GRB 190919B: Rapid optical rise explained as a flaring activity
Karpov, Sergey; Hu, Y. -D.; Castro-Tirado, Alberto J.; Rigoselli, Michela; Mašek, Martin; Hiriart, David; Lee, William H.; Topinka, Martin; Prouza, Michael; Hudec, René; Jelínek, Martin; Maleňáková, Alžběta; Štrobl, Jan; Ebr, Jan; Cunniffe, Ronan; Thoene, Christina; Janeček, Petr; Fernandez-García, Emilio; Vítek, Stanislav; Trávníček, Petr
Czech Republic, Italy, Spain, Mexico
Abstract
Following the detection of a long GRB 190919B by INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory), we obtained an optical photometric sequence of its optical counterpart. The light curve of the optical emission exhibits an unusually steep rise ∼100 s after the initial trigger. This behaviour is not expected from a `canonical' GRB optical afterglow. As an explanation, we propose a scenario consisting of two superimposed flares: an optical flare originating from the inner engine activity followed by the hydrodynamic peak of an external shock. The inner-engine nature of the first pulse is supported by a marginal detection of flux in hard X-rays. The second pulse eventually concludes in a slow constant decay, which, as we show, follows the closure relations for a slow cooling plasma expanding into the constant interstellar medium and can be seen as an optical afterglow sensu stricto.