GCN Circular 11026

S. R. Oates (MSSL-UCL) and D. C. Morris (GWU/GSFC) report on behalf of  
the Swift/UVOT team:

The Swift/UVOT began observerving the field of GRB 100728B 101s
after the BAT trigger (Morris et al., GCN Circ. 11009
Not found
 
 
). We detect the
optical afterglow in the white, v, b and u filters only.

The refined UVOT position is RA (J2000)= 163.48809, DEC  
(J2000)=-45.47265,
which is equivalent to:

      RA (J2000) = 02:56:13.467
     Dec (J2000) = +00:16:52.18

with an estimated uncertainty of 0.52 arcsec (radius, 90% confidence).

This position is consistent with the XRT refined position (Evans et  
al., GCN Circ 11017
Not found
 
 
)
and with the optical afterglow positions reported by KAIT (Perley et  
al., GCN Circ 11007
Not found
 
 
, 11010
Not found
 
 
 & 11024),
PROMPT (Ivarsen et al., GCN Circ 11008
Not found
 
 
, 11011
Not found
 
 
) & the SON-NM  
observatory (Elenin et al., GCN Circ 11012
Not found
 
 
).
The non-detection in the UV filters suggests a photometric redshift of  
z~2.8.

Preliminary magnitudes and the 3 sigma upper limits are reported below  
for
individual optical images and summed UV images.

Filter    T_start(s)  T_stop(s)  Exposure    Mag/3SigUL
###########################################################
white     101          251         147        17.43 +/- 0.08
white     4713         4913	   197	      20.62 +/- 0.22
v         3687         3887        197	      20.19 +/- 0.56
b         4509         4709	   197	      20.47 +/- 0.33
u	  4303         4503	   197	      19.90 +/- 0.28
uvw1	  4098         6050	   294	      > 20.50
uvm2      3892         5529	   393	      > 20.52
uvw2      3482         5119	   393	      > 20.93
###########################################################

The above magnitudes are not corrected for the Galactic extinction
corresponding to a non-negligible reddening of E_{B-V} = 0.07 mag  
(Schlegel et al.,
1998, ApJS, 500, 525).  The photometry is on the UVOT photometric system
described in Poole et al. (2008, MNRAS, 383,627).