Modelling kilonova afterglows: Effects of the thermal electron population and interaction with GRB outflows

Given an increasing number of gamma-ray bursts accompanied by potential kilonovae there is a growing importance to advance modelling of kilonova afterglows. This electromagnetic signature might play an important role in the multi-messenger picture of binary neutron star mergers and offer a possibility to infer the ejecta properties and, consequently, the binary properties. In this work, we investigate how the presence of two electron populations that follow a Maxwellian (thermal) and a power-law (non-thermal) distributions affect kilonova afterglow light curves. The modelling is done using the semi-analytic afterglow model, $\texttt{PyBlastAfterglow}$. We consider a set of kilonova ejecta profiles from ab-initio numerical relativity binary neutron star merger simulations, targeted to GW170817. We find that the emission from thermal electrons dominates at early times. If the interstellar medium density is sufficiently high (${\simeq}0.1\,$cm$^{-3}$) it adds an early time peak to the light curve. As ejecta decelerates the spectral and temporal indexes change in a characteristic way that, if observed, can be used to reconstruct the ejecta velocity distribution. For the low interstellar medium density, inferred for GRB170817A, the emission from the thermal electron population is dimmer than that from the non-thermal. We also assess how kilonova afterglow light curves change if the interstellar medium has been altered by laterally expanding gamma-ray burst ejecta. For the latter we consider properties informed by observations of GRB170817A. We find that the main effect is the emission suppression at early time ${\lesssim}10^{3}\,$days, and at its maximum it reaches ${\sim}40\%$. The subsequent rebrightening, when these ejecta break through and shocks form, is very mild (${\lesssim}10\%$), smeared over time and may not be observable.Comment: 20 pages, 14 figure