Update on Neutron Star Smash-Up: Jet Hits a Roadblock

Millions of years ago, a pair of extremely dense stars, called neutron stars, collided in a violent smash-up that shook space and time. On August 17, 2017, both gravitational waves—ripples in space and time—and light waves emitted during that neutron star merger finally reached Earth. The gravitational waves came first and were detected by the twin detectors of the National Science Foundation (NSF)-funded Laser Interferometry Gravitational-wave Observatory (LIGO), aided by the European Virgo observatory. The light waves were observed seconds, days, and months later by dozens of telescopes on the ground and in space.

Now, scientists from Caltech and several other institutions are reporting that light with radio wavelengths continues to brighten more than 100 days after the August 17 event. These radio observations indicate that a jet, launched from the two neutron stars as they collided, is slamming into surrounding material and creating a slower-moving, billowy cocoon.

"We think the jet is dumping its energy into the cocoon," says Gregg Hallinan, an assistant professor of astronomy at Caltech. "At first, people thought the material from the collision was coming out in a jet like a firehose, but we are finding that that the flow of material is slower and wider, expanding outward like a bubble."

The findings, made with the Karl G. Jansky Very Large Array in New Mexico, the Australia Telescope Compact Array, and the Giant Metrewave Radio Telescope in India, are reported in a new paper in the December 20 online issue of the journal Nature. The lead author is Kunal Mooley (PhD '15), formerly of the University of Oxford and now a Jansky Fellow at Caltech.

The new data argue against a popular theory describing the aftermath of the neutron star merger—a theory that proposes the event created a fast-moving and beam-like jet thought to be associated with extreme blasts of energy called gamma-ray bursts, and in particular with short gamma-ray bursts, or sGRBs. Scientists think that sGRBs, which pop up every few weeks in our skies, arise from the merger of a pair of neutron stars or the merger of a neutron star with a black hole (an event that has yet to be detected by LIGO). An sGRB is seen when the jet points exactly in the direction of Earth.

On August 17, NASA's Fermi Gamma-ray Space Telescope and the European INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) missions detected gamma rays just seconds after the neutron stars merged. The gamma rays were much weaker than what is expected for sGRBS, so the researchers reasoned that a fast and narrowly focused jet was produced but must have been pointed slightly askew from the direction of Earth, or off-axis.

The radio emission—originally detected 16 days after the August 17 event and still measurable and increasing in strength as of December 2—tells a different story. If the jet had been fast and beam-like, the radio light would have weakened with time as the jet lost energy. The fact that the brightness of the radio light is increasing instead suggests the presence of a cocoon that is choking the jet. The reason for this is complex, but it has to do with the fact that the slower-moving, wider-angle material of the cocoon gives off more radio light than the faster-moving, sharply focused jet material.

"It's like the jet was fogged out," says Mooley. "The jet may be off-axis, but it is not a simple pointed beam or as fast as some people thought. It may be blocked off by material thrown off during the merger, giving rise to a cocoon and emitting light in many different directions."

This means that the August 17 event was not a typical sGRB as originally proposed.

"Standard sGRBs are 10,000 times brighter than we saw for this event," says Hallinan. "Many people thought this was because the gamma-ray emission was off-axis and thus much weaker. But it turns out that the gamma rays are coming from the cocoon rather than the jet. It is possible that the jet managed to eventually break out through the cocoon, but we haven't seen any evidence for this yet. It is more likely that it got trapped and snuffed out by the cocoon."

The possibility that a cocoon was involved in the August 17 event was originally proposed in a study led by Caltech's Mansi Kasliwal (MS '07, PhD '11), assistant professor of astronomy, and colleagues. She and her team from the NSF-funded Global Relay of Observatories Watching Transients Happen (GROWTH) project observed the event at multiple wavelengths using many different telescopes.

"The cocoon model explains puzzling features we have observed in the neutron star merger," says Kasliwal. "It fits observations across the electromagnetic spectrum, from the early blue light we witnessed to the radio waves and X-rays that turned on later. The cocoon model had predicted that the radio emission would continue to increase in brightness, and that's exactly what we see."  

The researchers say that future observations with LIGO, Virgo, and other telescopes will help further clarify the origins and mechanisms of these extreme events. The observatories should be able to detect additional neutron star mergers—and perhaps eventually, mergers of neutron stars and black holes.

Work at Caltech on this study, titled "A mildly relativistic wide-angle outflow in the neutron star merger GW170817," was funded by the NSF, the Sloan Research Foundation, and Research Corporation for Science Advancement. Other Caltech authors are Kishalay De, a graduate student, and Shri Kulkarni, George Ellery Hale Professor of Astronomy and Planetary Science.