TITLE:   GCN CIRCULAR
NUMBER:  8544
SUBJECT: GRB081121: Refined Swift/UVOT observations
DATE:    08/11/22 12:13:02 GMT
FROM:    Samantha Oates at MSSL  <sro@mssl.ucl.ac.uk>

S. R. Oates (MSSL-UCL) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 081121,
2816s after the BAT trigger (Oates et al., GCN Circ. 8537). We detect the
optical afterglow in the white, v, b, u filters and marginally in the
uvw1 filter at the position:

RA(J2000.0)  =   5:57:06.15
DEC(J2000.0) = -60:36:10.0

with an estimated uncertainty of 0.6 arcsec (radius, 90% confidence).
This position is consistent with the UVOT-enhanced XRT position
and the position reported by ROTSE-IIIC (Yuan et al., GCN Circ. 8536).

The marginal detection of the optical afterglow in uvw1, and the lack
of a detection in the uvm2 and uvw2 filters is consistent with a redshift
of z=2.512 reported by Magellan (Berger and Rauch, GCN Circ. 8542).

The magnitudes and 3 sigma upper limits are reported below:

Filter    T_start (s) T_stop  Exposure      Mag/3sig UL
---------------------------------------------------------
white       2816       2965     147       17.93 +/- 0.04
v           2972       3172     197       17.58 +/- 0.08
b           3792       3992     197       18.26 +/- 0.07
u           3587       3787     197       17.62 +/- 0.06
uvw1        3383       3582     197       20.60 +/- 0.48 (2.3-sigma)
uvm2        3177       4813     393       > 20.09
uvw2        4203       11200    790       > 20.78
---------------------------------------------------------

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.05 mag (Schlegel et al.,
1998, ApJS, 500, 525).  The photometry is on the UVOT flight system
described in Poole et al. (2008, MNRAS, 383,627).