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Gray transits of WD 1145+017 over the visible band

We have observed several relatively deep transits of the white dwarf WD 1145+017 with the GTC over the wavelength range 480 nm to 920 nm. The observations covered approximately one hour on 2016 Jan 18 and two hours on 2016 Jan 20. There was variable extinction of the white dwarf during much of that time, but punctuated by four sharp transits with depths ranging from 25% to 40%. The spectrum was dispersed with a grism, and the flux data were ultimately summed into four bands centered at 0.53, 0.62, 0.71, and 0.84 $μ$m. After careful normalization, we find that the flux light curves in all four bands are consistent with being the same, including through the deepest dips. We use these results to compute Ångström exponents, $α$, for the particles responsible for the extinction and find |$\langle α\rangle $| $\lesssim 0.06$, assuming that the extinction is relatively optically thin. We use the complex indices of refraction for common minerals to set constraints on the median sizes of possible dust grains, and find that for most common minerals, particle sizes $\lesssim 0.5 \, μ$m can be excluded.