GCN Circular 8770

P.A. Curran (UCL-MSSL), V. Mangano (INAF-IASF PA) and S.T. Holland
(GSFC) on behalf of the Swift/UVOT team.


The Swift Ultra-Violet/Optical Telescope (UVOT) began settled 
observations of the Swift localised GRB 090102 (#338895; Mangano et al., 
GCN 8762) ~400 seconds after the BAT trigger.

An afterglow, consistent with the enhanced XRT position (Beardmore et 
al., GCN 8765) and with the afterglow candidate of Klotz et al. (GCN 
8761), is detected at the position reported by  Mangano et al. (GCN 
8762). The source is detected in the initial optical observations but 
quickly fades (with alpha ~ 0.9 +/- 0.2 in white filter) below detection 
limits. The initial magnitudes (or 3 sigma limits) are:


Filter  t_mid(s)  Exp(s)  Mag

wh      470.      147.    18.04 +/- 0.04
v       382.      10.     17.02 +/- 0.29
b       660.      19.     18.35 +/- 0.27
u       635.      19.     17.89 +/- 0.26
uvw1    3547.     255.    >19.99
uvm2    6278.     197.    >19.67
uvw2    1035.     388.    >20.45

The detection in u along with the limits in uvw1 and bluer is consistent
with a redshift of approximately ~1.8 < z < ~2.5 and with the
spectroscopic redshift suggested by de Ugarte Postigo et al. (GCN 8766).

The values quoted above are in the UVOT photometric system (Poole et al.
2008, MNRAS, 383, 627).  They are not corrected for the expected
Galactic extinction corresponding to a reddening of E(B-V)=0.047 mag in
the direction of the burst (Schlegel et al. 1998).