Improved Constraints on H0 from a Combined Analysis of Gravitational-wave and Electromagnetic Emission from GW170817

• Published in: Astrophysical journal. Letters 2017-12, Vol.851 (2)
• Main Authors: Guidorzi, C., Margutti, R., Brout, D., Scolnic, D., Fong, W., Alexander, K. D., Cowperthwaite, P. S., Annis, J., Berger, E., Blanchard, P. K., Chornock, R., Coppejans, D. L., Eftekhari, T., Frieman, J. A., Huterer, D., Nicholl, M., Soares-Santos, M., Terreran, G., Villar, V. A., Williams, P. K. G.
• Format: Article
• Language: English
• Subjects: ASTRONOMY AND ASTROPHYSICS; Broadband; Correlation analysis; Cosmic microwave background; Distance measurement; Emission analysis; Emission measurements; gamma-ray burst: general; Gravitation; Gravitational waves; Inclination; Luminosity; Modelling; Radio emission; Uncertainty; Velocity; Wave analysis
• ISSN: 2041-8205; 2041-8213; 2041-8213
• DOI: 10.3847/2041-8213/aaa009

The luminosity distance measurement of GW170817 derived from gravitational-wave analysis in Abbott et al. (2017a, hereafter 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 broadband 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 Ek ∼ 1048 −3 × 1050 erg propagating into an environment with density n ∼ 10−2-10−4 cm−3, with preference for wider jets (opening angle θj = 15°). For these jets, our modeling indicates an off-axis angle θobs ∼ 25°-50°. We combine our constraints on θobs with the joint distance-inclination constraint from LIGO. Using the same ∼170 km s−1 peculiar velocity uncertainty assumed in A17:H0 but with an inclination constraint from the afterglow data, we get a value of km s−1 Mpc−1, which is higher than the value of km s−1 Mpc−1 found in A17:H0. Further, using a more realistic peculiar velocity uncertainty of 250 km s−1 derived from previous work, we find km s−1 Mpc−1 for H0 from this system. This is in modestly better agreement with the local distance ladder than the Planck cosmic microwave background, though such a significant discrimination will require ∼50 such events. Measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.