01/02/2009 Moriond 2009

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.

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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.

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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

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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

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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

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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

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Raw correlation between telescope response time and redshiftPearson 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!

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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!

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Non-evolving simulated GRB optical LF to demonstrate the Malmquist bias

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Simulated Malmquist bias on average redshift for different telescope response times

Long response times (fainter OA) correlated with smaller redshifts

Malmquist

Anti-Malmquist

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Response times plotted with average redshift of the potentially observable OA…using an evolving OA LF.

Anti-Malmquist

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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?

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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

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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

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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