GW170817 Most Likely Made a Black Hole

TL;DR

This study analyzes X-ray observations of GW170817 to determine whether its remnant is a neutron star or a black hole, concluding it most likely formed a black hole based on flux measurements and models.

Contribution

The paper provides new X-ray observational analysis and modeling to identify the nature of GW170817's remnant, favoring a black hole over a neutron star.

Findings

X-ray flux measurements are inconsistent with a long-lived neutron star remnant.

Absence of brightening over time suggests a black hole remnant.

Future observations can further distinguish between neutron star and black hole scenarios.

Abstract

There are two outstanding issues regarding the neutron-star merger event GW170817: the nature of the compact remnant and the interstellar shock. The mass of the remnant of GW170817, $\sim$2.7 $M_\odot$, implies the remnant could be either a massive, rotating, neutron star, or a black hole. We report Chandra Director's Discretionary Time observations made in 2017 December and 2018 January, and we reanalyze earlier observations from 2017 August and 2017 September, in order to address these unresolved issues. We estimate the X-ray flux from a neutron star remnant and compare that to the measured X-ray flux. If we assume that the spin-down luminosity of any putative neutron star is converted to pulsar wind nebula X-ray emission in the 0.5-8 keV band with an efficiency of $10^{-3}$, for a dipole magnetic field with $3 \times 10^{11}$ G < $B$ < $10^{14}$ G, a rising X-ray signal would result and would be brighter than that observed by day 107, we therefore conclude that the remnant of GW170817 is most likely a black hole. Independent of any assumptions of X-ray efficiency, however, if the remnant is a rapidly-rotating, magnetized, neutron star, the total energy in the external shock should rise by a factor $\sim$$10^2$ (to $\sim$$10^{52}$ erg) after a few years, therefore, Chandra observations over the next year or two that do not show substantial brightening will rule out such a remnant. The same observations can distinguish between two different models for the relativistic outflow, either an angular or radially varying structure.