radio faint grb afterglows sydney institute for astronomy (sifa)/ caastro – the university of...
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Radio faint GRB afterglows
Sydney Institute for Astronomy (SIfA)/ CAASTRO – The University of Sydney
Dr. Paul Hancockwith Bryan Gaensler, Tara Murphy, and Davide Burlon
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
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
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
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Redshift distribution
8
Parameter distributions
9
0.3-10 keV X-ray Flux(erg/cm2/s)
Red
shift
No-
Red
shift
R-band optical flux(μJy)
Cum
ula
tive
Fra
ctio
n
Red
shift
No-
Red
shift
Gamma ray Fluence(erg/cm2)
Red
shift
No-
Red
shift
Conclusion
› The detection rate is NOT being biased by
- Differences in redshift distribution
- Our ability to measure redshift
10
The effect of limited sensitivity
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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
Stacking Results
13
100-1000 times
brighter
What flux would we have expected?
14
Producing a model flux distribution
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Luminosity Models Flux Distribution=> Redshift Distribution =>
Model predictions
16
Stacked observations
Consistent with
predictions
~5x fainter than
predictions
18
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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
A spectral break leads to dark/faint GRBs
22Piran, 1999, Phys.Rep, 314, 575Greiner et al., 2010, A&A, 526, A30