Inclination estimates from off-axis GRB afterglow modelling

TL;DR

This paper investigates how including lateral spreading in off-axis GRB afterglow models influences the estimation of the system's inclination angle, which is crucial for understanding GW-EM events and cosmological parameters.

Contribution

It introduces the effects of lateral spreading into afterglow models and demonstrates its impact on inclination estimates for GW-EM neutron star merger events.

Findings

Lateral spreading significantly affects inclination estimates.

Inclusion of lateral spreading can alter the inferred jet structure.

Modeling with lateral spreading improves parameter constraints.

Abstract

For gravitational wave (GW) detected neutron star mergers, one of the leading candidates for electromagnetic (EM) counterparts is the afterglow from an ultra-relativistic jet. Where this afterglow is observed, it will likely be viewed off-axis, such as the afterglow following GW170817/GRB 170817A. The temporal behaviour of an off-axis observed GRB afterglow can be used to reveal the lateral jet structure, and statistical model fits can put constraints on the various model free-parameters. Amongst these parameters is the inclination of the system to the line of sight. Along with the GW detection, the afterglow modelling provides the best constraint on the inclination to the line-of-sight and can improve the estimates of cosmological parameters e.g. the Hubble constant, from GW-EM events. However, modelling of the afterglow depends on the assumed jet structure and, often overlooked, the effects of lateral spreading. Here we show how the inclusion of lateral spreading in the afterglow models can affect the estimated inclination of GW-EM events.