GRB 190919B: Rapid optical rise explained as a flaring activity

¹ Astronomical Institute (ASU CAS), Ondřejov, Czech Republic
e-mail: mates@asu.cas.cz
² INAF – Instituto di Astrofisica Spaziale e Fisica Cosmica, Milano, Italy
³ Department of Theoretical Physics and Astrophysics, Masaryk University, Brno, Czech Republic
⁴ Institute of Physics of Czech Academy of Sciences, Prague, Czech Republic
⁵ Astronomical Institute, MFF UK, Prague, Czech Republic
⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, Spain
⁷ Instituto de Astronomía de la UNAM, Ensenada, Baja Cfa., Mexico
⁸ Circuito Exterior s/n, Ciudad Universitaria, Delg. Coyoacán, México DF, Mexico
⁹ Faculty of Electrical Engneering (FEL-CVUT), Prague, Czech Republic

Received: 27 December 2021
Accepted: 8 March 2022

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.