We present optical and near-infrared ( NIR ) photometry of 28 gamma-ray bursts ( GRBs ) detected by the Swift satellite and rapidly observed by the Reionization and Transients Infrared/Optical ( RATIR ) camera .
We compare the optical flux at fiducial times of 5.5 and 11 hours after the high-energy trigger to that in the X-ray regime to quantify optical darkness .
46 \pm 9 per cent ( 13/28 ) of all bursts in our sample and 55 \pm 10 per cent ( 13/26 ) of long GRBs are optically dark , which is statistically consistently with previous studies .
Fitting RATIR optical and NIR spectral energy distributions ( SEDs ) of 19 GRBs , most ( 6/7 ) optically dark GRBs either occur at high-redshift ( z > 4.5 ) or have a high dust content in their host galaxies ( A _ { V } > 0.3 ) .
Performing K-S tests , we compare the RATIR sample to those previously presented in the literature , finding our distributions of redshift , optical darkness , host dust extinction and X-ray derived column density to be consistent .
The one reported discrepancy is with host galaxy dust content in the BAT6 sample , which appears inconsistent with our sample and other previous literature .
Comparing X-ray derived host galaxy hydrogen column densities to host galaxy dust extinction , we find that GRBs tend to occur in host galaxies with a higher metal-to-dust ratio than our own Galaxy , more akin to the Large and Small Magellanic Clouds .
Finally , to mitigate time evolution of optical darkness , we measure \beta _ { OX,rest } at a fixed rest frame time , t _ { rest } = 1.5 hours and fixed rest frame energies in the X-ray and optical regimes .
Choosing to evaluate optical flux at \lambda _ { rest } = 0.25 ~ { } \mu m , we remove high-redshift as a source of optical darkness , demonstrating that optical darkness must result from either high-redshift , dust content in the host galaxy along the GRB sight line , or a combination of the two .