A Long-lived Remnant Neutron Star after GW170817 Inferred from Its Associated Kilonova

Abstract

The successful joint observation of the gravitational wave event GW170817 and its multi-wavelength electromagnetic counterparts first enables human to witness a definite merger event of two neutron stars (NSs). This historical event confirms the origin of short-duration gamma-ray bursts (GRBs), and in particular, identifies the theoretically-predicted kilonova phenomenon that is powered by radioactive decays of r-process heavy elements. However, whether a long-lived remnant NS could be formed during this merger event remains unknown, although such a central engine has been suggested by afterglow observations of some short-duration GRBs. By invoking this long-lived remnant NS, we here propose a model of hybrid energy sources for the kilonova AT2017gfo associated with GW 170817. While the early emission of AT2017gfo is still powered radioactively as usually suggested, its late emission is primarily caused by delayed energy injection from the remnant NS. In our model, only one single opacity is required and an intermediate value of κ ≃ 0.97 cm 2 g −1 is revealed, which could be naturally provided by lanthanide-rich ejecta that is deeply ionized by the emission from a wind of the NS. These self-consistent results indicate that a long-lived remnant NS, which must own a very stiff equation of state, had been formed during the merger event of GW170817. This provides a very stringent constraint on the strong interaction in nuclear-quark matter. It is further implied that such GW events could provide a probe of the early spin and magnetic evolutions of NSs, e.g., the burying of surface magnetic fields.

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

“…The case of GW170817/GRB 170817A is inconclusive. Even though observations are consistent with having a black hole merger product [40][41][42][43] , the existence of a long-lived NS engine is not ruled out and could be helpful in interpreting some phenomena of the event [44][45][46][47] .…”

Kilohertz gravitational waves from binary neutron star remnants: Time-domain model and constraints on extreme matter

“…The distribution of f2 is cut at κ T 2 < 70 to exclude binaries that undergo prompt collapse at merger. [37][38][39][40][41].…”

Jet–Cocoon Outflows from Neutron Star Mergers: Structure, Light Curves, and Fundamental Physics

“…In addition, the central engine might not become dormant after a few seconds, as assumed here. A long-active central engine, such as a magnetar, may result in extra energy injection in both the jet (Geng et al 2018) and the ejecta (Yu et al 2018), altering the simple geometry and dynamics that we assumed.…”