Synthetic radio images of structured GRB and kilonova afterglows

Abstract

ABSTRACT In this paper, we compute and analyse synthetic radio images of gamma-ray bursts (GRBs) and kilonova afterglows. For modelling 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. 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 GRB 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. However, this effect becomes negligible around the light curve peak of the GRB afterglow. For a far off-axis observer, neglecting the presence of the kilonova afterglow may lead to systematic errors in the inference of GRB 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 $t\gtrsim 1500\,$ d.