Optical measurement of superluminal motion in the neutron-star merger GW170817

Understand

The afterglow of the binary neutron star merger GW170817 gave evidence for a structured relativistic jet and a link between such mergers and short gamma-ray bursts.

• Superluminal motion, found using radio very long baseline interferometry (VLBI), together with the afterglow light curve provided constraints on the viewing angle (14-28 degrees), the opening angle of the jet core (less than about 5 degrees), and a modest limit on the initial Lorentz factor of the jet core (more than 4).
• Here we report on another superluminal motion measurement, at seven times the speed of light, leveraging Hubble Space Telescope precision astrometry and previous radio VLBI data of GW170817.
• We thereby obtain a unique measurement of the Lorentz factor of the wing of the structured jet, as well as substantially improved constraints on the viewing angle (19-25 degrees) and the initial Lorentz factor of the jet core (more than 40).