Table 3: Summary of optical and ultraviolet observations. All data but the point in the R band were measured by Swift-UVOT. The R-band observations is by Tristram et al. (2005). All measurements were corrected for the Galactic extinction ( $A_{\rm V} = 0.123$ mag; Schlegel et al. 1998), assuming the Milky Way extinction curve by Pei (1992). The optical-to-X-ray spectral index is $\beta_{\rm OX} = \log(F_\nu^{\rm opt}/F_\nu^{\rm X}) / \log(\nu_{\rm X}/\nu_{\rm opt})$. The X-ray frequency was set to $\nu _{\rm X} = 1.5$ keV, the logarithmic mean of the XRT observing range. The X-ray flux was computed by interpolating the XRT light curve to the time of the optical limit. The last column reports the ratio of the extrapolated X-ray flux to the optical one (assuming no spectral breaks between the two bands; see Fig. 7).

Time since burst		 Filter Wavelength Exposure time Extinction Magnitude Flux density $\beta_\mathbf{OX}$ X-ray/optical ratio
(s)   (Å) (s) (mag)   ($\mu$Jy)    

3300		 UVW2 1930 10 0.29 > 17.41 < 99.11 < 0.52 $\rm {>} 23^{+8.9}_{-6.4}$
3306 V 5460 100 0.12 > 18.66 < 107.5 < 0.45 $\rm {>} 65 ^{+31}_{-21}$
3350 UVM2 2220 100 0.36 > 18.54 < 34.17 < 0.32 $\rm {>} 75 ^{+30}_{-21}$
3470 UVW1 2600 100 0.27 > 18.75 < 27.80 < 0.29 $\rm {>} 103^{+43}_{-30}$
5219 U 3450 750 0.20 > 19.65 < 11.00 < 0.24 $\rm {>} 176^{+77}_{-54}$
9390 B 4350 714 0.16 > 20.87 < 17.47 < 0.46 $\rm {>} 52 ^{+24}_{-16}$

24 840		 $R_{\rm MOA}$ 6399 600 0.10 > 20.20 < 25.70 < 0.74 $\rm {>} 10^{+5.0}_{-3.4}$


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