The mystery of the missing GRB redshifts

It is clear that optical selection effects have distorted the “true" GRB redshift distribution to its presently observed biased distribution. We a statistically optimal model that implies GRB host galaxy dust extinction could account for up to 40% of missing optical afterglows and redshifts in z = 0 − 3, but the bias is negligible at very high-z. The limiting sensitivity of the telescopes, and the time to acquire spectroscopic/photometric redshifts, are significant sources of bias for the very high-z sample. We caution on constraining star formation rate and luminosity evolution using the GRB redshift distribution without accounting for these selection effects.