1. Radio faint GRB afterglows Sydney Institute for Astronomy (SIfA)/ CAASTRO – The University of Sydney Dr. Paul Hancock with Bryan Gaensler, Tara Murphy, and Davide Burlon

3. Overview ›Intro to GRBs ›Radio properties of GRBs ›Radio detection rate 3

4. Gamma Ray Bursts ›Intense bursts of gamma-rays detected by satellites such as Swift / FERMI ›Associated with the core collapse of massive stars 4

5. GRB afterglows ›Long GRBs associated with SNIbc T Totani Piran, 2003, Nature, 422, 268

6. Only 30% of GRBs are detected at radio frequencies ›304 GRBs observed (VLA+ATCA) ›Only 30% have a detected radio afterglow ›Detections and upper limits overlap in flux 6 GRB980703A GRB980329A Days Since Burst "Typical" Bright Chandra&Frail 2012

7. Sensitivity limitations? ›Assumption is that detection rate is a function of sensitivity. ›Implicitly: -There is a single population of GRBs -Flux (Luminosity) distribution may be broad but is single peaked -Better sensitivity would result in more detections ›Thus -The destinction between bright/faint is artificial -the mean flux of the faint GRBs is not far below this artificial divide 7

8. Redshift distribution 8

9. Parameter distributions 9 0.3-10 keV X-ray Flux (erg/cm 2 /s) Redshift No-Redshift R-band optical flux (μJy) Cumulative Fraction Redshift No-Redshift Gamma ray Fluence (erg/cm 2 ) Redshift No-Redshift

10. Conclusion ›The detection rate is NOT being biased by -Differences in redshift distribution -Our ability to measure redshift 10

11. The effect of limited sensitivity 11

12. 1x 12 hour observation SNR ~ 5 Visibility stacking 12 12x 1 hour observations SNR ~ 1 For a population of sources, visibility stacking can measure the (weighted) mean flux of the population. Hancock et al., 2011, ApJ,735, L35

13. Stacking Results 13 100-1000 times brighter

14. What flux would we have expected? 14

15. Producing a model flux distribution 15 Luminosity ModelsFlux Distribution=> Redshift Distribution =>

16. Model predictions 16

17. Stacked observations Consistent with predictions ~5x fainter than predictions

18. 18

19. 19

20. Radio faint afterglows are fainter at early times too! 20 0.3-10 keV X-ray Flux (erg/cm 2 /s) Bright Faint Beaming-corrected Gamma-ray Energy (erg) Bright Faint Median-TimeBrightFaint Radio* (mJy)0-5d4< 0.03 R-band (μJy)11h41.66.0 X-ray (erg/cm 2 /s)11h25.36.4 Gamma-ray (x10 52 erg)sec-min102 R-band optical flux (μJy) Bright Faint Cumulative Fraction GRBs with radio faint afterglows are less luminous at all wavelengths * radio data is mean not median

21. Two (more) populations of GRBs ›Long-soft GRBs are either radio bright or radio faint ›There are intrinsic differences between the bright and faint GRBs 21

22. A spectral break leads to dark/faint GRBs 22 Piran, 1999, Phys.Rep, 314, 575Greiner et al., 2010, A&A, 526, A30