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

7

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

ulat

ive

Frac

tion

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

11

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

15

Luminosity Models Flux Distribution=> Redshift Distribution =>

Model predictions

16

Stacked observations

Consistent with

predictions

~5x fainter than

predictions

18

19

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