What Powered the Optical Transient AT2017gfo Associated with GW170817?

TL;DR

This paper explores the energy sources powering the optical transient AT2017gfo associated with GW170817, suggesting a long-lived neutron star remnant as the primary energy contributor, challenging the standard radioactive decay model.

Contribution

It proposes that a long-lived neutron star remnant, rather than radioactive decay alone, explains the observed emission, providing new insights into neutron star merger physics.

Findings

A long-lived neutron star remnant can account for both early and late emission.

Radioactive decay alone struggles to explain the peak luminosity and timing.

Constraints on neutron star equations of state are derived from the remnant's properties.

Abstract

The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star (NS) mergers. We argue that the standard interpretation of AT2017gfo as being powered by radioactive decays of r-process elements faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, as it is not easy to achieve the required high mass, and especially the low opacity of the ejecta required to fit the data. A plausible solution would be to invoke an additional energy source, which is probably provided by the merger product. We consider energy injection from two types of the merger products: (1) a post-merger black hole powered by fallback accretion; and (2) a long-lived NS remnant. The former case can only account for the early emission of AT2017gfo, with the late emission still powered by radioactive decay. In the latter case, both early- and late-emission components can be well interpreted as due to energy injection from a spinning-down NS, with the required mass and opacity of the ejecta components well consistent with known numerical simulation results. We suggest that there is a strong indication that the merger product of GW170817 is a long-lived (supramassive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs.