Synthetic radio images of structured GRB and kilonova afterglows

TL;DR

This paper presents synthetic radio images of GRB and kilonova afterglows, revealing how their structures and apparent motions evolve over time, and highlighting the importance of considering kilonova effects in interpreting observations.

Contribution

It introduces detailed models of radio images for GRB and kilonova afterglows, analyzing their structure and motion, and discusses implications for observational inference.

Findings

Kilonova afterglow sky maps are doughnut-like early and ring-like late.

Flux centroid moves away from the observer initially.

Kilonova afterglow influences inferred source properties, reducing apparent velocity and increasing size.

Abstract

In this paper, we compute and analyze synthetic radio images of gamma-ray bursts and kilonova afterglows. For modeling the former, we consider GRB170817A-inspired set of parameters, while for the latter, we employ ejecta profiles from numerical-relativity simulations. We find that the kilonova afterglow sky map has a doughnut-like structure at early times that becomes more ring-like at late times. This is caused by the fact that the synchrotron emission from electrons following Maxwellian distribution function dominates the early, beamed, emission while emissions from electrons following power-law distribution is important at late times. For an on-axis observer, the image flux centroid moves on the image plane initially away from the observer. Flux centroid displacement The image sizes, we find, are the largest for equal mass merger simulations with the soft equation of state. The presence of a kilonova afterglow affects the properties inferred from the source sky map even if the gamma-ray burst afterglow dominates the total flux density. The main effect is the reduction of the mean apparent velocity of the source, and an increase in the source size. Thus, neglecting the presence of the kilonova afterglow may lead to systematic errors in the inference of gamma-ray burst properties from the sky map observations. Notably, at the observing angle inferred for GRB170817A the presence of kilonova afterglow would affect the sky map properties only at very late times $t\gtrsim1500\,$days.