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Radio faint GRB afterglows

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. Overview. Intro to GRBs Radio properties of GRBs Radio detection rate. Gamma Ray Bursts.

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Radio faint GRB afterglows

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

  2. Overview • Intro to GRBs • Radio properties of GRBs • Radio detection rate

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

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

  5. 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 Bright GRB980703A "Typical" GRB980329A Days Since Burst Chandra&Frail 2012

  6. 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. Redshift distribution

  8. Parameter distributions No-Redshift Redshift Cumulative Fraction R-band optical flux (μJy) No-Redshift Redshift No-Redshift Redshift Gamma ray Fluence (erg/cm2) 0.3-10 keV X-ray Flux(erg/cm2/s)

  9. Conclusion • The detection rate is NOT being biased by • Differences in redshift distribution • Our ability to measure redshift

  10. The effect of limited sensitivity

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

  12. Stacking Results 100-1000 times brighter

  13. What flux would we have expected?

  14. Producing a model flux distribution Luminosity Models => Redshift Distribution => Flux Distribution

  15. Model predictions

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

  17. 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

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

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