Last year saw the first event that astronomers detected using both photons and gravitational waves. The event, a gamma-ray burst, was triggered by the merger of two neutron stars, forming a single mass of neutrons that was large enough to collapse into a black hole shortly afterwards. Before the black hole appeared, however, lots of material was ejected into space, where it formed heavier elements.

Now, researchers are reporting follow-up observations that suggest the black hole has formed jets of material that are moving at a substantial fraction of the speed of light. The jets have moved fast enough that we have been able to watch them drive through the expanding shell of debris and pass beyond it into open space, helping reveal more details about what's been going on post-collision.

Superluminal

The neutron-star collision was picked up nearly simultaneously by the Fermi Space Telescope, which detected gamma rays produced during the collision, and the LIGO/VIRGO gravitational-wave detectors, which named the event GW170817. In response to the initial detection, many telescopes observed the initial debris in a variety of wavelengths. These observations confirmed that these collisions act as factories for heavy, neutron-rich elements that would be difficult to form in supernovae.

Since the collision, this material has formed a spherical shell that's expanding outward from the site where the collision took place. Researchers have tracked the light emitted by the shell, seeing it gradually ramp up to a peak output about 150 days after the collision, after which the emissions declined relatively quickly. This led to an obvious question: what was driving the gradual increase in light emitted by the material?

That's the question tackled by the new research, which focuses on radio emissions from the site of the collision. The work is based on observations done using what's called the High Sensitivity Array, which is an array of two arrays (the Very Long Baseline Array and Karl G. Jansky Very Large Array) and the Robert C. Byrd Green Bank Telescope. Combined, this hardware provides enough resolution to track the motion of radio sources within the debris field. Data was obtained 150 days apart on either side of the peak of emissions.

The earlier data, from day 75, showed radio emissions that were relatively diffuse. By contrast, the emissions at day 230 were much easier to resolve and, critically, weren't in the same location. In fact, from the perspective of Earth, the difference in the locations indicate that whatever was causing the emissions was "superluminal," or appeared to move faster than the speed of light.

What's going on here? If the radio sources were simply moving within the shell of debris, then there would be nothing like this level of motion. Instead, the data favors rapidly moving material. And, conveniently, the product of the neutron star collision is likely to make some near-light-speed material.

Jets!

Neutron stars and black holes can have extremely intense magnetic fields, which can accelerate charged particles. As a result, they often produce what are called "jets" of particles that are shot out of their magnetic poles at nearly the speed of light.

In this case, the authors test two different models of jets. In one, the jet isn't very focused—it's more like a broad cone that simply shoots particles into the shell of material ejected by the collision. In the second, the jets are narrow and force their way through the shell until they exit into the space beyond. The radio telescope data—specifically the distance between the site of emissions during the two observations—favors the latter, provided that one of the jets is pointed close to straight at Earth (at a 30° angle or less).

Overall, the authors suggest that the gradual increase in emissions came during the period when the jet was forcing its way through the shell of material, depositing its energy there. The peak came after it broke through and the jet itself dominated the light produced. After this point, the material in the jet was able to escape the immediate environment and slow down, leading to the rapid decline in light production.

While we've seen jets produced by a variety of objects, this is likely to be one of the few times when we've gotten a chance to observe one almost from the moment of its formation. Given how frequent and important they are for objects ranging from neutron stars to supermassive black holes, these observations and others of GW170817 will undoubtedly inform our models of the phenomenon. And, once LIGO starts taking data again, there's a good chance we'll get additional examples to consider.

Nature, 2018. DOI: 10.1038/s41586-018-0486-3 (About DOIs).