Constraining the observer angle of the kilonova AT2017gfo associated with GW170817: Implications for the Hubble constant

TL;DR

This paper constrains the observer viewing angle of the kilonova AT2017gfo associated with GW170817 using broad-band photometry, leading to an improved estimate of the Hubble constant, with implications for gravitational wave cosmology.

Contribution

First independent constraint on the viewing angle of GW170817's kilonova using broad-band photometry, improving H0 estimation by 24% and demonstrating robustness against model uncertainties.

Findings

Observer angle constrained to 32.5° +11.7°/-9.7°

H0 improved by 24% when combining GW and kilonova data

Broad wavelength coverage enhances angle constraints

Abstract

There is a strong degeneracy between the luminosity distance ($D_L$) and the observer viewing angle ($\theta_{\rm obs}$; hereafter viewing angle) of the gravitational wave (GW) source with an electromagnetic counterpart, GW170817. Here, for the first time, we present independent constraints on $\theta_{\rm obs} = 32.5 ^{\circ +11.7}_{-9.7}$ from broad-band photometry of the kilonova (kN) AT2017gfo associated with GW170817. These constraints are consistent with independent results presented in the literature using the associated gamma ray burst GRB170817A. Combining the constraints on $\theta_{\rm obs}$ with the GW data, we find an improvement of 24$\%$ on $H_0$. The observer angle constraints are insensitive to other model parameters, e.g. the ejecta mass, half-opening angle of the lanthanide-rich region and the temperature. A broad wavelength coverage extending to the near infrared is helpful to robustly constrain $\theta_{\rm obs}$. While the improvement on $H_0$ presented here is smaller than the one from high angular resolution imaging of the radio counterpart of GW170817, kN observations are significantly more feasible at the typical distances of such events from current and future LIGO-Virgo Collaboration observing runs ($D_L \sim 100$ Mpc). Our results are insensitive to the assumption on the peculiar velocity of the kN host galaxy.