Measuring the viewing angle of GW170817 with electromagnetic and gravitational waves

TL;DR

This paper measures the viewing angle of GW170817 using combined gravitational wave and electromagnetic observations, providing key insights into the geometry of the merger and its associated emissions.

Contribution

It introduces a method to determine the binary's viewing angle by combining GW data with a direct host galaxy distance measurement, including full posterior samples for community use.

Findings

Viewing angle is approximately 32 degrees with uncertainties.

Lower limit on viewing angle is 13 degrees, robust to prior assumptions.

Results support off-axis structured jet models for the merger emissions.

Abstract

The joint detection of gravitational waves (GWs) and electromagnetic (EM) radiation from the binary neutron star merger GW170817 ushered in a new era of multi-messenger astronomy. Joint GW-EM observations can be used to measure the parameters of the binary with better precision than either observation alone. Here, we use joint GW-EM observations to measure the viewing angle of GW170817, the angle between the binary's angular momentum and the line of sight. We combine a direct measurement of the distance to the host galaxy of GW170817 (NGC 4993) of $40.7\pm 2.36$ Mpc with the Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo GW data and find that the viewing angle is $32^{+10}_{-13}\,\pm 1.7$ degrees (90% confidence, statistical, and systematic errors). We place a conservative lower limit on the viewing angle of $\ge 13^\circ$, which is robust to the choice of prior. This measurement provides a constraint on models of the prompt $\gamma$-ray and radio/X-ray afterglow emission associated with the merger; for example, it is consistent with the off-axis viewing angle inferred for a structured jet model. We provide for the first time the full posterior samples from Bayesian parameter estimation of LIGO/Virgo data to enable further analysis by the community.