The two-Gaussian jet structure and its formation process: Constraints from the afterglows of GRB 170817A

Abstract

The short gamma-ray burst (SGRB) GRB 170817A was the first observed GRB associated with a gravitational-wave event, providing a valuable opportunity to further study the producing mechanism of SGRBs. Its multi-wavelength afterglow emissions probably reveal the angular structure of the jet to us. And, if the jet structure is described phenomenologically by a Gaussian function, large sample statistical studies on SGRBs show that we probably need to introduce two-Gaussian functions to unify the emissions from GRB 170817A with other normal SGRBs. In other words, GRB 170817A's prompt emission is likely from an additional component of a jet with a larger angle. Therefore, in order to test the compatibility between this two-Gaussian jet structure and the afterglow data of GRB 170817A, we calculate the multi-wavelength afterglow light curves from such an off-axis jet structure, and find that the introduction of the outer Gaussian component can well explain the early uptrend of the GRB 170817A afterglow data. On this basis, we also calculate the propagation and breakout processes of the jet in the merger ejecta, which shows that these processes can naturally lead to the two-Gaussian jet structure obtained by the observational constraint, thus showing the overall self-consistency of the model. Furthermore, the observational constraint on the jet propagation process also provides a new clue for understanding the properties of merger ejecta.