X-ray-powered macronovae

TL;DR

This paper proposes an X-ray-powered model for macronovae that explains infrared excesses observed after short gamma-ray bursts, allowing for smaller ejecta masses and broadening understanding of these phenomena.

Contribution

It introduces a novel X-ray-powered mechanism for macronovae, reducing the required ejecta mass and expanding the parameter space compared to previous models.

Findings

X-ray excess can power infrared emission via thermal re-emission.

Ejecta mass estimated around 10^{-3} to 10^{-2} solar masses.

Model explains multiple macronova candidates and informs gravitational wave counterpart searches.

Abstract

A macronova (or kilonova) was observed as an infrared excess several days after short gamma-ray burst, GRB 130603B. Although the $r$-process radioactivity is widely discussed as an energy source, it requires huge mass of ejecta from a neutron star (NS) binary merger. We propose a new model that the X-ray excess gives rise to the simultaneously observed infrared excess via thermal re-emission and explore what constraints this would place on the mass and velocity of the ejecta. This X-ray-powered model explains both the X-ray and infrared excesses with a single energy source by the central engine like a black hole, and allows for broader parameter region, in particular smaller ejecta mass $\sim10^{-3}-10^{-2}M_{\odot}$ with iron mixed as suggested by general relativistic simulations for typical NS-NS mergers, than the previous models. We also discuss the other macronova candidates in GRB 060614 and GRB 080503, and implications for the search of electromagnetic counterparts to gravitational waves.