# The on-axis view of GRB 170817A

## TL;DR

This paper models the jet structure of GRB 170817A, showing it is representative of typical short gamma-ray bursts and suggesting observed diversity is mainly due to viewing angles and environment rather than intrinsic differences.

## Contribution

It provides a detailed on-axis view of GRB 170817A's jet, linking its properties to the broader short GRB population and supporting a quasi-universal jet structure hypothesis.

## Key findings

- GRB 170817A's jet is typical of SGRBs.
- Observed diversity mainly due to extrinsic factors.
- Supports a quasi-universal jet structure hypothesis.

## Abstract

The peculiar short gamma-ray burst GRB 170817A has been firmly associated to the gravitational wave event GW170817, which has been unaninmously interpreted as due to the coalescence of a double neutron star binary. The unprecedented behaviour of the non-thermal afterglow led to a debate about its nature, which was eventually settled by high-resolution VLBI observations, which strongly support the off-axis structured jet scenario. Using information on the jet structure derived from multi-wavelength fitting of the afterglow emission and of the apparent VLBI image centroid motion, we compute the appearance of a GRB 170817A-like jet as seen by an on-axis observer and we compare it to the previously observed population of SGRB afterglows and prompt emission events. We find that the intrinsic properties of the GRB 170817A jet are representative of a typical event in the SGRB population, hinting at a quasi-universal jet structure. The diversity in the SGRB afterglow population could therefore be ascribed in large part to extrinsic (redshift, density of the surrounding medium, viewing angle) rather than intrinsic properties. Although more uncertain, the comparison can be extended to the prompt emission properties, leading to similar conclusions.