Optical follow-up of GRB 180720B: the first Gamma-Ray Burst observed in gamma-rays from the ground

While the gamma-rays from the bursts are usually detected by space telescopes, GRB 180720B marks the first ever observation of gamma-rays produced by a burst from the ground. We perform the optical follow-up and multispectral analysis of the long Gamma-Ray Burst GRB 180720B in order to establish the physical properties of the relativistic fireball related to the burst. The major part of the observational data presented in this paper was performed by the robotic telescope D50 at the Ond\v rejov Observatory near Prague, Czech republic. The system started observing 9.8 hours after the trigger and continued detecting the optical afterglow for the following 3 days.