The binary neutron-star merger GW170817 was discovered in August 2017. Many telescopes saw different kinds of light after the discovery, but only NASA’s Chandra X-ray Observatory is still making a detection. Early Chandra data revealed the presence of a narrow jet that has slowed down and expanded over time. The new study presents X-ray evidence for a shock — similar to a sonic boom — in the aftermath of the neutron-star merger. An artist’s...