1. UV/Optical Properties of GRBs with Swift UVOT P. Roming (Penn State University)

2. Burst Detection Distribution 229 observed GRBs –UVOT detected ~26% (individual exposure) –UVOT detected ~40% (combined exposure) –Ground-based detected ~60% (typically redder detections) –No detections ~40%

3. Why Are So Many “Dark”? Late observations Rapid temporal decay Circumburst extinction High redshift Ly- α blanketing & absorption Suppression of the reverse shock Wrong part of the sky

4. Time-to-burst Distribution 110 s 86 s

5. Color Observation Distribution Observations < 2000 sObservations > 2000 s

6. Peak Magnitude Brightness 18.02

8. Multicolor Light Curves of GRBs α=1.79α=1.59 α=1.51α=1.70 α=1.96α=1.94 α=1.58

9. Early “Flaring” in UV/Optical

10. Temporal Slope Distribution Color Independent Color Dependent 0.96 KS test = 0.087

11. Burst Color Evolution

12. Optical-to-X Comparison Spearman rank correlation (p = 8.8x10 -4 )

13. Optical-to- γ Comparison Spearman rank correlation (p = 0.0184)

14. Redshift Distribution

15. UVOT Detected BAT Bursts

16. Observations of GRB 060313

17. Event Mode Light Curves

18. Conclusions UV/optical single instrument samples becoming large enough to perform statistical analysis UV/optical regime trailing the X-ray but coming of age More data needed in the sample (i.e. multi-color information, SHBs light curves)