GRB150309A Ks and g bands images

We performed the early optical observations using the BOOTES robotic telescope and late-time afterglow observations using the GTC. A potential faint host galaxy is also detected at optical wavelength using the GTC.We modelled the potential host galaxy of GRB 150309A in order to explore the environment of the burst. The time-resolved spectral analysis of Fermi data indicates a hybrid jet composition consisting of a matter-dominated fireball and magnetic-dominated Poynting flux.

GTC observations of the afterglow revealed that the counterpart of GRB 150309A was very red, with H-Ks>2.1mag (95% confidence). The red counterpart was not discovered in any bluer filters of Swift UVOT/BOOTES, indicative of high redshift origin. This possibility was discarded based on multiple arguments, such as spectral analysis of X-ray afterglow constrain z < 4.15 and a moderate redshift value obtained using spectral energy distribution (SED) modelling of the potential galaxy.

The broadband (X-ray to NIR bands) afterglow SED implies a very dusty host galaxy with deeply embedded GRB (suggesting AV>~35mag). The environment of GRB 150309A demands a high extinction towards the line of sight, demanding dust obscuration is the most probable origin of optical darkness and the very red afterglow of GRB 150309A. This result makes GRB 150309A the highest extinguished GRB known to date.