Improved constraints on H0 from a combined analysis of gravitational-wave and electromagnetic emission from GW170817

TL;DR

This paper refines the measurement of the Hubble constant by modeling the afterglow emission of GW170817 to better constrain the system's inclination, leading to a more precise H0 estimate that aligns more closely with local measurements.

Contribution

The study introduces an improved method to constrain the inclination angle of GW170817 using broad-band afterglow modeling, enhancing the precision of H0 measurement from gravitational-wave data.

Findings

H0 estimated at 74.0 ± 11.5/7.5 km/s/Mpc with peculiar velocity uncertainty of 170 km/sec.

H0 estimated at 75.5 ± 11.6/9.6 km/s/Mpc with a more realistic peculiar velocity uncertainty of 250 km/sec.

Results show better agreement with local distance ladder measurements than Planck CMB estimates.

Abstract

The luminosity distance measurement of GW170817 derived from GW analysis in Abbott et al. 2017 (here, A17:H0) is highly correlated with the measured inclination of the NS-NS system. To improve the precision of the distance measurement, we attempt to constrain the inclination by modeling the broad-band X-ray-to-radio emission from GW170817, which is dominated by the interaction of the jet with the environment. We update our previous analysis and we consider the radio and X-ray data obtained at $t<40$ days since merger. We find that the afterglow emission from GW170817 is consistent with an off-axis relativistic jet with energy $10^{48}\,\rm{erg}<E_{k}\le 3\times 10^{50} \,\rm{erg}$ propagating into an environment with density $n\sim10^{-2}-10^{-4} \,\rm{cm^{-3}}$, with preference for wider jets (opening angle $\theta_j=15$ deg). For these jets, our modeling indicates an off-axis angle $\theta_{\rm obs}\sim25-50$ deg. We combine our constraints on $\theta_{\rm obs}$ with the joint distance-inclination constraint from LIGO. Using the same $\sim 170$ km/sec peculiar velocity uncertainty assumed in A17:H0 but with an inclination constraint from the afterglow data, we get a value of $H_0=$$74.0 \pm \frac{11.5}{7.5}$ $\mbox{km/s/Mpc}$, which is higher than the value of $H_0=$$70.0 \pm \frac{12.0}{8.0}$ $\mbox{km/s/Mpc}$ found in A17:H0. Further, using a more realistic peculiar velocity uncertainty of 250 km/sec derived from previous work, we find $H_0=$$75.5 \pm \frac{11.6}{9.6}$ km/s/Mpc for H0 from this system. We note that this is in modestly better agreement with the local distance ladder than the Planck CMB, though a significant such discrimination will require $\sim 50$ such events. Future measurements at $t>100$ days of the X-ray and radio emission will lead to tighter constraints.