[Paper Review] What powered AT2017gfo associated with GW170817

Shao-Ze Li, Liang-Duan Liu+2 more|arXiv (Cornell University)|Apr 18, 2018

Gamma-ray bursts and supernovae2 citations

TL;DR

The paper proposes that the optical transient AT2017gfo, linked to GW170817, was powered by energy injection from a long-lived neutron star remnant rather than solely by r-process radioactive decay. This model explains both the early and late emission components consistently with ejecta mass and opacity from numerical simulations, suggesting a supra-massive, low-magnetic-field neutron star formed in the merger.

ABSTRACT

The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star mergers. We argue that the standard interpretation of AT2017gfo as being powered by r-process radioactive decay faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, since 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 neutron star (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 (supra-massive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs.

Motivation & Objective

• To resolve the tension in explaining AT2017gfo’s peak luminosity and peak time under the standard r-process decay model.
• To investigate whether an additional energy source from the merger remnant could better explain the observed light curve.
• To evaluate the viability of two merger products—post-merger black hole with fallback accretion and a long-lived neutron star—as energy sources.
• To constrain the properties of the neutron star remnant using observational constraints on ejecta mass and opacity.
• To provide a stringent constraint on the equation of state of neutron stars based on the inferred remnant properties.

Proposed method

• Modeling the light curve of AT2017gfo using energy injection from a spinning-down neutron star remnant as the primary power source.
• Comparing the predicted light curve from neutron star spin-down with observed photometric data across multiple bands.
• Using ejecta mass and opacity values derived from numerical simulations of neutron star mergers to constrain the model parameters.
• Evaluating the alternative scenario of fallback accretion onto a post-merger black hole and assessing its ability to explain early emission only.
• Assessing the consistency of the neutron star model with observed peak time, peak luminosity, and spectral evolution.
• Applying constraints from the observed low opacity and required ejecta mass to infer properties of the remnant, such as magnetic field strength and lifetime.

Experimental results

Research questions

• RQ1 Can the observed light curve of AT2017gfo be consistently explained by r-process radioactive decay alone, given the required ejecta mass and opacity?
• RQ2 Does energy injection from a long-lived neutron star remnant provide a better fit to the full light curve of AT2017gfo than standard radioactive decay?
• RQ3 What are the implications of the observed emission for the nature of the merger remnant—specifically, its lifetime and magnetic field strength?
• RQ4 How do the observed luminosity and peak time constrain the mass and opacity of the ejecta in the context of neutron star merger simulations?
• RQ5 What does the data imply about the equation of state of neutron stars, particularly regarding the stability and maximum mass of supra-massive neutron stars?

Key findings

• The standard r-process decay model struggles to simultaneously reproduce the peak luminosity and peak time of AT2017gfo due to conflicting requirements on ejecta mass and opacity.
• Energy injection from a post-merger black hole with fallback accretion can explain only the early emission, not the late-time component.
• A long-lived neutron star remnant provides a consistent explanation for both early and late emission components through spin-down powered energy injection.
• The required ejecta mass and opacity in the neutron star model are well consistent with results from numerical simulations of neutron star mergers.
• The data strongly suggest the merger remnant is a supra-massive or permanently stable neutron star with a low magnetic field.
• This result provides a stringent constraint on the equation of state of neutron stars, favoring stiff equations of state that support long-lived massive neutron stars.