The Canonical Swift/UVOT Lightcurve

1. Sam Oates (UCL-MSSL) On behalf of the Swift/UVOT team UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS MULLARD SPACE SCIENCE LABORATORY The Canonical Swift/UVOT Lightcurve

2. The Sample Criteria To investigate the nature of optical/UV afterglows, we required a large number of well sampled, good quality UVOT lightcurves. • Peak magnitude in v <18th mag • GRBs without significant colour evolution • Observations must have commenced within the first 400s after the trigger • Most GRBs in the sample were observed within the first 100s • Observed until at least 105s after the trigger 26 GRBs contained in the sample. All GRBs in the sample were reduced systematically The count rates in each filter were normalized to the v The normalized lightcurves were binned with a ∆t/t = 0.2

3. The Lightcurves • Ordered by peak magnitude 12th Magnitude • General behaviour: • 10 lightcurves peak or shallow at the start of observations • 16 decay from the start of observations Brightest GRBs decay the quickest 17.8 Magnitude Mean peak time:400s

4. Temporal Index vs. Peak Magnitude Temporal Index Before 500s Temporal Index After 500s No correlation Spearman rank correlation: -0.23 at 73% Significant correlation Spearman rank correlation: 0.59 at 99.9%

5. Temporal Index: Before 500s vs. after 500s • Behaviour before 500s: • Wide range of values • Best fit mean: • α<500s = -0.48 dispersion 0.68 α<500s= 0 • Behaviour after 500s: • All lightcurves are decaying • Best fit mean: • α>500s = -0.87 dispersion 0.31 Behaviour after 500s is not affected by the behaviour before 500s Spearman rank correlation -0.17 with a poor statistical significance at 60% α<500s= α>500s

6. What could be the cause of the early rise? X • Reverse shock? • After the peak a decay α=(3p=1)/4 is expected • α=-1.75 for p=2 and α=-2.5 for p=3 • GRB 050726, GRB 061007 and GRB 070529 are the only GRBs consistent with α<-1.75 (at 95% confidence level) • Passage of νm? • Expected to produce a rise α=0.5 followed by a decay α=3(1-p)/4 ~ -1 • Implies colour evolution at early times • Would expect to see a step change in the UVOT lightcurve as the observing filter changes from white to v. • Dust Destruction? • Initially reddened and dim • Would expect to see the afterglow brighten and become less red as the dust is destroyed • GRB 060607a is the only GRB with a red excess X X

7. GRB 060607a GRB 070420

8. What could be the cause of the early rise? • Peak of the forward shock?Expected to produce a rise α= 2 or 3 followed by a decay α=3(1-p)/4 ~ -1Dependant on the Lorentz factor of the shell The shells of the GRBs with observed rises have lower Lorentz factors and are decelerated at a larger distance (Equation 1., Molinari et al. 2006)

9. Comparison with XRT canonical model Optical/UV afterglows do not follow the same behaviour as the X-ray afterglows, at least in the early afterglow.

10. What could be the reason for the correlation between magnitude and decay? (Assuming all GRBs have a similar energy budget) • A steeper value for p for the brighter GRBs (?) • Energy is released over a longer period for faint GRBs • The observers viewing angle of a jet will affect the optical afterglow observed. • The larger the observers viewing angle the shallower the temporal decay and the lower the peak magnitude. (e.g Panaitescu & Vestrand 2008)

11. Luminosity at restframe wavelength 1600Å

12. Luminosity at restframe wavelength 1600Å Mean = 11.38 Standard deviation = 0.66 Mean = 10.16 Standard deviation = 0.70 Mean = 8.91 Standard deviation = 0.82 The log luminosities at each epoch indicate a single distribution

13. Conclusions • The optical afterglow before 500s may rise or decay. • All optical afterglows after 500s are decaying. • The brightest GRBs decay the quickest. • The peak in the optical afterglows is most likely due to the start of the forward shock • The typical optical afterglow behaves very differently to the XRT canonical model especially before 500s.