The very faint K-band afterglow of GRB 020819 and the dust extinction hypothesis of the dark bursts

We report rapid follow-up K'-band observations of the error box of the bright High Energy Transient Explorer burst GRB 020819. We find that any afterglow was fainter than K' = 19 only 9 hr after the burst. Because no optical afterglow was found, GRB 020819 represents a typical dark burst.'' At first, we discuss if extinction by cosmic dust in the GRB host galaxy could explain the faintness of the afterglow of GRB 020819. We then turn to the entire ensemble of K-band dark afterglows. We find that extinction by cosmic dust in the GRB host galaxies is still a possible explanation for the faintness of many afterglows. In all investigated cases a combination of only a modest extinction with a modest redshift can explain the observations. However, the required extinction is very high if these bursts occurred at redshifts smaller than unity, perhaps arguing for alternative models to explain the nature of the dark bursts.