TITLE: GCN CIRCULAR NUMBER: 9450 SUBJECT: GRB090530: Swift/UVOT afterglow detection DATE: 09/05/30 17:08:39 GMT FROM: Patricia Schady at MSSL/Swift 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) and a decaying source is detected in all UVOT filters within the XRT error circle (Beardmore et al., GCN Circ. 9445), 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). 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).