Dust reddening and extinction curves towards gamma-ray bursts at z > 4
  Dust is known to be produced in the envelopes of AGB stars, the expanded
shells of supernova (SN) remnants, and in situ grain growth in the ISM,
although the corresponding efficiency of each of these dust formation
mechanisms at different redshifts remains a topic of debate. During the first
Gyr after the Big Bang, it is widely believed that there was not enough time to
form AGB stars in high numbers, so that the dust at this epoch is expected to
be purely from SNe, or subsequent grain growth in the ISM. The time period
corresponding to z ~5-6 is thus expected to display the transition from SN-only
dust to a mixture of both formation channels as we know it today. Here we aim
to use afterglow observations of GRBs at redshifts larger than $z > 4$ in order
to derive host galaxy dust column densities along their line-of-sight and to
test if a SN-type dust extinction curve is required for some of the bursts. GRB
afterglow observations were performed with the 7-channel GROND Detector at the
2.2m MPI telescope in La Silla, Chile and combined with data gathered with XRT.
We increase the number of measured $A_V$ values for GRBs at z > 4 by a factor
of ~2-3 and find that, in contrast to samples at mostly lower redshift, all of
the GRB afterglows have a visual extinction of $A_V$ < 0.5 mag. Analysis of the
GROND detection thresholds and results from a Monte-Carlo simulation show that,
although we partly suffer from an observational bias against highly
extinguished sight-lines, GRB host galaxies at 4 < z < 6 seem to contain on
average less dust than at z ~ 2. Additionally, we find that all of the GRBs can
be modeled with locally measured extinction curves and that the SN-like dust
extinction curve provides a better fit for only two of the afterglow SEDs. For
the first time we also report a photometric redshift of $z = 7.88$ for GRB
100905A, making it one of the most distant GRBs known to date.
