UV/Optical Properties of GRBs

with Swift UVOT

P. Roming (Penn State University)

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%

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

Time-to-burst Distribution

110 s

86 s

Color Observation Distribution

Observations < 2000 s Observations > 2000 s

Peak Magnitude Brightness

18.02

Multicolor Light Curves of GRBs

α=1.79 α=1.59

α=1.51 α=1.70

α=1.96 α=1.94

α=1.58

Early “Flaring” in UV/Optical

Temporal Slope Distribution

Color IndependentColor Dependent

0.96

KS test

= 0.087

Burst Color Evolution

Optical-to-X Comparison

Spearman rank correlation (p = 8.8x10-4)

Optical-to-γ Comparison

Spearman rank correlation (p = 0.0184)

Redshift Distribution

UVOT Detected BAT Bursts

Observations of GRB 060313

Event Mode Light Curves

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)