Context :

Aims : We study the dark nature of GRB 130528A through multi-wavelength observations and conclude that the main reason for the optical darkness is local extinction inside of the host galaxy .

Methods : 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 .

Results : 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 ] 3727 \AA at a redshift of 1.250 \pm 0.001 , implying a star formation rate ( SFR ) ( M _ { \odot } /yr ) \textgreater 6.18 M _ { \odot } /yr without correcting for dust extinction .
The probable line-of-sight extinction towards GRB 130528A is revealed through analysis of the afterglow SED , resulting in a value of A ^ { GRB } _ { V } \geq 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 ( \chi ^ { 2 } / d . o . f . =0.564 ) by a luminous ( M _ { B } =-21.16 ) , low-extinction ( A _ { V } =0 , rest frame ) , and aged ( 2.6 Gyr ) 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 SFR ( SSFR ) \sim 5.3 M _ { \odot } /yr ( L / L ^ { \star } ) ^ { -1 } , this also could explain the age of the old stellar population of host galaxy .

Conclusions :