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Transcript
44th Rencontres de Moriond Very High Energy Phenomena in the Universe Why the Swift GRB redshift distribution is changing in time Dr David Coward & Alan Imerito University of Western Australia 01/02/2009 Moriond 2009 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. 01/02/2009 Moriond 2009 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 - extinctionGRB environment, host galaxy type and distance • Preferentially measure redshifts from optically bright GRBs. 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009 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! 01/02/2009 Moriond 2009 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! 01/02/2009 Moriond 2009 Non-evolving simulated GRB optical LF to demonstrate the Malmquist bias 01/02/2009 Moriond 2009 Simulated Malmquist bias on average redshift for different telescope response times Malmquist Anti-Malmquist Long response times (fainter OA) correlated with smaller redshifts 01/02/2009 Moriond 2009 Response times plotted with average redshift of the potentially observable OA…using an evolving OA LF. Anti-Malmquist 01/02/2009 Moriond 2009 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? 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009 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 01/02/2009 Moriond 2009