Modelling of Long-Term Afterglow Counterparts to Gravitational Wave Events: The Full View of GRB 170817A

TL;DR

This paper enhances afterglow modeling of GW170817's gamma-ray burst by incorporating astrometric data, faint source statistics, and electron populations, improving understanding of late-time emission and external medium properties.

Contribution

It introduces comprehensive modeling updates including astrometric and statistical methods, applied to GW170817 to refine physical parameters and assess late-time emission evidence.

Findings

Weak evidence for a new late-time emission component

Improved fit to centroid evolution and external medium density

Enhanced modeling techniques for multi-messenger astrophysics

Abstract

The arrival of gravitational wave astronomy and a growing number of time-domain focused observatories are set to lead to a increasing number of detections of short gamma-ray bursts (GRBs) launched with a moderate inclination to Earth. Being nearby events, these are also prime candidates for very long-term follow-up campaigns and very-long-baseline interferometry (VLBI), which has implications for multi-messenger modelling, data analysis, and statistical inference methods applied to these sources. Here we present a comprehensive modelling update that directly incorporates into afterglowpy astrometric observations of the GRB position, Poissonian statistics for faint sources, and modelling of a trans-relativistic population of electrons. We use the revolutionary event GW170817 to demonstrate the impact of these extensions both for the best-fit physics parameters and model selection methods that assess the statistical significance of additional late-time emission components. By including in our analysis the latest Chandra X-ray observations of GRB 170817A, we find only weak evidence (less than two sigma) for a new emission component at late times, which makes for a slightly more natural fit to the centroid evolution and prediction for the external medium density.