2014A&A...569A..93J

We study the dark nature of GRB130528A through multi-wavelength observations and conclude that the main reason for the optical darkness is local extinction inside of the host galaxy. Automatic observations were performed at the Burst Optical Observer and Transient Exploring System (BOOTES)-4/MET robotic telescope. We also triggered target of opportunity (ToO) observations at Observatorio de Sierra Nevada (OSN), IRAM Plateau de Bure Interferometer (PdBI) and Gran Telescopio Canarias (GTC + OSIRIS). The host galaxy photometric observations in optical to near-infrared (nIR) wavelengths were achieved through large ground-based aperture telescopes, such as 10.4m Gran Telescopio Canarias (GTC), 4.2m William Herschel Telescope (WHT), 6m Bolshoi Teleskop Alt-azimutalnyi (BTA) telescope, and 2m Liverpool Telescope (LT). Based on these observations, spectral energy distributions (SED) for the host galaxy and afterglow were constructed. Thanks to millimetre (mm) observations at PdBI, we confirm the presence of a mm source within the XRT error circle that faded over the course of our observations and identify the host galaxy. However, we do not find any credible optical source within early observations with BOOTES-4/MET and 1.5m OSN telescopes. Spectroscopic observation of this galaxy by GTC showed a single faint emission line that likely corresponds to [OII] 3727Å at a redshift of 1.250±0.001, implying a star formation rate (M_☉/yr)>6.18M_☉/yr without correcting for dust extinction. The probable line-of-sight extinction towards GRB130528A is revealed through analysis of the afterglow SED, resulting in a value of A^GRB_V≥0.9 at the rest frame; this is comparable to extinction levels found among other dark GRBs. The SED of the host galaxy is explained well (χ²/d.o.f.=0.564) by a luminous (M_B=-21.16), low-extinction (A_V=0, rest frame), and aged (2.6Gyr) stellar population. We can explain this apparent contradiction in global and line-of-sight extinction if the GRB birth place happened to lie in a local dense environment. In light of having relatively small specific star formation rate ∼5.3M_☉/yr(L/L^*)^–1, this also could explain the age of the old stellar population of host galaxy.