TITLE: GCN CIRCULAR NUMBER: 17751 SUBJECT: GRB 150424A: Swift/UVOT Observations DATE: 15/04/24 15:03:18 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC) and A. P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150424A 99 s after the BAT trigger (Beardmore et al., GCN Circ. 17743). A source consistent with the position of the suggested optical afterglow (Perley and McConnell, GCN Circ. 17745) is detected in the initial UVOT exposures. The source remains nearly constant in 6 exposures with the white filter starting 99 s after the trigger and ending 6786 s after the trigger. The magnitude is approximately that reported by Perley and McConnell. This may be a detection of the host galaxy. The preliminary UVOT position is: RA (J2000) = 10:09:13.38 = 152.30577 (deg.) Dec (J2000) = -26:37:51.1 = -26.63086 (deg.) with an estimated uncertainty of 0.6 arcsec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limit using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 99 6786 609 20.38 +/- 0.07 v 641 5881 432 >19.8 b 567 6701 432 20.71 +/- 0.21 u 311 5061 462 20.13 +/- 0.20 w1 690 6291 413 20.00 +/- 0.24 m2 4452 6086 393 19.91 +/- 0.29 w2 616 5677 216 20.03 +/- 0.31 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.06 in the direction of the burst (Schlegel et al. 1998). [GCN OPS NOTE(24apr15): Per author's request the missing minus sign on the hh:mm:ss format of the value was added; the decimal-deg value was correct.]