Reverse Shock Emission and Ionization Break Out Powered by Post-merger Millisecond Magnetars

TL;DR

This paper explores the emission signatures across X-ray, UV, optical, and radio wavelengths from post-merger magnetars, aiming to understand ejecta ionization and opacity, with implications for observing r-process material in neutron star mergers.

Contribution

It investigates the potential of multi-wavelength timing observations to constrain the opacities of r-process ejecta from neutron star mergers, considering the effects of reverse shocks and ionization.

Findings

UV emission can ionize ejecta at early times.

Timing of emissions depends weakly on opacities.

Constraints on opacities from observations are limited.

Abstract

There is accumulating evidence that at least a fraction of binary neutron star mergers result in rapidly spinning magnetars, with subrelativistic neutron-rich ejecta as massive as a small fraction of solar mass. The ejecta could be heated continuously by the Poynting flux emanated from the central magnetars. Such Poynting flux could become lepton-dominated so that a reverse shock develops. It was demonstrated that such a picture is capable of accounting for the optical transient PTF11agg (Wang & Dai 2013b). In this paper we investigate the X-ray and ultraviolet (UV) radiation as well as the optical and radio radiation studied by Wang & Dai (2013b). UV emission is particularly important because it has the right energy to ionize the hot ejecta at times $t\lesssim 600$ s. It is thought that the ejecta of binary neutron star mergers are a remarkably pure sample of r-process material, about which our understanding is still incomplete. In this paper we evaluate the possibility of observationally determining the bound-bound and bound-free opacities of the r-process material by timing the X-ray, UV, and optical radiation. It is found that these timings depend on the opacities weakly and therefore only loose constraints on the opacities can be obtained.