44th Rencontres de Moriond Very High Energy Phenomena in the Universe

1. 44th Rencontres de MoriondVery High Energy Phenomena in the Universe Dr David Coward & Alan Imerito University of Western Australia Why the Swift GRB redshift distribution is changing in time Moriond 2009

2. The GRB redshift distribution The spatial distribution of GRBs is a powerful probe of GRB rate evolution. Potentially be used as an independent tracer of massive star formation in the early Universe. Potentially be used to probe the evolution of GRB environments. Moriond 2009

3. What we observe • Optical/NIR afterglows have been found for nearly 80% of GRBs • Only 40–50% of these have measured redshifts (now over 100) • Optically dark bursts - extinction- GRB environment, host galaxy type and distance • Preferentially measure redshifts from optically bright GRBs. Moriond 2009

4. GRB redshift statistics • Pre-Swift - <z> about 1.4 • Early Swift - <z> about 2.8 (in 2005-2006) • Swift is more sensitive to higher-z longer duration GRBs • Recent Swift -<z> about 2 (2008) • Statistical moments of the redshift distribution should converge to constants given enough statistics Moriond 2009

5. Time series analysis of GRB redshifts • Search for evolution in statistical moments • time-dependent selection effects • 92 long GRB redshifts from 2005-2008 • Motivation - redshifts measured from bright optical afterglow absorption spectra - expect biases • This is linked to the efficiency of GRB follow-up telescopes to acquire absorption spectra Moriond 2009

6. Swift triggered redshift time series Red squares - redshifts Solid line - nearest neighbour averaging of reshift Open circles - optical afterglow magnitude at discovery Average z is evolving on time-scale of years - must be an observation bias Moriond 2009

7. Response times for spectroscopy enabled GRB follow-up telescopes Optical afterglow brightness decays as 1/T Average time to acquire absorption spectra for the VLT has reduced from about 1000 min in 2005 to 100 min in 2008 The so-called learning curve effect Moriond 2009

8. Raw correlation between telescope response time and redshift Pearson Rho = 0.41 Plotting the time-series windowed averages clearly identifies the +ve trend More probable for a long response time to be correlated with a more distant GRB. Does this make sense! Moriond 2009

9. Malmquist bias revisited • Malmquist bias - for flux limited surveys - high-z events originate from the bright end of the GRB optical LF. • At small-z, can see both faint and bright end of LF. • Long telescope response times -> fainter OA because of 1/T -> only seen at relatively smaller z • Short telescope response times -> brighter OA -> seen at relatively higher z • What we find is the opposite…an “anti-Malmquist” bias! Moriond 2009

10. Non-evolving simulated GRB optical LF to demonstrate the Malmquist bias Moriond 2009

11. Simulated Malmquist bias on average redshift for different telescope response times Malmquist Anti-Malmquist Long response times (fainter OA) correlated with smaller redshifts Moriond 2009

12. Response times plotted with average redshift of the potentially observable OA…using an evolving OA LF. Anti-Malmquist Moriond 2009

13. To produce an anti-Malmquist bias in the simulations we employ an OA LF that evolves with z. GRBs OA optical brightness must be evolving with z? Are the high-z bursts intrinsically brighter or less obscured? Moriond 2009

14. Summary • Response times of large telescopes to acquire a redshift are decreasing in 2005-2008 period • Average GRB redshift is reducing over the same period • Longer average telescope response times are correlated with larger average redshifts • An “anti-Malmquist” bias is observed: that is GRBs at high-z are easier to see than expected • To reconcile this trend, simulations suggest that GRBs at high-z must be relatively brighter than those at small-z • The analysis implies that GRB optical selection effects are potentially an important tool for probing GRB environments Moriond 2009

15. Future work • Use OA data to confirm how the OA brightness affects the probability of obtaining a redshift • Differentiate between dust obscuration and intrinsic GRB brightness • Is the change of GRB optical obscuration with z linked with the history of massive star evolution? • Selection effects in astronomy are often considered a problem…in this case they might actually reveal new insight into the origin and evolution of of GRBs Moriond 2009

16. Acknowledgements • Australian Research Council • UWA • Moriond 09 organisers - Hady schenten and many others • I have only seen snow twice in my lifetime…Moriond 09 is the second time Moriond 2009