AIP Conf. Proc. 1358, 373).