GCN Circular 9450

P. Schady (MSSL-UCL) and J. K. Cannizzo (NASA/UMBC) report on behalf of 
the Swift UVOT team:

The Swift/UVOT began settled observations of the field of GRB 090530 158s 
after the BAT trigger (Cannizzo et al., GCN Circ. 9438
Not found
 
 
) and a decaying 
source is detected in all UVOT filters within the XRT error circle 
(Beardmore et al., GCN Circ. 9445
Not found
 
 
), putting an upper limit on the redshift 
of z < 1.7. The source initially rises, and at around 100s after the BAT 
trigger decays at a constant rate of 0.73+/-0.03 for the duration of 
intial UVOT observations, out to ~20ks after the BAT trigger.  The best 
UVOT position determined from a co-added white band exposure is

RA (J2000)  11:57:40.50   =  179.41873 (deg)
Dec (J2000) +26:35:38.4   =  +26.59400 (deg)

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence, 
statistical + systematic), consistent with the ROTSE-IIIb afterglow 
position (Flewelling et al., GCN Circ. 9439
Not found
 
 
).

The magnitudes for the observations currently available are as follows:

Filter   T_start(s)  T_stop(s)  Exp(s)       Mag
white       83         233       147    17.43 +/- 0.03
v           624        644       19     17.69 +/- 0.29
b           550        570       19     18.60 +/- 0.27
u           295        545       246    17.46 +/- 0.05
uvw1        5735       5935      197    20.15 +/- 0.37
uvm2        5530       11687     1082   20.31 +/- 0.22
uvw2        5121       17468     1279   21.31 +/- 0.32

The values quoted above are not corrected for the Galactic extinction due 
to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel 
et al. 1998). The photometry is on the UVOT photometric system described 
in Poole et al. (2008, MNRAS, 383, 627).