TITLE: GCN CIRCULAR NUMBER: 7145 SUBJECT: GRB 070429B: Possible UVOT Detection of an Optical Afterglow DATE: 07/12/10 15:32:47 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC Holland, S. T. (CRESST/USRA/GSFC), de Pasquale, M. (UCL/MSSL), and Markwardt, C.~B. (CRESST/GSFC/UMD) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of the short-hard GRB 070429B starting 230 s after the BAT trigger (Markwardt, et al., GCN Circ. 6358). We have re-examined this data in light of the redshift for the Antonelli, et al. (GCN Circ. 6372) source "A" reported by Perley, et al. (GCN Circ. 7140). We find weak evidence in the UVOT data for a fading source located inside the refined XRT error circle (Beardmore, et al., GCN Circ. 6360) at RA = 21:52:03.68 (J2000) Dec = -38:49:43.6 (J2000) with an uncertainty of 0.6 arcsec (90% confidence). This is 0.6 arcsec from the ground-based position of source "A". For a redshift of z = 0.904, and (H_0,Omega_M,Omega_Lambda) = (70,0.3,0.7) this corresponds to 4.7 proper kpc. This possible source is detected with a 3.9-sigma significance in a summed image (summed over all seven filters) between 591 and 2661 s after the BAT trigger. It is not detected in subsequent all-filter summed images. This possible UVOT source has the following magnitudes and upper limits. All magnitudes are on the UVOT system described in Poole, et al. (2007, MNRAS, in press). Filter T_start T_stop Exposure Mag Err Significance v 2002 2661 94 19.3 0.4 2.5 6805 7004 393 >20.1 3-sigma upper limit b 1460 2598 156 20.5 0.5 2.2 34,978 35,884 885 >22.0 3-sigma upper limit u 403 42,285 2260 >21.8 3-sigma upper limit uvw1 379 41,789 3075 >22.4 3-sigma upper limit uvm2 354 20,226 2192 >22.0 3-sigma upper limit uvw2 456 30,222 2378 >22.4 3-sigma upper limit white 591 977 117 21.6 0.5 2.2 6395 8027 393 >22.1 3-sigma upper limit The observations where the suspected source is weakly detected are all at less than one hour after the BAT trigger. The earliest reported ground-based detection of source "A" is Cucchiara, et al. (GCN Circ. 6368) who find r = 23.18 +/- 0.15 at approximately 4 hours. Perley, et al. (GCN Circ. 7140) find R = 23.24 +/- 0.05 at approximately 7 months, which implies that the source did not fade, and conclude that the "A" source is the host galaxy. They also find g = 24.29 +/- 0.14 at this time. If we assume that the SDSS g band is approximately similar to the UVOT v band then the expected v magnitude of the host at 2332 s (assuming a constant magnitude) is v ~= 24.3, which is 5 mag fainter than the UVOT detection at that time. This corresponds to a mean decay index of alpha ~= -2.5. The observed decay power-law indices from the UVOT data are alpha_v <= -0.6 +/- 0.3, alpha_b <= -0.5 +/- 0.2, and alpha_white <= -0.2 +/- 0.2. The shallow decay slope in the white filter, and the faintness of the White detection at early time (t_mid = 784 s) relative to the b and v detections at later times (t_mid = 2332 (v), 2029 (b)) suggest that the afterglow candidate may have increased in luminosity at some point between approximately 784 and 2029 s. We do not detect any source at the position of source "B" (Antonelli, et al., GCN Circ. 6372) down to the following limiting magnitudes. Filter T_start T_stop Exposure 3-Sigma Upper Limit v 230 31,793 2011 >21.5 b 427 35,115 1463 >22.3 u 403 42,285 2260 >21.9 uvw1 379 41,789 3075 >22.4 uvm2 354 20,226 2192 >22.0 uvw2 456 30,222 2378 >22.4 white 247 36,269 1088 >22.7 The values quoted above are not corrected for the expected Galactic extinction corresponding to a reddening of E_{B-V} = 0.03 mag (Schlegel, et al. 1998).