Inverse compton scattered merger-nova: late X-ray counterpart of gravitational wave signals from NS-NS/BH mergers

TL;DR

This paper predicts a late X-ray emission from neutron star mergers caused by inverse Compton scattering of merger-nova photons, which could serve as an electromagnetic counterpart to gravitational wave signals and help understand jet and cocoon properties.

Contribution

It introduces a new model for late X-ray counterparts from NS-NS/BH mergers based on inverse Compton scattering in the cocoon region, linking EM signals to merger dynamics.

Findings

X-ray emission peaks days after merger

Detectable up to 200 Mpc with current X-ray telescopes

Provides insights into jet and cocoon structures

Abstract

The recent observations of GW170817 and its electromagnetic (EM) counterparts show that double neutron star mergers could lead to rich and bright EM emissions. Recent numerical simulations suggest that neutron star and neutron star/black hole (NS-NS/BH) mergers would leave behind a central remnant surrounded by a mildly isotropic ejecta. The central remnant could launch a collimated jet and when the jet propagating through the ejecta, a mildly relativistic cocoon would be formed and the interaction between the cocoon and the ambient medium would accelerate electrons via external shock in a wide angle. So that the merger-nova photons (i.e., thermal emission from the ejecta) would be scattered into higher frequency via inverse compton (IC) process when they propagating through the cocoon shocked region. We find that the IC scattered component peaks at X-ray band and it will reach its peak luminosity in order of days (simultaneously with the merger-nova emission). With current X-ray detectors, such a late X-ray component could be detected out to 200 Mpc, depending on the merger remnant properties. It could serve as an important electromagnetic counterpart of gravitational wave signals from NS-NS/BH mergers. Nevertheless, simultaneous detection of such a late X-ray signal and the merger-nova signal could shed light on the cocoon properties and the concrete structure of the jet.