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We use the afterglow detection statistics of the systematic follow-up observations performed with GROND since mid-2007 in order to derive the fraction of 'dark bursts' according to different methods, and to distinguish between various scenarios for 'dark bursts'. For long-duration Swift bursts with a detected X-ray afterglow, we achieve a 90% (35/39) detection rate of optical/NIR afterglows whenever our observations started within less than 240 min after the burst. Complementing our GROND data with Swift/XRT spectra we construct broad-band spectral energy distributions and derive rest-frame extinctions. We detect 25-40% 'dark bursts', depending on the definition used. The faint optical afterglow emission of 'dark bursts' is mainly due to a combination of two contributing factors: (i) moderate intrinsic extinction at moderate redshifts, and (ii) about 22% of 'dark' bursts at redshift >5. [ABSTRACT FROM AUTHOR]

During the first decade after the discovery of the first GRB afterglow, optical light curves have been considered to be characterised by a quite smooth behaviour, usually being well described with single or broken power law decays from some hours up to weeks after the trigger. Only a few densely sampled events showed re-brightenings or fluctuations around the power law continuum in the optical. After the launch of Swift, the optical light curve smoothness paradigm has been questioned thanks to the faster and richer available optical follow up. That triggered the interest of the whole GRB community. GRB 081029 is an outstanding example of this standard view crisis. Its optical-NIR light curve is characterised by an extremely sharp intense re-brightening at late times that cannot be explained in the framework of the simplest standard fireball models. Thanks to the 7-band contemporaneous imager GROND, we have been able to follow the temporal and spectral evolution of this afterglow with an unprecedented resolution. This leads us to rule out some of the most common models proposed for explaining the re-brightenings and to constrain alternative scenarios (late time central engine activity vs. environment properties). After 3 years of activity, the capabilities of GROND are showing that this behaviour not unique making GRB 081029 a fundamental laboratory for interpreting the afterglow emission processes. [ABSTRACT FROM AUTHOR]

More than 13 years after the discovery of the first afterglows, the nature of dark gamma-ray bursts (GRB) has still eluded explanation: while each long-duration GRB typically has an X-ray afterglow, optical/NIR emission is only seen for 40-60% of those. Here we use the afterglow detection statistics of the systematic follow-up observations performed with GROND since mid-2007 in order to derive the fraction of 'dark bursts'. We find that the faint optical afterglow emission of 'dark bursts' is due to a combination of two components: (i) moderate intrinsic extinction at moderate redshifts, and (ii) about 25% of bursts at redshift >5. [ABSTRACT FROM AUTHOR]

We report on early observations of the optical afterglow of GRB 070411 obtained by the Cˇrni Vrh 0.6-m, the ROTSE IIIc 0.45-m, and the Tautenburg 1.34-m Schmidt telescope starting within 1 to 5 minutes after the trigger. The derived light curve presents a rising phase, then a plateau followed by a steep decay and finally a jet break. [ABSTRACT FROM AUTHOR]